options.go

// Copyright 2011 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.

package kvstore

import (
	"bytes"
	"fmt"
	"io"
	"runtime"
	"strconv"
	"strings"
	"time"

	"github.com/cockroachdb/errors"
	"github.com/edgelesssys/ego-kvstore/internal/base"
	"github.com/edgelesssys/ego-kvstore/internal/cache"
	"github.com/edgelesssys/ego-kvstore/internal/humanize"
	"github.com/edgelesssys/ego-kvstore/internal/keyspan"
	"github.com/edgelesssys/ego-kvstore/internal/manifest"
	"github.com/edgelesssys/ego-kvstore/objstorage/remote"
	"github.com/edgelesssys/ego-kvstore/rangekey"
	"github.com/edgelesssys/ego-kvstore/sstable"
	"github.com/edgelesssys/ego-kvstore/vfs"
)

const (
	cacheDefaultSize       = 8 << 20 // 8 MB
	defaultLevelMultiplier = 10
)

// Compression exports the base.Compression type.
type Compression = sstable.Compression

// Exported Compression constants.
const (
	DefaultCompression = sstable.DefaultCompression
	NoCompression      = sstable.NoCompression
	SnappyCompression  = sstable.SnappyCompression
)

// FilterType exports the base.FilterType type.
type FilterType = base.FilterType

// Exported TableFilter constants.
const (
	TableFilter = base.TableFilter
)

// FilterWriter exports the base.FilterWriter type.
type FilterWriter = base.FilterWriter

// FilterPolicy exports the base.FilterPolicy type.
type FilterPolicy = base.FilterPolicy

// TablePropertyCollector exports the sstable.TablePropertyCollector type.
type TablePropertyCollector = sstable.TablePropertyCollector

// BlockPropertyCollector exports the sstable.BlockPropertyCollector type.
type BlockPropertyCollector = sstable.BlockPropertyCollector

// BlockPropertyFilter exports the sstable.BlockPropertyFilter type.
type BlockPropertyFilter = base.BlockPropertyFilter

// ShortAttributeExtractor exports the base.ShortAttributeExtractor type.
type ShortAttributeExtractor = base.ShortAttributeExtractor

// UserKeyPrefixBound exports the sstable.UserKeyPrefixBound type.
type UserKeyPrefixBound = sstable.UserKeyPrefixBound

// IterKeyType configures which types of keys an iterator should surface.
type IterKeyType int8

const (
	// IterKeyTypePointsOnly configures an iterator to iterate over point keys
	// only.
	IterKeyTypePointsOnly IterKeyType = iota
	// IterKeyTypeRangesOnly configures an iterator to iterate over range keys
	// only.
	IterKeyTypeRangesOnly
	// IterKeyTypePointsAndRanges configures an iterator iterate over both point
	// keys and range keys simultaneously.
	IterKeyTypePointsAndRanges
)

// String implements fmt.Stringer.
func (t IterKeyType) String() string {
	switch t {
	case IterKeyTypePointsOnly:
		return "points-only"
	case IterKeyTypeRangesOnly:
		return "ranges-only"
	case IterKeyTypePointsAndRanges:
		return "points-and-ranges"
	default:
		panic(fmt.Sprintf("unknown key type %d", t))
	}
}

// IterOptions hold the optional per-query parameters for NewIter.
//
// Like Options, a nil *IterOptions is valid and means to use the default
// values.
type IterOptions struct {
	// LowerBound specifies the smallest key (inclusive) that the iterator will
	// return during iteration. If the iterator is seeked or iterated past this
	// boundary the iterator will return Valid()==false. Setting LowerBound
	// effectively truncates the key space visible to the iterator.
	LowerBound []byte
	// UpperBound specifies the largest key (exclusive) that the iterator will
	// return during iteration. If the iterator is seeked or iterated past this
	// boundary the iterator will return Valid()==false. Setting UpperBound
	// effectively truncates the key space visible to the iterator.
	UpperBound []byte
	// TableFilter can be used to filter the tables that are scanned during
	// iteration based on the user properties. Return true to scan the table and
	// false to skip scanning. This function must be thread-safe since the same
	// function can be used by multiple iterators, if the iterator is cloned.
	TableFilter func(userProps map[string]string) bool
	// SkipPoint may be used to skip over point keys that don't match an
	// arbitrary predicate during iteration. If set, the Iterator invokes
	// SkipPoint for keys encountered. If SkipPoint returns true, the iterator
	// will skip the key without yielding it to the iterator operation in
	// progress.
	//
	// SkipPoint must be a pure function and always return the same result when
	// provided the same arguments. The iterator may call SkipPoint multiple
	// times for the same user key.
	SkipPoint func(userKey []byte) bool
	// PointKeyFilters can be used to avoid scanning tables and blocks in tables
	// when iterating over point keys. This slice represents an intersection
	// across all filters, i.e., all filters must indicate that the block is
	// relevant.
	//
	// Performance note: When len(PointKeyFilters) > 0, the caller should ensure
	// that cap(PointKeyFilters) is at least len(PointKeyFilters)+1. This helps
	// avoid allocations in Pebble internal code that mutates the slice.
	PointKeyFilters []BlockPropertyFilter
	// RangeKeyFilters can be usefd to avoid scanning tables and blocks in tables
	// when iterating over range keys. The same requirements that apply to
	// PointKeyFilters apply here too.
	RangeKeyFilters []BlockPropertyFilter
	// KeyTypes configures which types of keys to iterate over: point keys,
	// range keys, or both.
	KeyTypes IterKeyType
	// RangeKeyMasking can be used to enable automatic masking of point keys by
	// range keys. Range key masking is only supported during combined range key
	// and point key iteration mode (IterKeyTypePointsAndRanges).
	RangeKeyMasking RangeKeyMasking

	// OnlyReadGuaranteedDurable is an advanced option that is only supported by
	// the Reader implemented by DB. When set to true, only the guaranteed to be
	// durable state is visible in the iterator.
	// - This definition is made under the assumption that the FS implementation
	//   is providing a durability guarantee when data is synced.
	// - The visible state represents a consistent point in the history of the
	//   DB.
	// - The implementation is free to choose a conservative definition of what
	//   is guaranteed durable. For simplicity, the current implementation
	//   ignores memtables. A more sophisticated implementation could track the
	//   highest seqnum that is synced to the WAL and published and use that as
	//   the visible seqnum for an iterator. Note that the latter approach is
	//   not strictly better than the former since we can have DBs that are (a)
	//   synced more rarely than memtable flushes, (b) have no WAL. (a) is
	//   likely to be true in a future CockroachDB context where the DB
	//   containing the state machine may be rarely synced.
	// NB: this current implementation relies on the fact that memtables are
	// flushed in seqnum order, and any ingested sstables that happen to have a
	// lower seqnum than a non-flushed memtable don't have any overlapping keys.
	// This is the fundamental level invariant used in other code too, like when
	// merging iterators.
	//
	// Semantically, using this option provides the caller a "snapshot" as of
	// the time the most recent memtable was flushed. An alternate interface
	// would be to add a NewSnapshot variant. Creating a snapshot is heavier
	// weight than creating an iterator, so we have opted to support this
	// iterator option.
	OnlyReadGuaranteedDurable bool
	// UseL6Filters allows the caller to opt into reading filter blocks for L6
	// sstables. Helpful if a lot of SeekPrefixGEs are expected in quick
	// succession, that are also likely to not yield a single key. Filter blocks in
	// L6 can be relatively large, often larger than data blocks, so the benefit of
	// loading them in the cache is minimized if the probability of the key
	// existing is not low or if we just expect a one-time Seek (where loading the
	// data block directly is better).
	UseL6Filters bool

	// Internal options.

	logger Logger
	// Level corresponding to this file. Only passed in if constructed by a
	// levelIter.
	level manifest.Level
	// disableLazyCombinedIteration is an internal testing option.
	disableLazyCombinedIteration bool
	// snapshotForHideObsoletePoints is specified for/by levelIter when opening
	// files and is used to decide whether to hide obsolete points. A value of 0
	// implies obsolete points should not be hidden.
	snapshotForHideObsoletePoints uint64

	// NB: If adding new Options, you must account for them in iterator
	// construction and Iterator.SetOptions.
}

// GetLowerBound returns the LowerBound or nil if the receiver is nil.
func (o *IterOptions) GetLowerBound() []byte {
	if o == nil {
		return nil
	}
	return o.LowerBound
}

// GetUpperBound returns the UpperBound or nil if the receiver is nil.
func (o *IterOptions) GetUpperBound() []byte {
	if o == nil {
		return nil
	}
	return o.UpperBound
}

func (o *IterOptions) pointKeys() bool {
	if o == nil {
		return true
	}
	return o.KeyTypes == IterKeyTypePointsOnly || o.KeyTypes == IterKeyTypePointsAndRanges
}

func (o *IterOptions) rangeKeys() bool {
	if o == nil {
		return false
	}
	return o.KeyTypes == IterKeyTypeRangesOnly || o.KeyTypes == IterKeyTypePointsAndRanges
}

func (o *IterOptions) getLogger() Logger {
	if o == nil || o.logger == nil {
		return DefaultLogger
	}
	return o.logger
}

