dracut-tmpfs-root

This is a dracut module to allow for booting to a tmpfs root created by extracting an archive (currently only tar.gz).

• A simplified breakdown of the process
  1. Make a temporary directory to mount the source volume
  2. Mount a tmpfs filesystem on /sysroot
  3. Untar the archive from the source volume to /sysroot
  4. Unmount the source volume
  5. Execute switch-root to /sysroot

Install the module

Simply drop the 90tmpfs-root directory into your dracut modules directory. On CentOS/RHEL, this directory is /usr/lib/dracut/modules.d.

Use the module in an initrd image

You must modify the dracut command slightly to include the module:

dracut -N -a tmpfs-root -f /boot/initrd-tmpfs.img

• -N builds a non-host-only initrd (TODO: this requirement may be removed in the future)
• -a tells dracut to include the tmpfs-root module
• -f is, of course, the output image file

Create a suitable image archive

How you get your image is up to you. For now, the image must be a gzipped tarball with permissions preserved (SELinux contexts will be restored from /etc/selinux/... after the image is extracted).

For KVM VMs, you can use guestfish:

guestfish --ro -a /work/disk.img -i tar-out / /work/rootfs.tar.gz compress:gzip

Or simply tarball an offline root filesystem:

mount /dev/centos/root /mnt
tar cpzf rootfs.tgz -C /mnt

Kernel cmdline arguments

All are required. (TODO: Sane defaults)

• root: This takes a special syntax which this modules uses to hijack the root mount process: root=tmpfs:/dev/vda1
  • A local disk can be specified by:
    • Explicit device root=tmpfs:/dev/sda1
    • Disk Label root=tmpfs:LABEL=BOOT
    • Disk UUID root=tmpfs:UUID=00361d4f-646b-478c-9947-73b9bec8531e
    • Partition Label root=tmpfs:PARTLABEL=BOOTPART
    • Partition UUID root=tmpfs:PARTUUID=8b4ec1c1-0ec7-4bc2-acef-077e9283362a
  • An NFS export can be specified by formats supported by Dracut nfs-lib:
    • root=tmpfs:NFS=nfs:10.0.0.11:/nfsexport
    • root=tmpfs:NFS=nfs4:10.0.0.11:/nfsexport
    • root=tmpfs:NFS=nfs:10.0.0.11:/nfsexport,tcp,hard,vers=3
• rd.tmpfs.imgfile: The relative path to the archive file on the volume set for root.
• rd.tmpfs.size: The size of the tmpfs root filesystem. For a description of the size mount arguments, see https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt

A sample kernel cmdline with tmpfs root

linux16 /vmlinuz-3.10.0-693.11.6.el7.x86_64 root=tmpfs:/dev/vda1 rd.tmpfs.size=3G rd.tmpfs.imgfile=rootfs.tgz ro crashkernel=auto rhgb LANG=en_US.UTF-8

Using an LVM volume

linux16 /vmlinuz-3.10.0-693.11.6.el7.x86_64 root=tmpfs:/dev/mapper/centos-root rd.tmpfs.size=5G rd.tmpfs.imgfile=images/rootfs.tar.gz rd.lvm.lv=centos/root ro crashkernel=auto rhgb LANG=en_US.UTF-8

Using an NFS export

linux16 /vmlinuz-3.10.0-693.11.6.el7.x86_64 root=tmpfs:NFS=nfs:10.0.0.11:/nfsexport,tcp rd.tmpfs.size=5G rd.tmpfs.imgfile=images/rootfs.tar.gz ip=10.0.0.31:::24:compute1:eth1:none rd.neednet=1 ro crashkernel=auto rhgb LANG=en_US.UTF-8