Features/SnapshottingImprovements

Summary

Refactor the savevm, loadvm and info snapshots commands to facilitate the transtion to QMP.

This page is being maintained by the GSoC student Miguel Di Ciurcio Filho as part the QMP project. All feedback is appreciated.

Issues

Problem	Difficulty	Proposed solution	Status
Snapshot existence is not verified before bdrv_snapshot_goto()	Easy	Improve the resilience of the load_vmstate() function, doing further and better ordered tests.	Patch applied to the block branch: http://lists.gnu.org/archive/html/qemu-devel/2010-07/msg01104.html
Monitor shows wrong information about snapshots	Easy	Consolidate the snapshot information to show only snapshots fully available Do not display block device information anymore	Patch: http://lists.gnu.org/archive/html/qemu-devel/2010-07/msg01591.html Need to code fixes suggested by Markus
Do not allow empty names	Easy	If the user does not specify a name, we must create a name from a template, like: vm-20100801121212.	Patch: http://lists.gnu.org/archive/html/qemu-devel/2010-07/msg01701.html Need some corrections pointed by Luiz and Markus
Snapshot overwriting	Easy	Don't save the snapshot if the name given by the user already exists. If the user really wants to overwrite it, add a -f parameter to savevm. Do not to support savevm <ID> anymore. We are trying not to expose ID information to the user.	Patch: http://lists.gnu.org/archive/html/qemu-devel/2010-07/msg01701.html Need some corrections pointed by Luiz and Markus Wait for libvirt to confirm if we are break anything
ID and TAG confusion	Medium	When specifying an snapshot to be loaded or saved, only use names/tags. Have separate commands to load by ID and name, or add parameters to explicitly distinguish one from another.	Patch: http://lists.gnu.org/archive/html/qemu-devel/2010-07/msg01701.html Need some corrections pointed by Luiz and Markus
snapshot ID collision	Medium	Use an stronger ID numbering system instead of simple sequential decimal numbers UUID seams good enough
inheritance	Hard	Add char parent_id_str[128] to QEMUSnapshotInfo and char *parent_id_str to QCowSnapshot Use the extra_data_size field of the qcow2 header to store the parent_id_str When loading and saving snapshots, verify if extra_data_size > 0 and feed the information info QEMUSnapshotInfo.parent_id_str libvirt wants this feature Update the info snapshots output to hide ID and PARENT_ID information Add another command to show full details of snapshots The ID information must not be exposed to the user, unless when using qemu-img to inspect a disk image.

Snapshot existence is not verified before bdrv_snapshot_goto()

This patch improves the resilience of the load_vmstate() function, doing further and better ordered tests.

In load_vmstate(), if there is any error on bdrv_snapshot_goto(), except if the error is on VM state device, load_vmstate() will return zero and the VM will be started with major corruption chances.

Monitor shows wrong information about snapshots

The output generated by 'info snapshots' shows only snapshots that exist on the block device that saves the VM state. This output can cause an user to erroneously try to load an snapshot that is not available on all block devices.

$ qemu-img snapshot -l xxtest.qcow2
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1                                1.5M 2010-07-26 16:51:52   00:00:08.599
2                                1.5M 2010-07-26 16:51:53   00:00:09.719
3                                1.5M 2010-07-26 17:26:49   00:00:13.245
4                                1.5M 2010-07-26 19:01:00   00:00:46.763

$ qemu-img snapshot -l xxtest2.qcow2
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
3                                   0 2010-07-26 17:26:49   00:00:13.245
4                                   0 2010-07-26 19:01:00   00:00:46.763 

Current output:

$ qemu -hda xxtest.qcow2 -hdb xxtest2.qcow2 -monitor stdio -vnc :0
QEMU 0.12.4 monitor - type 'help' for more information
(qemu) info snapshots
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1                                1.5M 2010-07-26 16:51:52   00:00:08.599
2                                1.5M 2010-07-26 16:51:53   00:00:09.719
3                                1.5M 2010-07-26 17:26:49   00:00:13.245
4                                1.5M 2010-07-26 19:01:00   00:00:46.763 

Snapshots 1 and 2 do not exist on xxtest2.qcow, but they are displayed anyway.