// SpanIterOptions creates a SpanIterOptions from this IterOptions.
func (o *IterOptions) SpanIterOptions() keyspan.SpanIterOptions {
	if o == nil {
		return keyspan.SpanIterOptions{}
	}
	return keyspan.SpanIterOptions{
		RangeKeyFilters: o.RangeKeyFilters,
	}
}

// scanInternalOptions is similar to IterOptions, meant for use with
// scanInternalIterator.
type scanInternalOptions struct {
	IterOptions

	visitPointKey   func(key *InternalKey, value LazyValue, iterInfo IteratorLevel) error
	visitRangeDel   func(start, end []byte, seqNum uint64) error
	visitRangeKey   func(start, end []byte, keys []rangekey.Key) error
	visitSharedFile func(sst *SharedSSTMeta) error

	// skipSharedLevels skips levels that are shareable (level >=
	// sharedLevelStart).
	skipSharedLevels bool

	// includeObsoleteKeys specifies whether keys shadowed by newer internal keys
	// are exposed. If false, only one internal key per user key is exposed.
	includeObsoleteKeys bool

	// rateLimitFunc is used to limit the amount of bytes read per second.
	rateLimitFunc func(key *InternalKey, value LazyValue) error
}

// RangeKeyMasking configures automatic hiding of point keys by range keys. A
// non-nil Suffix enables range-key masking. When enabled, range keys with
// suffixes ≥ Suffix behave as masks. All point keys that are contained within a
// masking range key's bounds and have suffixes greater than the range key's
// suffix are automatically skipped.
//
// Specifically, when configured with a RangeKeyMasking.Suffix _s_, and there
// exists a range key with suffix _r_ covering a point key with suffix _p_, and
//
//	_s_ ≤ _r_ < _p_
//
// then the point key is elided.
//
// Range-key masking may only be used when iterating over both point keys and
// range keys with IterKeyTypePointsAndRanges.
type RangeKeyMasking struct {
	// Suffix configures which range keys may mask point keys. Only range keys
	// that are defined at suffixes greater than or equal to Suffix will mask
	// point keys.
	Suffix []byte
	// Filter is an optional field that may be used to improve performance of
	// range-key masking through a block-property filter defined over key
	// suffixes. If non-nil, Filter is called by Pebble to construct a
	// block-property filter mask at iterator creation. The filter is used to
	// skip whole point-key blocks containing point keys with suffixes greater
	// than a covering range-key's suffix.
	//
	// To use this functionality, the caller must create and configure (through
	// Options.BlockPropertyCollectors) a block-property collector that records
	// the maxmimum suffix contained within a block. The caller then must write
	// and provide a BlockPropertyFilterMask implementation on that same
	// property. See the BlockPropertyFilterMask type for more information.
	Filter func() BlockPropertyFilterMask
}

// BlockPropertyFilterMask extends the BlockPropertyFilter interface for use
// with range-key masking. Unlike an ordinary block property filter, a
// BlockPropertyFilterMask's filtering criteria is allowed to change when Pebble
// invokes its SetSuffix method.
//
// When a Pebble iterator steps into a range key's bounds and the range key has
// a suffix greater than or equal to RangeKeyMasking.Suffix, the range key acts
// as a mask. The masking range key hides all point keys that fall within the
// range key's bounds and have suffixes > the range key's suffix. Without a
// filter mask configured, Pebble performs this hiding by stepping through point
// keys and comparing suffixes. If large numbers of point keys are masked, this
// requires Pebble to load, iterate through and discard a large number of
// sstable blocks containing masked point keys.
//
// If a block-property collector and a filter mask are configured, Pebble may
// skip loading some point-key blocks altogether. If a block's keys are known to
// all fall within the bounds of the masking range key and the block was
// annotated by a block-property collector with the maximal suffix, Pebble can
// ask the filter mask to compare the property to the current masking range
// key's suffix. If the mask reports no intersection, the block may be skipped.
//
// If unsuffixed and suffixed keys are written to the database, care must be
// taken to avoid unintentionally masking un-suffixed keys located in the same
// block as suffixed keys. One solution is to interpret unsuffixed keys as
// containing the maximal suffix value, ensuring that blocks containing
// unsuffixed keys are always loaded.
type BlockPropertyFilterMask interface {
	BlockPropertyFilter

	// SetSuffix configures the mask with the suffix of a range key. The filter
	// should return false from Intersects whenever it's provided with a
	// property encoding a block's minimum suffix that's greater (according to
	// Compare) than the provided suffix.
	SetSuffix(suffix []byte) error
}

// WriteOptions hold the optional per-query parameters for Set and Delete
// operations.
//
// Like Options, a nil *WriteOptions is valid and means to use the default
// values.
type WriteOptions struct {
	// Sync is whether to sync writes through the OS buffer cache and down onto
	// the actual disk, if applicable. Setting Sync is required for durability of
	// individual write operations but can result in slower writes.
	//
	// If false, and the process or machine crashes, then a recent write may be
	// lost. This is due to the recently written data being buffered inside the
	// process running Pebble. This differs from the semantics of a write system
	// call in which the data is buffered in the OS buffer cache and would thus
	// survive a process crash.
	//
	// The default value is true.
	Sync bool
}

// Sync specifies the default write options for writes which synchronize to
// disk.
var Sync = &WriteOptions{Sync: true}

// NoSync specifies the default write options for writes which do not
// synchronize to disk.
var NoSync = &WriteOptions{Sync: false}

// GetSync returns the Sync value or true if the receiver is nil.
func (o *WriteOptions) GetSync() bool {
	return o == nil || o.Sync
}

// LevelOptions holds the optional per-level parameters.
type LevelOptions struct {
	// BlockRestartInterval is the number of keys between restart points
	// for delta encoding of keys.
	//
	// The default value is 16.
	BlockRestartInterval int

	// BlockSize is the target uncompressed size in bytes of each table block.
	//
	// The default value is 4096.
	BlockSize int

	// BlockSizeThreshold finishes a block if the block size is larger than the
	// specified percentage of the target block size and adding the next entry
	// would cause the block to be larger than the target block size.
	//
	// The default value is 90
	BlockSizeThreshold int

	// Compression defines the per-block compression to use.
	//
	// The default value (DefaultCompression) uses snappy compression.
	Compression Compression

	// FilterPolicy defines a filter algorithm (such as a Bloom filter) that can
	// reduce disk reads for Get calls.
	//
	// One such implementation is bloom.FilterPolicy(10) from the pebble/bloom
	// package.
	//
	// The default value means to use no filter.
	FilterPolicy FilterPolicy

	// FilterType defines whether an existing filter policy is applied at a
	// block-level or table-level. Block-level filters use less memory to create,
	// but are slower to access as a check for the key in the index must first be
	// performed to locate the filter block. A table-level filter will require
	// memory proportional to the number of keys in an sstable to create, but
	// avoids the index lookup when determining if a key is present. Table-level
	// filters should be preferred except under constrained memory situations.
	FilterType FilterType

	// IndexBlockSize is the target uncompressed size in bytes of each index
	// block. When the index block size is larger than this target, two-level
	// indexes are automatically enabled. Setting this option to a large value
	// (such as math.MaxInt32) disables the automatic creation of two-level
	// indexes.
	//
	// The default value is the value of BlockSize.
	IndexBlockSize int

	// The target file size for the level.
	TargetFileSize int64
}

// EnsureDefaults ensures that the default values for all of the options have
// been initialized. It is valid to call EnsureDefaults on a nil receiver. A
// non-nil result will always be returned.
func (o *LevelOptions) EnsureDefaults() *LevelOptions {
	if o == nil {
		o = &LevelOptions{}
	}
	if o.BlockRestartInterval <= 0 {
		o.BlockRestartInterval = base.DefaultBlockRestartInterval
	}
	if o.BlockSize <= 0 {
		o.BlockSize = base.DefaultBlockSize
	} else if o.BlockSize > sstable.MaximumBlockSize {
		panic(errors.Errorf("BlockSize %d exceeds MaximumBlockSize", o.BlockSize))
	}
	if o.BlockSizeThreshold <= 0 {
		o.BlockSizeThreshold = base.DefaultBlockSizeThreshold
	}
	if o.Compression <= DefaultCompression || o.Compression >= sstable.NCompression {
		o.Compression = SnappyCompression
	}
	if o.IndexBlockSize <= 0 {
		o.IndexBlockSize = o.BlockSize
	}
	if o.TargetFileSize <= 0 {
		o.TargetFileSize = 2 << 20 // 2 MB
	}
	return o
}

// Options holds the optional parameters for configuring pebble. These options
// apply to the DB at large; per-query options are defined by the IterOptions
// and WriteOptions types.
type Options struct {
	// EncryptionKey is the master key for encryption at rest. Must be 16, 24, or 32 bytes.
	EncryptionKey []byte

