[Qemu-devel] drive-mirror sync points

[Qemu-devel] drive-mirror sync points

[Wolfgang Richter]

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Paolo/anyone who knows -

Are drive-mirror sync points (NBD flush commands) reflecting guest write
barriers?  Are guest write barriers respected by drive-mirror?  If so, that
would make drive-mirror much more palatable for disk introspection work (a
drop-in usable feature of QEMU!).

-- 
Wolf

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Re: [Qemu-devel] drive-mirror sync points

[Paolo Bonzini]

Il 13/05/2013 19:50, Wolfgang Richter ha scritto:
> Paolo/anyone who knows -
> 
> Are drive-mirror sync points (NBD flush commands) reflecting guest write
> barriers?  Are guest write barriers respected by drive-mirror?  If so,
> that would make drive-mirror much more palatable for disk introspection
> work (a drop-in usable feature of QEMU!).

Kind of.  Sync points reflect points where the destination image is a
copy of the source image at some point in time.  Destination image write
barriers hence reflect places where the destination image is consistent
with guest write barriers, but not all guest write barriers become
destination image write barriers.

Paolo

Re: [Qemu-devel] drive-mirror sync points

[Richard W.M. Jones]

On Mon, May 13, 2013 at 01:50:00PM -0400, Wolfgang Richter wrote:
> Paolo/anyone who knows -
> 
> Are drive-mirror sync points (NBD flush commands) reflecting guest write
> barriers?  Are guest write barriers respected by drive-mirror?  If so, that
> would make drive-mirror much more palatable for disk introspection work (a
> drop-in usable feature of QEMU!).

I'm also interested in this question.  Further extensions to this
(*not* drive-mirror on its own AIUI) which stefanha is working on
should allow libguestfs to perform point-in-time snapshots of images,
which will mean that we can do complex and long-running inspection
operations on live guests.

Rich.

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Re: [Qemu-devel] drive-mirror sync points

[Richard W.M. Jones]

On Mon, May 13, 2013 at 10:46:55PM +0100, Richard W.M. Jones wrote:
> On Mon, May 13, 2013 at 01:50:00PM -0400, Wolfgang Richter wrote:
> > Paolo/anyone who knows -
> > 
> > Are drive-mirror sync points (NBD flush commands) reflecting guest write
> > barriers?  Are guest write barriers respected by drive-mirror?  If so, that
> > would make drive-mirror much more palatable for disk introspection work (a
> > drop-in usable feature of QEMU!).
> 
> I'm also interested in this question.  Further extensions to this
> (*not* drive-mirror on its own AIUI) which stefanha is working on
> should allow libguestfs to perform point-in-time snapshots of images,
> which will mean that we can do complex and long-running inspection
> operations on live guests.

OK, I got my attributions wrong there.  I see that Dietmar Maurer
wrote the original version of the patch and Stefan modified that.
Anyway I'm very interested in point-in-time snapshots.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming blog: http://rwmj.wordpress.com
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Re: [Qemu-devel] drive-mirror sync points

[Wolfgang Richter]

On May 13, 2013, at 5:46 PM, "Richard W.M. Jones" <rjones@redhat.com> wrote:

> On Mon, May 13, 2013 at 01:50:00PM -0400, Wolfgang Richter wrote:
>> Paolo/anyone who knows -
>> 
>> Are drive-mirror sync points (NBD flush commands) reflecting guest write
>> barriers?  Are guest write barriers respected by drive-mirror?  If so, that
>> would make drive-mirror much more palatable for disk introspection work (a
>> drop-in usable feature of QEMU!).
> 
> I'm also interested in this question.  Further extensions to this
> (*not* drive-mirror on its own AIUI) which stefanha is working on
> should allow libguestfs to perform point-in-time snapshots of images,
> which will mean that we can do complex and long-running inspection
> operations on live guests.

And I'm trying to do complex, long-running inspection on live guests without needing point-in-time snapshots :-)