New output:

(qemu) info snapshots
ID        TAG                 VM SIZE                DATE       VM CLOCK
3                                1.5M 2010-07-26 17:26:49   00:00:13.245
4                                1.5M 2010-07-26 19:01:00   00:00:46.763 

Do not allow empty names

When running:

(qemu) savevm

The snapshot is saved without a name and can only be loaded using the generated ID. The idea is to not have IDs exposed, so every snapshot must have a name.

Snapshot overwriting

Is this the expected behavior? Don't think so.

By id

(qemu) savevm 
(qemu) savevm 
(qemu) info snapshots 
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1                                3.9M 2010-07-21 17:08:31   00:00:03.696
2                                3.9M 2010-07-21 17:08:33   00:00:05.419 <-- 17:08:33
(qemu) savevm 2 <overwrittes the snapshot with id 2>
(qemu) info snapshots 
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1                                3.9M 2010-07-21 17:08:31   00:00:03.696
2                                3.9M 2010-07-21 17:08:53   00:00:25.918 <-- 17:08:53

Ideas for improvements:

• Do not allow the user to specify an ID when saving a VM, just a TAG.

By tag

(qemu) savevm test1
(qemu) savevm test2
 (qemu) info snapshots 
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         test1                  3.9M 2010-07-21 17:12:11   00:00:08.126 <-- 17:12:11
2         test2                  3.9M 2010-07-21 17:12:19   00:00:16.053
(qemu) savevm test1
(qemu) info snapshots 
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
2         test2                  3.9M 2010-07-21 17:12:19   00:00:16.053
1         test1                  3.9M 2010-07-21 17:12:33   00:00:29.221 <-- 17:12:33

Ideas for improvements:

• Do not allow the user to overwrite an snapshot, even if saving with the same name (will have different IDs anyway)

ID and TAG confusion

What other projects do?

• VirtualBox does not allow specifying an ID, just a name and description of an snapshot. http://www.virtualbox.org/manual/ch01.html#snapshots
• VMware does not allow specifying and ID, just a name and description. http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1015180
• XenServer does not allow specifying an ID, just a name. http://support.citrix.com/servlet/KbServlet/download/23735-102-646135/Administrators_Guide.pdf - page 127

Starting a VM with no previous snapshot on the disks.

(qemu) savevm 2
(qemu) savevm
(qemu) info snapshots 
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         2                      1.5M 2010-07-21 16:55:48   00:00:15.581
2                                1.5M 2010-07-21 16:55:55   00:00:21.501
(qemu) loadvm 2 <loads the VM with the id 2>

It is not clear witch snapshot is going to be loaded.

Ideas for improvements:

• savevm tests the argument to check if it is an ID or a TAG. Make it accept only a TAG and do not allow the user to specify and ID.
• loadvm does the same thing. The user must specify if an ID or TAG is wanted. Suggestions:

(qemu) loadvm id=3
(qemu) loadvm tag=xxx
(qemu) loadvm xxx <error>
(qemu) loadvm 3 <error>

In case loadvm cannot be changed due to retro-compatibility, introduce a new command. Suggestions:

(qemu) load_vm id=3
(qemu) load_vm tag=xxx
(qemu) loadvm_byid 3
(qemu) loadvm_bytag xxx

snapshot ID collision

A VM is loaded with two devices, one device with three previous snapshots and the other device with two previous snapshots.

I take a new snapshot of the VM without a name: the first device will save the snapshot as id_str 4 and the second device with id_str 3.

I play around for a while and I want to restore the VM to my latest state. When 'info snapshots' is run, only the first device is show with the VM state, and the latest ID shown will be 4.