	// Sync sstables periodically in order to smooth out writes to disk. This
	// option does not provide any persistency guarantee, but is used to avoid
	// latency spikes if the OS automatically decides to write out a large chunk
	// of dirty filesystem buffers. This option only controls SSTable syncs; WAL
	// syncs are controlled by WALBytesPerSync.
	//
	// The default value is 512KB.
	BytesPerSync int

	// Cache is used to cache uncompressed blocks from sstables.
	//
	// The default cache size is 8 MB.
	Cache *cache.Cache

	// Cleaner cleans obsolete files.
	//
	// The default cleaner uses the DeleteCleaner.
	Cleaner Cleaner

	// Comparer defines a total ordering over the space of []byte keys: a 'less
	// than' relationship. The same comparison algorithm must be used for reads
	// and writes over the lifetime of the DB.
	//
	// The default value uses the same ordering as bytes.Compare.
	Comparer *Comparer

	// DebugCheck is invoked, if non-nil, whenever a new version is being
	// installed. Typically, this is set to pebble.DebugCheckLevels in tests
	// or tools only, to check invariants over all the data in the database.
	DebugCheck func(*DB) error

	// Disable the write-ahead log (WAL). Disabling the write-ahead log prohibits
	// crash recovery, but can improve performance if crash recovery is not
	// needed (e.g. when only temporary state is being stored in the database).
	//
	// TODO(peter): untested
	DisableWAL bool

	// ErrorIfExists causes an error on Open if the database already exists.
	// The error can be checked with errors.Is(err, ErrDBAlreadyExists).
	//
	// The default value is false.
	ErrorIfExists bool

	// ErrorIfNotExists causes an error on Open if the database does not already
	// exist. The error can be checked with errors.Is(err, ErrDBDoesNotExist).
	//
	// The default value is false which will cause a database to be created if it
	// does not already exist.
	ErrorIfNotExists bool

	// ErrorIfNotPristine causes an error on Open if the database already exists
	// and any operations have been performed on the database. The error can be
	// checked with errors.Is(err, ErrDBNotPristine).
	//
	// Note that a database that contained keys that were all subsequently deleted
	// may or may not trigger the error. Currently, we check if there are any live
	// SSTs or log records to replay.
	ErrorIfNotPristine bool

	// EventListener provides hooks to listening to significant DB events such as
	// flushes, compactions, and table deletion.
	EventListener *EventListener

	// Experimental contains experimental options which are off by default.
	// These options are temporary and will eventually either be deleted, moved
	// out of the experimental group, or made the non-adjustable default. These
	// options may change at any time, so do not rely on them.
	Experimental struct {
		// The threshold of L0 read-amplification at which compaction concurrency
		// is enabled (if CompactionDebtConcurrency was not already exceeded).
		// Every multiple of this value enables another concurrent
		// compaction up to MaxConcurrentCompactions.
		L0CompactionConcurrency int

		// CompactionDebtConcurrency controls the threshold of compaction debt
		// at which additional compaction concurrency slots are added. For every
		// multiple of this value in compaction debt bytes, an additional
		// concurrent compaction is added. This works "on top" of
		// L0CompactionConcurrency, so the higher of the count of compaction
		// concurrency slots as determined by the two options is chosen.
		CompactionDebtConcurrency uint64

		// IngestSplit, if it returns true, allows for ingest-time splitting of
		// existing sstables into two virtual sstables to allow ingestion sstables to
		// slot into a lower level than they otherwise would have.
		IngestSplit func() bool

		// ReadCompactionRate controls the frequency of read triggered
		// compactions by adjusting `AllowedSeeks` in manifest.FileMetadata:
		//
		// AllowedSeeks = FileSize / ReadCompactionRate
		//
		// From LevelDB:
		// ```
		// We arrange to automatically compact this file after
		// a certain number of seeks. Let's assume:
		//   (1) One seek costs 10ms
		//   (2) Writing or reading 1MB costs 10ms (100MB/s)
		//   (3) A compaction of 1MB does 25MB of IO:
		//         1MB read from this level
		//         10-12MB read from next level (boundaries may be misaligned)
		//         10-12MB written to next level
		// This implies that 25 seeks cost the same as the compaction
		// of 1MB of data.  I.e., one seek costs approximately the
		// same as the compaction of 40KB of data.  We are a little
		// conservative and allow approximately one seek for every 16KB
		// of data before triggering a compaction.
		// ```
		ReadCompactionRate int64

		// ReadSamplingMultiplier is a multiplier for the readSamplingPeriod in
		// iterator.maybeSampleRead() to control the frequency of read sampling
		// to trigger a read triggered compaction. A value of -1 prevents sampling
		// and disables read triggered compactions. The default is 1 << 4. which
		// gets multiplied with a constant of 1 << 16 to yield 1 << 20 (1MB).
		ReadSamplingMultiplier int64

		// TableCacheShards is the number of shards per table cache.
		// Reducing the value can reduce the number of idle goroutines per DB
		// instance which can be useful in scenarios with a lot of DB instances
		// and a large number of CPUs, but doing so can lead to higher contention
		// in the table cache and reduced performance.
		//
		// The default value is the number of logical CPUs, which can be
		// limited by runtime.GOMAXPROCS.
		TableCacheShards int

		// KeyValidationFunc is a function to validate a user key in an SSTable.
		//
		// Currently, this function is used to validate the smallest and largest
		// keys in an SSTable undergoing compaction. In this case, returning an
		// error from the validation function will result in a panic at runtime,
		// given that there is rarely any way of recovering from malformed keys
		// present in compacted files. By default, validation is not performed.
		//
		// Additional use-cases may be added in the future.
		//
		// NOTE: callers should take care to not mutate the key being validated.
		KeyValidationFunc func(userKey []byte) error

		// ValidateOnIngest schedules validation of sstables after they have
		// been ingested.
		//
		// By default, this value is false.
		ValidateOnIngest bool

		// LevelMultiplier configures the size multiplier used to determine the
		// desired size of each level of the LSM. Defaults to 10.
		LevelMultiplier int

		// MultiLevelCompactionHeuristic determines whether to add an additional
		// level to a conventional two level compaction. If nil, a multilevel
		// compaction will never get triggered.
		MultiLevelCompactionHeuristic MultiLevelHeuristic

		// MaxWriterConcurrency is used to indicate the maximum number of
		// compression workers the compression queue is allowed to use. If
		// MaxWriterConcurrency > 0, then the Writer will use parallelism, to
		// compress and write blocks to disk. Otherwise, the writer will
		// compress and write blocks to disk synchronously.
		MaxWriterConcurrency int

		// ForceWriterParallelism is used to force parallelism in the sstable
		// Writer for the metamorphic tests. Even with the MaxWriterConcurrency
		// option set, we only enable parallelism in the sstable Writer if there
		// is enough CPU available, and this option bypasses that.
		ForceWriterParallelism bool

		// CPUWorkPermissionGranter should be set if Pebble should be given the
		// ability to optionally schedule additional CPU. See the documentation
		// for CPUWorkPermissionGranter for more details.
		CPUWorkPermissionGranter CPUWorkPermissionGranter

		// EnableValueBlocks is used to decide whether to enable writing
		// TableFormatPebblev3 sstables. This setting is only respected by a
		// specific subset of format major versions: FormatSSTableValueBlocks,
		// FormatFlushableIngest and FormatPrePebblev1MarkedCompacted. In lower
		// format major versions, value blocks are never enabled. In higher
		// format major versions, value blocks are always enabled.
		EnableValueBlocks func() bool

		// ShortAttributeExtractor is used iff EnableValueBlocks() returns true
		// (else ignored). If non-nil, a ShortAttribute can be extracted from the
		// value and stored with the key, when the value is stored elsewhere.
		ShortAttributeExtractor ShortAttributeExtractor

		// RequiredInPlaceValueBound specifies an optional span of user key
		// prefixes that are not-MVCC, but have a suffix. For these the values
		// must be stored with the key, since the concept of "older versions" is
		// not defined. It is also useful for statically known exclusions to value
		// separation. In CockroachDB, this will be used for the lock table key
		// space that has non-empty suffixes, but those locks don't represent
		// actual MVCC versions (the suffix ordering is arbitrary). We will also
		// need to add support for dynamically configured exclusions (we want the
		// default to be to allow Pebble to decide whether to separate the value
		// or not, hence this is structured as exclusions), for example, for users
		// of CockroachDB to dynamically exclude certain tables.
		//
		// Any change in exclusion behavior takes effect only on future written
		// sstables, and does not start rewriting existing sstables.
		//
		// Even ignoring changes in this setting, exclusions are interpreted as a
		// guidance by Pebble, and not necessarily honored. Specifically, user
		// keys with multiple Pebble-versions *may* have the older versions stored
		// in value blocks.
		RequiredInPlaceValueBound UserKeyPrefixBound

		// DisableIngestAsFlushable disables lazy ingestion of sstables through
		// a WAL write and memtable rotation. Only effectual if the the format
		// major version is at least `FormatFlushableIngest`.
		DisableIngestAsFlushable func() bool

