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dracut module to enable booting from tmpfs mount

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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

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