Ideas for improvements:

• Use an stronger ID model, not sequential and large enough to be virtually unique.

loading without checking all devices

So, I fire up loadvm 4. qemu will try to load by id or name anyway, will succeed loading the VM state from the first device, but will fail to load an non-existent snapshot id from the second device and will still be running with the second device out of sync with the VM state.

equal IDs on different devices with different states (id collision)

Running again:

(qemu) savevm 4

Continuing on the same case. The previously saved state identified by id_str 4 will be deleted and replaced by a new one, but only in the first device of my example above. The second device will get a new snapshot, since it does not have a id_str equal to 4.

If I run loadvm 4 again, the snapshot id 3 in the second device will be left there in an state diferent from the snapshot id 3 from the first device.

Ideas for improvements:

• Use an stronger ID model, not sequential and large enough to be virtually unique.

inheritance

The only way to know witch snapshot came before of after another is to look at the date, but it is not possible to fully deduct the relationship. e.g.:

(qemu) savevm a
(qemu) savevm b
(qemu) savevm c
(qemu) savevm d
(qemu) loadvm b
(qemu) savevm e
(qemu) info snapshots 
Snapshot devices: ide0-hd0
Snapshot list (from ide0-hd0):
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         a                      3.9M 2010-07-21 17:32:55   00:00:06.144
2         b                      3.9M 2010-07-21 17:33:00   00:00:11.103
3         c                      3.9M 2010-07-21 17:33:13   00:00:22.953
4         d                      3.9M 2010-07-21 17:33:16   00:00:26.289
5         e                      3.9M 2010-07-21 17:33:28   00:00:15.548

Snapshot e is newer than d, but it is an state saved right after b, and not d as implied by the ids and dates.

Ideas for improvements:

• Store the previous snapshot taken as parent_id of a new snapshot.

QMP

Here is a list of suggested commands to manipulate snapshots information on a QEMU VM.

query-snapshot-status

Returns true when the VM can be snapshotted, false otherwise.

-> {"execute": "query-snapshot-status"}
<- {"return": true }

• Not sure if QMP allow returning just a boolean.

snapshot-create

Runs savevm.

-> {"execute": "snapshot-create", "arguments": {"name": "vm_before_upgrade"} }
<- { "return": {} }

• "name" is mandatory and must be unique.
• Needs to return any errors.

snapshot-delete

Runs delvm.

-> {"execute": "snapshot-delete", "arguments": {"name": "vm_before_upgrade"} }
<- { "return": {} }

• "name" is mandatory and must be unique.
• Needs to return any errors.

snapshot-load

Runs loadvm.

-> {"execute": "snapshot-load", "arguments": {"name": "vm_before_upgrade"} }
<- { "return": {} }

• "name" is mandatory and must be unique.
• Needs to return any errors.

query-snapshots

-> {"execute": "query-snapshots"}
<- {
"return": [
   {           
     "name": "vm_before_upgrade",
     "parent_name": "",
     "id": "3fab28c9-a6bd-4659-b9bc-683780d8a2d5",
     "parent_id": "00000000-0000-0000-0000-000000000000",
     "date": 20100415164856
   },
   {           
     "name": "vm_before_upgrade-2",
     "parent_name": "vm_before_upgrade",
     "id": "e693dce2-6e12-4474-9bbf-2b3f97323423",
     "parent_id": "00000000-0000-0000-0000-000000000000",
     "date": 20100415100000
   }
  ]        
}

• Do we really need to expose IDs or names should be enough?
• Does JSON has a date type of just using a integer like YYYYMMDDHHMMSS would be better?

Instead of a list, how about this:

-> {"execute": "query-snapshots"}
<- {
"return": [
   {           
      "vm_before_upgrade": { 
		      "id": "3fab28c9-a6bd-4659-b9bc-683780d8a2d5", 
		      "date": 20100415164856, 
		      "child": { "vm_before_upgrade-2": 
  					"id": "e693dce2-6e12-4474-9bbf-2b3f97323423",
 				        "date": 20100415100000 
			       }
                           }
        "random-backup": {
 		      "id": "abcdefcc-a6bd-4659-b9bc-683780d8a2d5",
 		      "date": 20111111111111
                         }
     }
  ]