[libvirt-users] Create qcow2 v3 volumes via libvirt

[Eric Blake]

On 05/01/2018 03:11 PM, Gionatan Danti wrote:
> Il 01-05-2018 10:56 Daniel P. Berrangé ha scritto:
>> qcow2 is widely used in production at large scale in general. Just not
>> with internal snapshots - almost everything uses external snapshots,
>> aka backing file chains.
>>
>> The QEMU community still tends to discourage use of internal snapshots.
>> There are not even any QMP monitor commands to use them - you are forced
>> to use the legacy HMP interface to QEMU for mgmt. All of the workaround
>> providing interesting block storage mgmt is focused on external snapshots
>> (aka the backing_file option). There are some technical downsides to
>> internal snapshots IIUC, such as inability to free the space used by the
>> internal snapshot when it is deleted, loading/saving snapshots blocks
>> execution of the guest OS, and probably more I've forgotten about.
>>
>> The only nice thing about internal snapshots is simplicity of mgmt, and
>> that is a very nice thing indeed, which is why virt-manager has code
>> to support that - it was much easier to add that code for external
>> snapshots. Just a shame about all the downsides :-(
> 
> So internal snapshots remain something very useful for lab/tests, but 
> are not recommended for regular use in production environment, right?

That's fairly accurate.  Also, mixing internal and external snapshots at 
the same time is likely to trigger some known data-loss problems, if you 
are not extremely careful, so for now, just pick one or the other and 
stick to it (I hope to someday enhance qemu to refuse operations that 
would risk data loss, or to perform a slower version of the operation 
with the same result instead of its current fast but wrong 
implementations, when dealing with mixed-internal/external backing chains).

https://lists.gnu.org/archive/html/qemu-devel/2018-04/msg00865.html

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org