		// RemoteStorage enables use of remote storage (e.g. S3) for storing
		// sstables. Setting this option enables use of CreateOnShared option and
		// allows ingestion of external files.
		RemoteStorage remote.StorageFactory

		// If CreateOnShared is non-zero, new sstables are created on remote storage
		// (using CreateOnSharedLocator and with the appropriate
		// CreateOnSharedStrategy). These sstables can be shared between different
		// Pebble instances; the lifecycle of such objects is managed by the
		// remote.Storage constructed by options.RemoteStorage.
		//
		// Can only be used when RemoteStorage is set (and recognizes
		// CreateOnSharedLocator).
		CreateOnShared        remote.CreateOnSharedStrategy
		CreateOnSharedLocator remote.Locator

		// CacheSizeBytesBytes is the size of the on-disk block cache for objects
		// on shared storage in bytes. If it is 0, no cache is used.
		SecondaryCacheSizeBytes int64

		// IneffectualPointDeleteCallback is called in compactions/flushes if any
		// single delete is being elided without deleting a point set/merge.
		IneffectualSingleDeleteCallback func(userKey []byte)

		// SingleDeleteInvariantViolationCallback is called in compactions/flushes if any
		// single delete has consumed a Set/Merge, and there is another immediately older
		// Set/SetWithDelete/Merge. The user of Pebble has violated the invariant under
		// which SingleDelete can be used correctly.
		//
		// Consider the sequence SingleDelete#3, Set#2, Set#1. There are three
		// ways some of these keys can first meet in a compaction.
		//
		// - All 3 keys in the same compaction: this callback will detect the
		//   violation.
		//
		// - SingleDelete#3, Set#2 meet in a compaction first: Both keys will
		//   disappear. The violation will not be detected, and the DB will have
		//   Set#1 which is likely incorrect (from the user's perspective).
		//
		// - Set#2, Set#1 meet in a compaction first: The output will be Set#2,
		//   which will later be consumed by SingleDelete#3. The violation will
		//   not be detected and the DB will be correct.
		SingleDeleteInvariantViolationCallback func(userKey []byte)
	}

	// Filters is a map from filter policy name to filter policy. It is used for
	// debugging tools which may be used on multiple databases configured with
	// different filter policies. It is not necessary to populate this filters
	// map during normal usage of a DB.
	Filters map[string]FilterPolicy

	// FlushDelayDeleteRange configures how long the database should wait before
	// forcing a flush of a memtable that contains a range deletion. Disk space
	// cannot be reclaimed until the range deletion is flushed. No automatic
	// flush occurs if zero.
	FlushDelayDeleteRange time.Duration

	// FlushDelayRangeKey configures how long the database should wait before
	// forcing a flush of a memtable that contains a range key. Range keys in
	// the memtable prevent lazy combined iteration, so it's desirable to flush
	// range keys promptly. No automatic flush occurs if zero.
	FlushDelayRangeKey time.Duration

	// FlushSplitBytes denotes the target number of bytes per sublevel in
	// each flush split interval (i.e. range between two flush split keys)
	// in L0 sstables. When set to zero, only a single sstable is generated
	// by each flush. When set to a non-zero value, flushes are split at
	// points to meet L0's TargetFileSize, any grandparent-related overlap
	// options, and at boundary keys of L0 flush split intervals (which are
	// targeted to contain around FlushSplitBytes bytes in each sublevel
	// between pairs of boundary keys). Splitting sstables during flush
	// allows increased compaction flexibility and concurrency when those
	// tables are compacted to lower levels.
	FlushSplitBytes int64

	// FormatMajorVersion sets the format of on-disk files. It is
	// recommended to set the format major version to an explicit
	// version, as the default may change over time.
	//
	// At Open if the existing database is formatted using a later
	// format major version that is known to this version of Pebble,
	// Pebble will continue to use the later format major version. If
	// the existing database's version is unknown, the caller may use
	// FormatMostCompatible and will be able to open the database
	// regardless of its actual version.
	//
	// If the existing database is formatted using a format major
	// version earlier than the one specified, Open will automatically
	// ratchet the database to the specified format major version.
	FormatMajorVersion FormatMajorVersion

	// FS provides the interface for persistent file storage.
	//
	// The default value uses the underlying operating system's file system.
	FS vfs.FS

	// Lock, if set, must be a database lock acquired through LockDirectory for
	// the same directory passed to Open. If provided, Open will skip locking
	// the directory. Closing the database will not release the lock, and it's
	// the responsibility of the caller to release the lock after closing the
	// database.
	//
	// Open will enforce that the Lock passed locks the same directory passed to
	// Open. Concurrent calls to Open using the same Lock are detected and
	// prohibited.
	Lock *Lock

	// The count of L0 files necessary to trigger an L0 compaction.
	L0CompactionFileThreshold int

	// The amount of L0 read-amplification necessary to trigger an L0 compaction.
	L0CompactionThreshold int

	// Hard limit on L0 read-amplification, computed as the number of L0
	// sublevels. Writes are stopped when this threshold is reached.
	L0StopWritesThreshold int

	// The maximum number of bytes for LBase. The base level is the level which
	// L0 is compacted into. The base level is determined dynamically based on
	// the existing data in the LSM. The maximum number of bytes for other levels
	// is computed dynamically based on the base level's maximum size. When the
	// maximum number of bytes for a level is exceeded, compaction is requested.
	LBaseMaxBytes int64

	// Per-level options. Options for at least one level must be specified. The
	// options for the last level are used for all subsequent levels.
	Levels []LevelOptions

	// LoggerAndTracer will be used, if non-nil, else Logger will be used and
	// tracing will be a noop.

	// Logger used to write log messages.
	//
	// The default logger uses the Go standard library log package.
	Logger Logger
	// LoggerAndTracer is used for writing log messages and traces.
	LoggerAndTracer LoggerAndTracer

	// MaxManifestFileSize is the maximum size the MANIFEST file is allowed to
	// become. When the MANIFEST exceeds this size it is rolled over and a new
	// MANIFEST is created.
	MaxManifestFileSize int64

	// MaxOpenFiles is a soft limit on the number of open files that can be
	// used by the DB.
	//
	// The default value is 1000.
	MaxOpenFiles int

	// The size of a MemTable in steady state. The actual MemTable size starts at
	// min(256KB, MemTableSize) and doubles for each subsequent MemTable up to
	// MemTableSize. This reduces the memory pressure caused by MemTables for
	// short lived (test) DB instances. Note that more than one MemTable can be
	// in existence since flushing a MemTable involves creating a new one and
	// writing the contents of the old one in the
	// background. MemTableStopWritesThreshold places a hard limit on the size of
	// the queued MemTables.
	//
	// The default value is 4MB.
	MemTableSize uint64

	// Hard limit on the number of queued of MemTables. Writes are stopped when
	// the sum of the queued memtable sizes exceeds:
	//   MemTableStopWritesThreshold * MemTableSize.
	//
	// This value should be at least 2 or writes will stop whenever a MemTable is
	// being flushed.
	//
	// The default value is 2.
	MemTableStopWritesThreshold int

	// Merger defines the associative merge operation to use for merging values
	// written with {Batch,DB}.Merge.
	//
	// The default merger concatenates values.
	Merger *Merger

	// MaxConcurrentCompactions specifies the maximum number of concurrent
	// compactions. The default is 1. Concurrent compactions are performed
	// - when L0 read-amplification passes the L0CompactionConcurrency threshold
	// - for automatic background compactions
	// - when a manual compaction for a level is split and parallelized
	// MaxConcurrentCompactions must be greater than 0.
	MaxConcurrentCompactions func() int

	// DisableAutomaticCompactions dictates whether automatic compactions are
	// scheduled or not. The default is false (enabled). This option is only used
	// externally when running a manual compaction, and internally for tests.
	DisableAutomaticCompactions bool

	// NoSyncOnClose decides whether the Pebble instance will enforce a
	// close-time synchronization (e.g., fdatasync() or sync_file_range())
	// on files it writes to. Setting this to true removes the guarantee for a
	// sync on close. Some implementations can still issue a non-blocking sync.
	NoSyncOnClose bool

	// NumPrevManifest is the number of non-current or older manifests which
	// we want to keep around for debugging purposes. By default, we're going
	// to keep one older manifest.
	NumPrevManifest int

	// ReadOnly indicates that the DB should be opened in read-only mode. Writes
	// to the DB will return an error, background compactions are disabled, and
	// the flush that normally occurs after replaying the WAL at startup is
	// disabled.
	ReadOnly bool

	// TableCache is an initialized TableCache which should be set as an
	// option if the DB needs to be initialized with a pre-existing table cache.
	// If TableCache is nil, then a table cache which is unique to the DB instance
	// is created. TableCache can be shared between db instances by setting it here.
	// The TableCache set here must use the same underlying cache as Options.Cache
	// and pebble will panic otherwise.
	TableCache *TableCache

