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rbd2qcow2

Backup program which INCREMENTALLY back up CEPH RBD images to a chain of qcow2 files

First of all, requirements:

  • Python >= 3.5
  • Python3 RBD and Python3 Rados binding.
  • qemu-nbd and qemu-img. I advice to use qemu-nbd which supports discarding and zero-detection.

This tool basically will look for all RBD images (by default) in specified CEPH pool. For each image, it makes RBD snapshot. This snapshot is pulled basically in a same way as rbd export-diff --from-snap does. And this diff is transformed to qcow2-image, forming chain of qcow2 images. Each qcow2 image stores diff from previous one.

The main features of such way of storing snapshot diffs:

  1. Ability to start VM from any qcow2 file in the chain. Since each qcow2-file in our case is the snapshot and also is the diff. Just like commits in GIT. Note, you MUST NOT modify any qcow2 file in a chain. If you do that, all backups in the chain after modified file will be broken. If you want to start VM immediatelly -- either create new qcow2-image which refers to chosen qcow2-image, or start qemu --snapshot.

  2. Usable backups even if whole CEPH cluster is broken down. rbd import-diff requires working CEPH cluster.

  3. qcow2 format is standard. Any qcow2 image may be easily put back to CEPH (or another storage) using qemu-img command without any vendor-specific tool.

  4. Ability to remove old incremental backups. This may be implemented using qcow2 rebase operation in qemu-img.files in order to remove old backups.

  5. Ability to pull diff using multiple parallel data streams. This is MUCH faster, since different chunks are pulled from different OSDs in parallel.

  6. This tool effectively transfers regions filled with zeroes. Both when pulling from RBD and when writing to qcow2. In order to effectively store zeroes in RBD you should enable DISCARD in qemu. Configure discard alignment in qemu correct to match RBD chunk size (i.e. 4MB by default) and use virtio-scsi or ide interface (and not just virtio) for disks. Also, you need to set up fstrim -v -a by cron (or equivalent) in your VM. Do not add discard option while mounting FS. This is inefficient actually. Recent Windows versions already discard zeroes by default. If everything done right, zeroes (i.e. trash typically from removed files) will not be transferred over the network.

    And also you need to have recent qemu-nbd which supports discard operation. If not, this tool will just write zeroes directly.

    Anyway, if you forget something from this notes, everything will work, but not so efficient.

  7. I pay attention to atomic disk operations. So, interrupted backups will never corrupt the chain.

Note:

  1. This script does not backup VM metadata (i.e. hardware description).

  2. This script does not make atomic snapshot of VM with two or more disks.

  3. qcow2 compressing is still not used (will be used in future). Unfortunatelly it can not be used while writing image. It's only possible using qemu-img convert. But I don't know how to compress only diff, not resulting image. It seems This will be done when I rewrite RBD layer from scratch and eliminate qemu-nbd.

  4. This tool uses qemu-nbd to write qcow2 images. Unfortunatelly, there is no library for such task. Only image format specification.

  5. Do not remove old backups. Since qcow2 images form a chain, removing any part from the chain will break it. If you need to remove old backups, use qemu-img rebase. In future, it will be implemented in this tool.

  6. Do not copy qcow2 files. They contain only diff, and so they can not be used without all previous images. Either copy image and all previous files, or use qemu-img convert to copy from some backed up qcow2 file to target one. I recommend to enable compression im qqmu-img convert if target file supports compression.

  7. Do not run this tool simultaneously on same set of images, especially with same target directory. It will possibly corrupt logic.

  8. This script is used in production ;)

  9. Patches, suggestions, questions, feature requests are welcome.

  10. This tool should work on CEPH versions Jewel and upper.