Features/SnapshottingImprovements

Summary

Adjust 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

Index	Problem	Difficulty	Proposed solution	Status
1	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://repo.or.cz/w/qemu/kevin.git/commit/d98644fe6fc05ce75c203c761920a6948b0ad853
2	Monitor shows wrong information about snapshots	Easy	Consolidate the snapshot information to show only snapshots fully available Do not display any block device information anymore	Patch v2: http://lists.gnu.org/archive/html/qemu-devel/2010-08/msg00346.html
3	Do not allow empty names	Easy	If the user does not specify a name, we must create a name from a template, using a timestamp like: vm-20100801121212.
4	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. In case of overwriting, do not reuse the ID, just the name.
5	snapshot ID collision	Easy	Use an stronger ID numbering system instead of simple sequential decimal numbers Take the ID generation out of the block drivers UUID seams a good mechanism, needs to depend on libuuid-dev	Patch v1: http://lists.gnu.org/archive/html/qemu-devel/2010-08/msg00204.html
6	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 in loadvm. Do not support savevm <ID> anymore. We are trying not to expose ID information to the user.
7	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.	Patch v1: http://lists.gnu.org/archive/html/qemu-devel/2010-08/msg00204.html

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

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)

snapshot ID collision

qemu-img create -f qcow2 diskA.qcow2 10G
qemu-img create -f qcow2 diskB.qcow2 10G

qemu -hda diskA.qcow2
(qemu) savevm a
(qemu) savevm b
(qemu) savevm c

qemu -hda diskB.qcow2
(qemu) savevm a
(qemu) savevm b

qemu -hda diskB.qcow2 -hdb diskA.qcow2
(qemu) loadvm b

diskA.qcow2 and diskB.qcow2 have both the same snapshot identification b and 2, but they were taken at different moments and must not be loaded. We need an stronger ID mechanism to identify snapshots among different disks. It is not a common case, but we reduce the chance of anyone having trouble.

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 an 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

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. libvirt wants this.

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
                         }
     }
  ]