	// TablePropertyCollectors is a list of TablePropertyCollector creation
	// functions. A new TablePropertyCollector is created for each sstable built
	// and lives for the lifetime of the table.
	TablePropertyCollectors []func() TablePropertyCollector

	// BlockPropertyCollectors is a list of BlockPropertyCollector creation
	// functions. A new BlockPropertyCollector is created for each sstable
	// built and lives for the lifetime of writing that table.
	BlockPropertyCollectors []func() BlockPropertyCollector

	// WALBytesPerSync sets the number of bytes to write to a WAL before calling
	// Sync on it in the background. Just like with BytesPerSync above, this
	// helps smooth out disk write latencies, and avoids cases where the OS
	// writes a lot of buffered data to disk at once. However, this is less
	// necessary with WALs, as many write operations already pass in
	// Sync = true.
	//
	// The default value is 0, i.e. no background syncing. This matches the
	// default behaviour in RocksDB.
	WALBytesPerSync int

	// WALDir specifies the directory to store write-ahead logs (WALs) in. If
	// empty (the default), WALs will be stored in the same directory as sstables
	// (i.e. the directory passed to pebble.Open).
	WALDir string

	// WALMinSyncInterval is the minimum duration between syncs of the WAL. If
	// WAL syncs are requested faster than this interval, they will be
	// artificially delayed. Introducing a small artificial delay (500us) between
	// WAL syncs can allow more operations to arrive and reduce IO operations
	// while having a minimal impact on throughput. This option is supplied as a
	// closure in order to allow the value to be changed dynamically. The default
	// value is 0.
	//
	// TODO(peter): rather than a closure, should there be another mechanism for
	// changing options dynamically?
	WALMinSyncInterval func() time.Duration

	// TargetByteDeletionRate is the rate (in bytes per second) at which sstable file
	// deletions are limited to (under normal circumstances).
	//
	// Deletion pacing is used to slow down deletions when compactions finish up
	// or readers close and newly-obsolete files need cleaning up. Deleting lots
	// of files at once can cause disk latency to go up on some SSDs, which this
	// functionality guards against.
	//
	// This value is only a best-effort target; the effective rate can be
	// higher if deletions are falling behind or disk space is running low.
	//
	// Setting this to 0 disables deletion pacing, which is also the default.
	TargetByteDeletionRate int

	// private options are only used by internal tests or are used internally
	// for facilitating upgrade paths of unconfigurable functionality.
	private struct {
		// strictWALTail configures whether or not a database's WALs created
		// prior to the most recent one should be interpreted strictly,
		// requiring a clean EOF. RocksDB 6.2.1 and the version of Pebble
		// included in CockroachDB 20.1 do not guarantee that closed WALs end
		// cleanly. If this option is set within an OPTIONS file, Pebble
		// interprets previous WALs strictly, requiring a clean EOF.
		// Otherwise, it interprets them permissively in the same manner as
		// RocksDB 6.2.1.
		strictWALTail bool

		// disableDeleteOnlyCompactions prevents the scheduling of delete-only
		// compactions that drop sstables wholy covered by range tombstones or
		// range key tombstones.
		disableDeleteOnlyCompactions bool

		// disableElisionOnlyCompactions prevents the scheduling of elision-only
		// compactions that rewrite sstables in place in order to elide obsolete
		// keys.
		disableElisionOnlyCompactions bool

		// disableLazyCombinedIteration is a private option used by the
		// metamorphic tests to test equivalence between lazy-combined iteration
		// and constructing the range-key iterator upfront. It's a private
		// option to avoid littering the public interface with options that we
		// do not want to allow users to actually configure.
		disableLazyCombinedIteration bool

		// A private option to disable stats collection.
		disableTableStats bool

		// testingAlwaysWaitForCleanup is set by some tests to force waiting for
		// obsolete file deletion (to make events deterministic).
		testingAlwaysWaitForCleanup bool

		// fsCloser holds a closer that should be invoked after a DB using these
		// Options is closed. This is used to automatically stop the
		// long-running goroutine associated with the disk-health-checking FS.
		// See the initialization of FS in EnsureDefaults. Note that care has
		// been taken to ensure that it is still safe to continue using the FS
		// after this closer has been invoked. However, if write operations
		// against the FS are made after the DB is closed, the FS may leak a
		// goroutine indefinitely.
		fsCloser io.Closer
	}
}

// DebugCheckLevels calls CheckLevels on the provided database.
// It may be set in the DebugCheck field of Options to check
// level invariants whenever a new version is installed.
func DebugCheckLevels(db *DB) error {
	return db.CheckLevels(nil)
}

// EnsureDefaults ensures that the default values for all options are set if a
// valid value was not already specified. Returns the new options.
func (o *Options) EnsureDefaults() *Options {
	if o == nil {
		o = &Options{}
	}
	if o.BytesPerSync <= 0 {
		o.BytesPerSync = 512 << 10 // 512 KB
	}
	if o.Cleaner == nil {
		o.Cleaner = DeleteCleaner{}
	}
	if o.Comparer == nil {
		o.Comparer = DefaultComparer
	}
	if o.Experimental.DisableIngestAsFlushable == nil {
		o.Experimental.DisableIngestAsFlushable = func() bool { return false }
	}
	if o.Experimental.L0CompactionConcurrency <= 0 {
		o.Experimental.L0CompactionConcurrency = 10
	}
	if o.Experimental.CompactionDebtConcurrency <= 0 {
		o.Experimental.CompactionDebtConcurrency = 1 << 30 // 1 GB
	}
	if o.Experimental.KeyValidationFunc == nil {
		o.Experimental.KeyValidationFunc = func([]byte) error { return nil }
	}
	if o.L0CompactionThreshold <= 0 {
		o.L0CompactionThreshold = 4
	}
	if o.L0CompactionFileThreshold <= 0 {
		// Some justification for the default of 500:
		// Why not smaller?:
		// - The default target file size for L0 is 2MB, so 500 files is <= 1GB
		//   of data. At observed compaction speeds of > 20MB/s, L0 can be
		//   cleared of all files in < 1min, so this backlog is not huge.
		// - 500 files is low overhead for instantiating L0 sublevels from
		//   scratch.
		// - Lower values were observed to cause excessive and inefficient
		//   compactions out of L0 in a TPCC import benchmark.
		// Why not larger?:
		// - More than 1min to compact everything out of L0.
		// - CockroachDB's admission control system uses a threshold of 1000
		//   files to start throttling writes to Pebble. Using 500 here gives
		//   us headroom between when Pebble should start compacting L0 and
		//   when the admission control threshold is reached.
		//
		// We can revisit this default in the future based on better
		// experimental understanding.
		//
		// TODO(jackson): Experiment with slightly lower thresholds [or higher
		// admission control thresholds] to see whether a higher L0 score at the
		// threshold (currently 2.0) is necessary for some workloads to avoid
		// starving L0 in favor of lower-level compactions.
		o.L0CompactionFileThreshold = 500
	}
	if o.L0StopWritesThreshold <= 0 {
		o.L0StopWritesThreshold = 12
	}
	if o.LBaseMaxBytes <= 0 {
		o.LBaseMaxBytes = 64 << 20 // 64 MB
	}
	if o.Levels == nil {
		o.Levels = make([]LevelOptions, 1)
		for i := range o.Levels {
			if i > 0 {
				l := &o.Levels[i]
				if l.TargetFileSize <= 0 {
					l.TargetFileSize = o.Levels[i-1].TargetFileSize * 2
				}
			}
			o.Levels[i].EnsureDefaults()
		}
	} else {
		for i := range o.Levels {
			o.Levels[i].EnsureDefaults()
		}
	}
	if o.Logger == nil {
		o.Logger = DefaultLogger
	}
	if o.EventListener == nil {
		o.EventListener = &EventListener{}
	}
	o.EventListener.EnsureDefaults(o.Logger)
	if o.MaxManifestFileSize == 0 {
		o.MaxManifestFileSize = 128 << 20 // 128 MB
	}
	if o.MaxOpenFiles == 0 {
		o.MaxOpenFiles = 1000
	}
	if o.MemTableSize <= 0 {
		o.MemTableSize = 4 << 20 // 4 MB
	}
	if o.MemTableStopWritesThreshold <= 0 {
		o.MemTableStopWritesThreshold = 2
	}
	if o.Merger == nil {
		o.Merger = DefaultMerger
	}
	o.private.strictWALTail = true
	if o.MaxConcurrentCompactions == nil {
		o.MaxConcurrentCompactions = func() int { return 1 }
	}
	if o.NumPrevManifest <= 0 {
		o.NumPrevManifest = 1
	}

	if o.FormatMajorVersion == FormatDefault {
		o.FormatMajorVersion = FormatMostCompatible
	}

	if o.FS == nil {
		o.WithFSDefaults()
	}
	if o.FlushSplitBytes <= 0 {
		o.FlushSplitBytes = 2 * o.Levels[0].TargetFileSize
	}
	if o.Experimental.LevelMultiplier <= 0 {
		o.Experimental.LevelMultiplier = defaultLevelMultiplier
	}
	if o.Experimental.ReadCompactionRate == 0 {
		o.Experimental.ReadCompactionRate = 16000
	}
	if o.Experimental.ReadSamplingMultiplier == 0 {
		o.Experimental.ReadSamplingMultiplier = 1 << 4
	}
	if o.Experimental.TableCacheShards <= 0 {
		o.Experimental.TableCacheShards = runtime.GOMAXPROCS(0)
	}
	if o.Experimental.CPUWorkPermissionGranter == nil {
		o.Experimental.CPUWorkPermissionGranter = defaultCPUWorkGranter{}
	}
	if o.Experimental.MultiLevelCompactionHeuristic == nil {
		o.Experimental.MultiLevelCompactionHeuristic = WriteAmpHeuristic{}
	}

	o.initMaps()
	return o
}

// WithFSDefaults configures the Options to wrap the configured filesystem with
// the default virtual file system middleware, like disk-health checking.
func (o *Options) WithFSDefaults() *Options {
	if o.FS == nil {
		o.FS = vfs.Default
	}
	o.FS, o.private.fsCloser = vfs.WithDiskHealthChecks(o.FS, 5*time.Second,
		func(info vfs.DiskSlowInfo) {
			o.EventListener.DiskSlow(info)
		})
	return o
}

