qemu snapshot 예

== 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 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

== 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.