Features/Snapshots

Live Snapshots

This document is describing the current design of live snapshots for QEMU. It is a work in progress and things may change as we progress.

Overall concept

The idea is to be able to issue a command to QEMU via the monitor or QMP, which causes QEMU to create a new snapshot image with the original image as the backing file, mounted read-only. This will allow the original image file to be backed up.

Roll-back to a previous version requires one to boot from the previous backing file, at which point the snapshot file becomes invalid. Unfortunately there is no way to detect that a backing file has been booted, making it important for administrators to take care to not rely on snapshot files being valid after a roll-back.

The snapshot image will have to be in a format which support backing files, ie QCOW2 (and QED when the code is integrated), however the original image can be of any supported format. Ie. it is possible to make a QCOW2 snapshot of a RAW image.

High level design

Snapshot procedure:

1. Run the VM
2. (Optional)Admin tool calls Agent in guest, requesting consistent state
3. (Optional)Admin tool pauses qemu (mandatory if more than one device will be snapshot simultaneously)
4. Issue snapshot command to QEMU: The command will specify which block device is to be snapshot, as well as the filename of the QCOW2 snapshot image (see below).
5. QEMU creates new snapshot image
6. VM is halted by QEMU (if management tool did not already pause vm)
7. Flush pending IOs
8. Switch VM to open new snapshot image, using original image as read only backing file.new image.
9. If VM was not paused at start of snapshot command, QEMU will Restart VM
10. If admin tool paused VM first, then admin tool must restart VM
11. (Optional)Admin tool calls Agent in guest, with run command

Monitor command

The monitor command is designed to be flexible enough to handle both internal and external snapshots, as well as snapshots to various different snapshot file formats.

snapshot_blkdev device [snapshot-file] [format]:

device	block device to snapshot
snapshot-file	target snapshot file, if no specified, the snapshot is meant to be internal to the original image. Internal snapshots are not expected to be supported in the initial implementation.
format	format of snapshot image, valid formats are QCOW2 & QED (when merged upstream If not specified, the image will default to QCOW2.

QMP command

The QMP command matches the behaviour of the human monitor command, except it is named slightly differently to match the fact that the command is synchronous.

blockdev-snapshot-sync device snapshot-file [format]

device	device name to snapshot (json-string)
snapshot-file	name of new image file (json-string)
format	format of new image (json-string, optional)

Future features

Internal snapshots to images which support internal snapshots (QCOW2 & QED) are not expected to be supported initially.

There have been requests and suggestions for a number of alternative and enhanced interfaces for accessing live snapshots as follows:

fd passed as target for snapshot file/device

To get around problems with selinux, in particular in conjunction with images based on NFS, there is a wish to be able to pass an already open file descriptor using the getfd interface.

However, this poses a number of problems. When creating the COW headers for the new image file, as the COW header needs to know the file name of the disk image it is pointing to. On Linux this can be obtained through /proc/self/fd/<X> but this is not available on all other operating systems.

An alternative solution would be to extend the getfd interface to take an optional file name. However this is ugly and error prone, as it would allow a broken/hostile controller to create an image which points to the wrong place, but which wouldn't be discovered until the time where the image was actually being booted from.

Allowing the controlling application to create the COW headers in the new image is not an acceptable solution. It is race prone and would cause problems for COW formats where the new COW headers include state as of when they are created.

Other proposed qemu features that solve the same problem

Snapshots2 and Livebackup

snapshots2
livebackup