// AddEventListener adds the provided event listener to the Options, in addition
// to any existing event listener.
func (o *Options) AddEventListener(l EventListener) {
	if o.EventListener != nil {
		l = TeeEventListener(l, *o.EventListener)
	}
	o.EventListener = &l
}

func (o *Options) equal() Equal {
	if o.Comparer.Equal == nil {
		return bytes.Equal
	}
	return o.Comparer.Equal
}

// initMaps initializes the Comparers, Filters, and Mergers maps.
func (o *Options) initMaps() {
	for i := range o.Levels {
		l := &o.Levels[i]
		if l.FilterPolicy != nil {
			if o.Filters == nil {
				o.Filters = make(map[string]FilterPolicy)
			}
			name := l.FilterPolicy.Name()
			if _, ok := o.Filters[name]; !ok {
				o.Filters[name] = l.FilterPolicy
			}
		}
	}
}

// Level returns the LevelOptions for the specified level.
func (o *Options) Level(level int) LevelOptions {
	if level < len(o.Levels) {
		return o.Levels[level]
	}
	n := len(o.Levels) - 1
	l := o.Levels[n]
	for i := n; i < level; i++ {
		l.TargetFileSize *= 2
	}
	return l
}

// Clone creates a shallow-copy of the supplied options.
func (o *Options) Clone() *Options {
	n := &Options{}
	if o != nil {
		*n = *o
	}
	return n
}

func filterPolicyName(p FilterPolicy) string {
	if p == nil {
		return "none"
	}
	return p.Name()
}

func (o *Options) String() string {
	var buf bytes.Buffer

	cacheSize := int64(cacheDefaultSize)
	if o.Cache != nil {
		cacheSize = o.Cache.MaxSize()
	}

	fmt.Fprintf(&buf, "[Version]\n")
	fmt.Fprintf(&buf, "  pebble_version=0.1\n")
	fmt.Fprintf(&buf, "\n")
	fmt.Fprintf(&buf, "[Options]\n")
	fmt.Fprintf(&buf, "  bytes_per_sync=%d\n", o.BytesPerSync)
	fmt.Fprintf(&buf, "  cache_size=%d\n", cacheSize)
	fmt.Fprintf(&buf, "  cleaner=%s\n", o.Cleaner)
	fmt.Fprintf(&buf, "  compaction_debt_concurrency=%d\n", o.Experimental.CompactionDebtConcurrency)
	fmt.Fprintf(&buf, "  comparer=%s\n", o.Comparer.Name)
	fmt.Fprintf(&buf, "  disable_wal=%t\n", o.DisableWAL)
	if o.Experimental.DisableIngestAsFlushable != nil && o.Experimental.DisableIngestAsFlushable() {
		fmt.Fprintf(&buf, "  disable_ingest_as_flushable=%t\n", true)
	}
	fmt.Fprintf(&buf, "  flush_delay_delete_range=%s\n", o.FlushDelayDeleteRange)
	fmt.Fprintf(&buf, "  flush_delay_range_key=%s\n", o.FlushDelayRangeKey)
	fmt.Fprintf(&buf, "  flush_split_bytes=%d\n", o.FlushSplitBytes)
	fmt.Fprintf(&buf, "  format_major_version=%d\n", o.FormatMajorVersion)
	fmt.Fprintf(&buf, "  l0_compaction_concurrency=%d\n", o.Experimental.L0CompactionConcurrency)
	fmt.Fprintf(&buf, "  l0_compaction_file_threshold=%d\n", o.L0CompactionFileThreshold)
	fmt.Fprintf(&buf, "  l0_compaction_threshold=%d\n", o.L0CompactionThreshold)
	fmt.Fprintf(&buf, "  l0_stop_writes_threshold=%d\n", o.L0StopWritesThreshold)
	fmt.Fprintf(&buf, "  lbase_max_bytes=%d\n", o.LBaseMaxBytes)
	if o.Experimental.LevelMultiplier != defaultLevelMultiplier {
		fmt.Fprintf(&buf, "  level_multiplier=%d\n", o.Experimental.LevelMultiplier)
	}
	fmt.Fprintf(&buf, "  max_concurrent_compactions=%d\n", o.MaxConcurrentCompactions())
	fmt.Fprintf(&buf, "  max_manifest_file_size=%d\n", o.MaxManifestFileSize)
	fmt.Fprintf(&buf, "  max_open_files=%d\n", o.MaxOpenFiles)
	fmt.Fprintf(&buf, "  mem_table_size=%d\n", o.MemTableSize)
	fmt.Fprintf(&buf, "  mem_table_stop_writes_threshold=%d\n", o.MemTableStopWritesThreshold)
	fmt.Fprintf(&buf, "  min_deletion_rate=%d\n", o.TargetByteDeletionRate)
	fmt.Fprintf(&buf, "  merger=%s\n", o.Merger.Name)
	fmt.Fprintf(&buf, "  read_compaction_rate=%d\n", o.Experimental.ReadCompactionRate)
	fmt.Fprintf(&buf, "  read_sampling_multiplier=%d\n", o.Experimental.ReadSamplingMultiplier)
	fmt.Fprintf(&buf, "  strict_wal_tail=%t\n", o.private.strictWALTail)
	fmt.Fprintf(&buf, "  table_cache_shards=%d\n", o.Experimental.TableCacheShards)
	fmt.Fprintf(&buf, "  table_property_collectors=[")
	for i := range o.TablePropertyCollectors {
		if i > 0 {
			fmt.Fprintf(&buf, ",")
		}
		// NB: This creates a new TablePropertyCollector, but Options.String() is
		// called rarely so the overhead of doing so is not consequential.
		fmt.Fprintf(&buf, "%s", o.TablePropertyCollectors[i]().Name())
	}
	fmt.Fprintf(&buf, "]\n")
	fmt.Fprintf(&buf, "  validate_on_ingest=%t\n", o.Experimental.ValidateOnIngest)
	fmt.Fprintf(&buf, "  wal_dir=%s\n", o.WALDir)
	fmt.Fprintf(&buf, "  wal_bytes_per_sync=%d\n", o.WALBytesPerSync)
	fmt.Fprintf(&buf, "  max_writer_concurrency=%d\n", o.Experimental.MaxWriterConcurrency)
	fmt.Fprintf(&buf, "  force_writer_parallelism=%t\n", o.Experimental.ForceWriterParallelism)
	fmt.Fprintf(&buf, "  secondary_cache_size_bytes=%d\n", o.Experimental.SecondaryCacheSizeBytes)
	fmt.Fprintf(&buf, "  create_on_shared=%d\n", o.Experimental.CreateOnShared)

