Revision as of 15:49, 7 August 2012

Virt Live Snapshots

Summary

Live snapshots allow a user to take a snapshot of a virtual machine while the guest is running, thus preserving the state and data of a VM at a specific point in time.

Owner

Name: Cole Robinson
Email: crobinso@redhat.com
Name: Jeff Cody
Email: jtc@redhat.com

Current status

Targeted release: Fedora 18
Last updated: June 6 2012
Percentage of completion: 90%

Detailed Description

QEMU has had snapshotting capabilities for a long time, but they have always required the guest to be paused/stopped for a short period of time while the storage was snapshoted. Live snapshots allow for qemu and libvirt to snapshot a the guest's storage with no downtime. This even works on guests with 'raw' disk images: libvirt will snapshot using an external qcow2 file, and transparently switch the guest over to running off the new external image.

Benefit to Fedora

Snapshots can be performed on guests using any storage configuration (utilizing external qcow2 snapshots)
No downtime during the snapshotting process.

Scope

Live snapshot support in qemu (DONE, 1.1 is in rawhide/f18)
Live snapshot support in qemu-ga (DONE)
Live snapshot support in libvirt (DONE)
Apps (all optional but would be nice if they are done)
- Snapshot support in ovirt? (available, used for live backup)
- Snapshot support in virt-manager? (not done, not a requirement)
- Snapshot support in boxes? (out of scope)

How To Test

Live backup

You can backup the storage of a VM from the host machine with no VM downtime.

Snapshot the VM:

 virsh snapshot-create-as myvm snapshot1 "snapshot1 description" --disk-only --atomic

Backup the original storage using your regular backup method.

Merge the disk changes that accumulated in the snapshot back with the original disk. --path is the original disk image, --base is the snapshot image the guest starting using after the live snapshot.

 virsh blockpull --domain myvm --path /var/lib/libvirt/images/myvm.img --base /var/lib/libvirt/images/myvm.1

'blockpull' needs to be done for every disk image that was snapshotted.

VM checkpointing

The below commands are not fully implemented in libvirt yet.

Any guest KVM guest can be snapshotted using external snapshot files. Simplest way:

 virsh snapshot-create-as myvm snapshot1 "snapshot1 description" --disk-only --atomic
 virsh snapshot-create-as myvm snapshot2 "snapshot2 description" --disk-only --atomic

You make some changes and want to switch back to an older snapshot state, you do:

 virsh snapshot-revert myvm snapshot1

XXX: however this doesn't work right now: 'revert to external disk snapshot not supported yet'

If you want to delete a snapshot, use:

 virsh snapshot-delete myvm snapshot2

XXX: also doesn't work: 'deletion of 1 external disk snapshots not supported yet'

If the guest has qemu-ga installed and configured, passing --quiesce to snapshot-create-as will make sure the guest's filesystems are frozen for extra safety.

--atomic should ensure all disks are snapshotted atomically

User Experience

Virtualization users will have a more production ready mechanism for snapshotting than QEMU previously supported (no loss of uptime). Exposing snapshotting in tools like ovirt and/or virt-manager will essentially be a new and compelling feature for those users.

Dependencies

None

Contingency Plan

Since this is brand new functionality, if it doesn't make it in time for F18, nothing has changed. We just drop this feature page.

Documentation

Release Notes

KVM and libvirt now support storage snapshotting of live guests with no downtime.

Comments and Discussion

See Talk:Features/Virt_Live_Snapshots

@@ Line 34: / Line 34: @@
 * Live snapshot support in libvirt (DONE)
 * Apps (all optional but would be nice if they are done)
-** Snapshot support in ovirt? (unknown)
+** Snapshot support in ovirt? (available, used for live backup)
-** Snapshot support in virt-manager? (not done)
+** Snapshot support in virt-manager? (not done, not a requirement)
-** Snapshot support in boxes? (probably out of scope)
+** Snapshot support in boxes? (out of scope)
 == How To Test ==
@@ Line 52: / Line 52: @@
 . What are the expected results of those actions?
 -->
+=== Live backup ===
+You can backup the storage of a VM from the host machine with no VM downtime.
+Snapshot the VM:
+  virsh snapshot-create-as myvm snapshot1 "snapshot1 description" --disk-only --atomic
+Backup the original storage using your regular backup method.
+Merge the disk changes that accumulated in the snapshot back with the original disk. --path is the original disk image, --base is the snapshot image the guest starting using after the live snapshot.
+  virsh blockpull --domain myvm --path /var/lib/libvirt/images/myvm.img --base /var/lib/libvirt/images/myvm.1
+'blockpull' needs to be done for every disk image that was snapshotted.
+=== VM checkpointing ===
+The below commands are not fully implemented in libvirt yet.
 Any guest KVM guest can be snapshotted using external snapshot files. Simplest way:
    virsh snapshot-create-as myvm snapshot1 "snapshot1 description" --disk-only --atomic
    virsh snapshot-create-as myvm snapshot2 "snapshot2 description" --disk-only --atomic
-(however --atomic claims to only work for a guest with 1 disk. isn't that what jtc's patches were meant to fix? maybe just a libvirt validation issue)
 You make some changes and want to switch back to an older snapshot state, you do:
@@ Line 63: / Line 83: @@
    virsh snapshot-revert myvm snapshot1
-(however this doesn't work right now?: revert to external disk snapshot not supported yet)
+XXX: however this doesn't work right now: 'revert to external disk snapshot not supported yet'
 If you want to delete a snapshot, use:
@@ Line 69: / Line 89: @@
    virsh snapshot-delete myvm snapshot2
-(that also doesn't work: deletion of 1 external disk snapshots not supported yet)?
+XXX: also doesn't work: 'deletion of 1 external disk snapshots not supported yet'
 If the guest has qemu-ga installed and configured, passing --quiesce to snapshot-create-as will make sure the guest's filesystems are frozen for extra safety.

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