[libvirt] Question about fsfreeze/fsthaw API

[Tomoki Sekiyama]

On 11/7/13 14:05 , "Doug Goldstein" <cardoe at gentoo.org> wrote:

>On Thu, Nov 7, 2013 at 11:39 AM, Eric Blake <eblake at redhat.com> wrote:
>> On 11/06/2013 05:31 PM, Tomoki Sekiyama wrote:
>>> Hi all,
>>>
>>> Is there any plans to add APIs to execute fsfreeze/fsthaw in qemu
>>>guests?
>>> (something like virDomainFSFreeze(domain,timeout,flags) and
>>>                 virDomainFSThaw(domain,timeout,flags))
>>
>> I'm wondering if it might be better to have a single command with a
>> callback argument, something like:
>>
>> virDomainQuiese(domain, timeout, callback, opaque, flags)
>>
>> which calls callback(domain, opaque) at the right point in time.  I'm
>> just a bit worried that since the freeze/thaw sequence is already
>> handled as a pair by the QUIESCE flag of snapshot creation that exposing
>> it as two non-atomic APIs may lead to inconsistent states that we'd have
>> a hard time tracking which commands are allowed in which state.  With
>> only a single command and a callback, we have guaranteed semantics that
>> all other API are locked out by our normal job mechanism, and that we
>> can pair the freeze/thaw under the hood just as we do in snapshots.
>
>I think this is a much better approach. We could arguably refactor the
>snapshot code to use this as well.
>

This sounds reasonable for me. Thanks for your suggestion.

Can we call virDomainBlockRebase() and BlockJobInfo()/BlockJobAbort() for
the
rebase operation in the callback? OpensStack is using these while image
snapshot for now. For other kinds of volumes, other APIs might also be
used.

Tomoki Sekiyama