	// Private options.
	//
	// These options are only encoded if true, because we do not want them to
	// appear in production serialized Options files, since they're testing-only
	// options. They're only serialized when true, which still ensures that the
	// metamorphic tests may propagate them to subprocesses.
	if o.private.disableDeleteOnlyCompactions {
		fmt.Fprintln(&buf, "  disable_delete_only_compactions=true")
	}
	if o.private.disableElisionOnlyCompactions {
		fmt.Fprintln(&buf, "  disable_elision_only_compactions=true")
	}
	if o.private.disableLazyCombinedIteration {
		fmt.Fprintln(&buf, "  disable_lazy_combined_iteration=true")
	}

	for i := range o.Levels {
		l := &o.Levels[i]
		fmt.Fprintf(&buf, "\n")
		fmt.Fprintf(&buf, "[Level \"%d\"]\n", i)
		fmt.Fprintf(&buf, "  block_restart_interval=%d\n", l.BlockRestartInterval)
		fmt.Fprintf(&buf, "  block_size=%d\n", l.BlockSize)
		fmt.Fprintf(&buf, "  block_size_threshold=%d\n", l.BlockSizeThreshold)
		fmt.Fprintf(&buf, "  compression=%s\n", l.Compression)
		fmt.Fprintf(&buf, "  filter_policy=%s\n", filterPolicyName(l.FilterPolicy))
		fmt.Fprintf(&buf, "  filter_type=%s\n", l.FilterType)
		fmt.Fprintf(&buf, "  index_block_size=%d\n", l.IndexBlockSize)
		fmt.Fprintf(&buf, "  target_file_size=%d\n", l.TargetFileSize)
	}

	return buf.String()
}

func parseOptions(s string, fn func(section, key, value string) error) error {
	var section string
	for _, line := range strings.Split(s, "\n") {
		line = strings.TrimSpace(line)
		if len(line) == 0 {
			// Skip blank lines.
			continue
		}
		if line[0] == ';' || line[0] == '#' {
			// Skip comments.
			continue
		}
		n := len(line)
		if line[0] == '[' && line[n-1] == ']' {
			// Parse section.
			section = line[1 : n-1]
			continue
		}

		pos := strings.Index(line, "=")
		if pos < 0 {
			const maxLen = 50
			if len(line) > maxLen {
				line = line[:maxLen-3] + "..."
			}
			return base.CorruptionErrorf("invalid key=value syntax: %q", errors.Safe(line))
		}

		key := strings.TrimSpace(line[:pos])
		value := strings.TrimSpace(line[pos+1:])

		// RocksDB uses a similar (INI-style) syntax for the OPTIONS file, but
		// different section names and keys. The "CFOptions ..." paths are the
		// RocksDB versions which we map to the Pebble paths.
		mappedSection := section
		if section == `CFOptions "default"` {
			mappedSection = "Options"
			switch key {
			case "comparator":
				key = "comparer"
			case "merge_operator":
				key = "merger"
			}
		}

		if err := fn(mappedSection, key, value); err != nil {
			return err
		}
	}
	return nil
}

// ParseHooks contains callbacks to create options fields which can have
// user-defined implementations.
type ParseHooks struct {
	NewCache        func(size int64) *Cache
	NewCleaner      func(name string) (Cleaner, error)
	NewComparer     func(name string) (*Comparer, error)
	NewFilterPolicy func(name string) (FilterPolicy, error)
	NewMerger       func(name string) (*Merger, error)
	SkipUnknown     func(name, value string) bool
}

// Parse parses the options from the specified string. Note that certain
// options cannot be parsed into populated fields. For example, comparer and
// merger.
func (o *Options) Parse(s string, hooks *ParseHooks) error {
	return parseOptions(s, func(section, key, value string) error {
		// WARNING: DO NOT remove entries from the switches below because doing so
		// causes a key previously written to the OPTIONS file to be considered unknown,
		// a backwards incompatible change. Instead, leave in support for parsing the
		// key but simply don't parse the value.

		switch {
		case section == "Version":
			switch key {
			case "pebble_version":
			default:
				if hooks != nil && hooks.SkipUnknown != nil && hooks.SkipUnknown(section+"."+key, value) {
					return nil
				}
				return errors.Errorf("pebble: unknown option: %s.%s",
					errors.Safe(section), errors.Safe(key))
			}
			return nil

		case section == "Options":
			var err error
			switch key {
			case "bytes_per_sync":
				o.BytesPerSync, err = strconv.Atoi(value)
			case "cache_size":
				var n int64
				n, err = strconv.ParseInt(value, 10, 64)
				if err == nil && hooks != nil && hooks.NewCache != nil {
					if o.Cache != nil {
						o.Cache.Unref()
					}
					o.Cache = hooks.NewCache(n)
				}
				// We avoid calling cache.New in parsing because it makes it
				// too easy to leak a cache.
			case "cleaner":
				switch value {
				case "archive":
					o.Cleaner = ArchiveCleaner{}
				case "delete":
					o.Cleaner = DeleteCleaner{}
				default:
					if hooks != nil && hooks.NewCleaner != nil {
						o.Cleaner, err = hooks.NewCleaner(value)
					}
				}
			case "comparer":
				switch value {
				case "leveldb.BytewiseComparator":
					o.Comparer = DefaultComparer
				default:
					if hooks != nil && hooks.NewComparer != nil {
						o.Comparer, err = hooks.NewComparer(value)
					}
				}
			case "compaction_debt_concurrency":
				o.Experimental.CompactionDebtConcurrency, err = strconv.ParseUint(value, 10, 64)
			case "delete_range_flush_delay":
				// NB: This is a deprecated serialization of the
				// `flush_delay_delete_range`.
				o.FlushDelayDeleteRange, err = time.ParseDuration(value)
			case "disable_delete_only_compactions":
				o.private.disableDeleteOnlyCompactions, err = strconv.ParseBool(value)
			case "disable_elision_only_compactions":
				o.private.disableElisionOnlyCompactions, err = strconv.ParseBool(value)
			case "disable_ingest_as_flushable":
				var v bool
				v, err = strconv.ParseBool(value)
				if err == nil {
					o.Experimental.DisableIngestAsFlushable = func() bool { return v }
				}
			case "disable_lazy_combined_iteration":
				o.private.disableLazyCombinedIteration, err = strconv.ParseBool(value)
			case "disable_wal":
				o.DisableWAL, err = strconv.ParseBool(value)
			case "flush_delay_delete_range":
				o.FlushDelayDeleteRange, err = time.ParseDuration(value)
			case "flush_delay_range_key":
				o.FlushDelayRangeKey, err = time.ParseDuration(value)
			case "flush_split_bytes":
				o.FlushSplitBytes, err = strconv.ParseInt(value, 10, 64)
			case "format_major_version":
				// NB: The version written here may be stale. Open does
				// not use the format major version encoded in the
				// OPTIONS file other than to validate that the encoded
				// version is valid right here.
				var v uint64
				v, err = strconv.ParseUint(value, 10, 64)
				if vers := FormatMajorVersion(v); vers > internalFormatNewest || vers == FormatDefault {
					err = errors.Newf("unknown format major version %d", o.FormatMajorVersion)
				}
				if err == nil {
					o.FormatMajorVersion = FormatMajorVersion(v)
				}
			case "l0_compaction_concurrency":
				o.Experimental.L0CompactionConcurrency, err = strconv.Atoi(value)
			case "l0_compaction_file_threshold":
				o.L0CompactionFileThreshold, err = strconv.Atoi(value)
			case "l0_compaction_threshold":
				o.L0CompactionThreshold, err = strconv.Atoi(value)
			case "l0_stop_writes_threshold":
				o.L0StopWritesThreshold, err = strconv.Atoi(value)
			case "l0_sublevel_compactions":
				// Do nothing; option existed in older versions of pebble.
			case "lbase_max_bytes":
				o.LBaseMaxBytes, err = strconv.ParseInt(value, 10, 64)
			case "level_multiplier":
				o.Experimental.LevelMultiplier, err = strconv.Atoi(value)
			case "max_concurrent_compactions":
				var concurrentCompactions int
				concurrentCompactions, err = strconv.Atoi(value)
				if concurrentCompactions <= 0 {
					err = errors.New("max_concurrent_compactions cannot be <= 0")
				} else {
					o.MaxConcurrentCompactions = func() int { return concurrentCompactions }
				}
			case "max_manifest_file_size":
				o.MaxManifestFileSize, err = strconv.ParseInt(value, 10, 64)
			case "max_open_files":
				o.MaxOpenFiles, err = strconv.Atoi(value)
			case "mem_table_size":
				o.MemTableSize, err = strconv.ParseUint(value, 10, 64)
			case "mem_table_stop_writes_threshold":
				o.MemTableStopWritesThreshold, err = strconv.Atoi(value)
			case "min_compaction_rate":
				// Do nothing; option existed in older versions of pebble, and
				// may be meaningful again eventually.
			case "min_deletion_rate":
				o.TargetByteDeletionRate, err = strconv.Atoi(value)
			case "min_flush_rate":
				// Do nothing; option existed in older versions of pebble, and
				// may be meaningful again eventually.
			case "point_tombstone_weight":
				// Do nothing; deprecated.
			case "strict_wal_tail":
				o.private.strictWALTail, err = strconv.ParseBool(value)
			case "merger":
				switch value {
				case "nullptr":
					o.Merger = nil
				case "pebble.concatenate":
					o.Merger = DefaultMerger
				default:
					if hooks != nil && hooks.NewMerger != nil {
						o.Merger, err = hooks.NewMerger(value)
					}
				}
			case "read_compaction_rate":
				o.Experimental.ReadCompactionRate, err = strconv.ParseInt(value, 10, 64)
			case "read_sampling_multiplier":
				o.Experimental.ReadSamplingMultiplier, err = strconv.ParseInt(value, 10, 64)
			case "table_cache_shards":
				o.Experimental.TableCacheShards, err = strconv.Atoi(value)
			case "table_format":
				switch value {
				case "leveldb":
				case "rocksdbv2":
				default:
					return errors.Errorf("pebble: unknown table format: %q", errors.Safe(value))
				}
			case "table_property_collectors":
				// TODO(peter): set o.TablePropertyCollectors
			case "validate_on_ingest":
				o.Experimental.ValidateOnIngest, err = strconv.ParseBool(value)
			case "wal_dir":
				o.WALDir = value
			case "wal_bytes_per_sync":
				o.WALBytesPerSync, err = strconv.Atoi(value)
			case "max_writer_concurrency":
				o.Experimental.MaxWriterConcurrency, err = strconv.Atoi(value)
			case "force_writer_parallelism":
				o.Experimental.ForceWriterParallelism, err = strconv.ParseBool(value)
			case "secondary_cache_size_bytes":
				o.Experimental.SecondaryCacheSizeBytes, err = strconv.ParseInt(value, 10, 64)
			case "create_on_shared":
				var createOnSharedInt int64
				createOnSharedInt, err = strconv.ParseInt(value, 10, 64)
				o.Experimental.CreateOnShared = remote.CreateOnSharedStrategy(createOnSharedInt)
			default:
				if hooks != nil && hooks.SkipUnknown != nil && hooks.SkipUnknown(section+"."+key, value) {
					return nil
				}
				return errors.Errorf("pebble: unknown option: %s.%s",
					errors.Safe(section), errors.Safe(key))
			}
			return err

		case strings.HasPrefix(section, "Level "):
			var index int
			if n, err := fmt.Sscanf(section, `Level "%d"`, &index); err != nil {
				return err
			} else if n != 1 {
				if hooks != nil && hooks.SkipUnknown != nil && hooks.SkipUnknown(section, value) {
					return nil
				}
				return errors.Errorf("pebble: unknown section: %q", errors.Safe(section))
			}

			if len(o.Levels) <= index {
				newLevels := make([]LevelOptions, index+1)
				copy(newLevels, o.Levels)
				o.Levels = newLevels
			}
			l := &o.Levels[index]

			var err error
			switch key {
			case "block_restart_interval":
				l.BlockRestartInterval, err = strconv.Atoi(value)
			case "block_size":
				l.BlockSize, err = strconv.Atoi(value)
			case "block_size_threshold":
				l.BlockSizeThreshold, err = strconv.Atoi(value)
			case "compression":
				switch value {
				case "Default":
					l.Compression = DefaultCompression
				case "NoCompression":
					l.Compression = NoCompression
				case "Snappy":
					l.Compression = SnappyCompression
				default:
					return errors.Errorf("pebble: unknown compression: %q", errors.Safe(value))
				}
			case "filter_policy":
				if hooks != nil && hooks.NewFilterPolicy != nil {
					l.FilterPolicy, err = hooks.NewFilterPolicy(value)
				}
			case "filter_type":
				switch value {
				case "table":
					l.FilterType = TableFilter
				default:
					return errors.Errorf("pebble: unknown filter type: %q", errors.Safe(value))
				}
			case "index_block_size":
				l.IndexBlockSize, err = strconv.Atoi(value)
			case "target_file_size":
				l.TargetFileSize, err = strconv.ParseInt(value, 10, 64)
			default:
				if hooks != nil && hooks.SkipUnknown != nil && hooks.SkipUnknown(section+"."+key, value) {
					return nil
				}
				return errors.Errorf("pebble: unknown option: %s.%s", errors.Safe(section), errors.Safe(key))
			}
			return err
		}
		if hooks != nil && hooks.SkipUnknown != nil && hooks.SkipUnknown(section+"."+key, value) {
			return nil
		}
		return errors.Errorf("pebble: unknown section: %q", errors.Safe(section))
	})
}

func (o *Options) checkOptions(s string) (strictWALTail bool, err error) {
	// TODO(jackson): Refactor to avoid awkwardness of the strictWALTail return value.
	return strictWALTail, parseOptions(s, func(section, key, value string) error {
		switch section + "." + key {
		case "Options.comparer":
			if value != o.Comparer.Name {
				return errors.Errorf("pebble: comparer name from file %q != comparer name from options %q",
					errors.Safe(value), errors.Safe(o.Comparer.Name))
			}
		case "Options.merger":
			// RocksDB allows the merge operator to be unspecified, in which case it
			// shows up as "nullptr".
			if value != "nullptr" && value != o.Merger.Name {
				return errors.Errorf("pebble: merger name from file %q != merger name from options %q",
					errors.Safe(value), errors.Safe(o.Merger.Name))
			}
		case "Options.strict_wal_tail":
			strictWALTail, err = strconv.ParseBool(value)
			if err != nil {
				return errors.Errorf("pebble: error parsing strict_wal_tail value %q: %w", value, err)
			}
		}
		return nil
	})
}

// Check verifies the options are compatible with the previous options
// serialized by Options.String(). For example, the Comparer and Merger must be
// the same, or data will not be able to be properly read from the DB.
func (o *Options) Check(s string) error {
	_, err := o.checkOptions(s)
	return err
}

// Validate verifies that the options are mutually consistent. For example,
// L0StopWritesThreshold must be >= L0CompactionThreshold, otherwise a write
// stall would persist indefinitely.
func (o *Options) Validate() error {
	// Note that we can presume Options.EnsureDefaults has been called, so there
	// is no need to check for zero values.

	var buf strings.Builder
	if o.Experimental.L0CompactionConcurrency < 1 {
		fmt.Fprintf(&buf, "L0CompactionConcurrency (%d) must be >= 1\n",
			o.Experimental.L0CompactionConcurrency)
	}
	if o.L0StopWritesThreshold < o.L0CompactionThreshold {
		fmt.Fprintf(&buf, "L0StopWritesThreshold (%d) must be >= L0CompactionThreshold (%d)\n",
			o.L0StopWritesThreshold, o.L0CompactionThreshold)
	}
	if uint64(o.MemTableSize) >= maxMemTableSize {
		fmt.Fprintf(&buf, "MemTableSize (%s) must be < %s\n",
			humanize.Bytes.Uint64(uint64(o.MemTableSize)), humanize.Bytes.Uint64(maxMemTableSize))
	}
	if o.MemTableStopWritesThreshold < 2 {
		fmt.Fprintf(&buf, "MemTableStopWritesThreshold (%d) must be >= 2\n",
			o.MemTableStopWritesThreshold)
	}
	if o.FormatMajorVersion > internalFormatNewest {
		fmt.Fprintf(&buf, "FormatMajorVersion (%d) must be <= %d\n",
			o.FormatMajorVersion, internalFormatNewest)
	}
	if o.TableCache != nil && o.Cache != o.TableCache.cache {
		fmt.Fprintf(&buf, "underlying cache in the TableCache and the Cache dont match\n")
	}
	if buf.Len() == 0 {
		return nil
	}
	return errors.New(buf.String())
}

// MakeReaderOptions constructs sstable.ReaderOptions from the corresponding
// options in the receiver.
func (o *Options) MakeReaderOptions() sstable.ReaderOptions {
	var readerOpts sstable.ReaderOptions
	if o != nil {
		readerOpts.Cache = o.Cache
		readerOpts.Comparer = o.Comparer
		readerOpts.Filters = o.Filters
		if o.Merger != nil {
			readerOpts.Merge = o.Merger.Merge
			readerOpts.MergerName = o.Merger.Name
		}
		readerOpts.LoggerAndTracer = o.LoggerAndTracer
	}
	return readerOpts
}

// MakeWriterOptions constructs sstable.WriterOptions for the specified level
// from the corresponding options in the receiver.
func (o *Options) MakeWriterOptions(level int, format sstable.TableFormat) sstable.WriterOptions {
	var writerOpts sstable.WriterOptions
	writerOpts.TableFormat = format
	if o != nil {
		writerOpts.Cache = o.Cache
		writerOpts.Comparer = o.Comparer
		if o.Merger != nil {
			writerOpts.MergerName = o.Merger.Name
		}
		writerOpts.TablePropertyCollectors = o.TablePropertyCollectors
		writerOpts.BlockPropertyCollectors = o.BlockPropertyCollectors
	}
	if format >= sstable.TableFormatPebblev3 {
		writerOpts.ShortAttributeExtractor = o.Experimental.ShortAttributeExtractor
		writerOpts.RequiredInPlaceValueBound = o.Experimental.RequiredInPlaceValueBound
		if format >= sstable.TableFormatPebblev4 && level == numLevels-1 {
			writerOpts.WritingToLowestLevel = true
		}
	}
	levelOpts := o.Level(level)
	writerOpts.BlockRestartInterval = levelOpts.BlockRestartInterval
	writerOpts.BlockSize = levelOpts.BlockSize
	writerOpts.BlockSizeThreshold = levelOpts.BlockSizeThreshold
	writerOpts.Compression = levelOpts.Compression
	writerOpts.FilterPolicy = levelOpts.FilterPolicy
	writerOpts.FilterType = levelOpts.FilterType
	writerOpts.IndexBlockSize = levelOpts.IndexBlockSize
	return writerOpts
}