Re: [Qemu-devel] [Qemu-block] [RFC PATCH V4] qemu-img: make convert async

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Jun 07, 2018 at 01:19:29PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 07.06.2018 13:10, Stefan Hajnoczi wrote:
> > On Fri, Jun 01, 2018 at 07:16:14PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > 20.02.2017 17:59, Peter Lieven wrote:
> > > > Am 20.02.2017 um 15:50 schrieb Stefan Hajnoczi:
> > > > > On Fri, Feb 17, 2017 at 05:00:24PM +0100, Peter Lieven wrote:
> > > > > > this is something I have been thinking about for almost 2 years now.
> > > > > > we heavily have the following two use cases when using qemu-img
> > > > > > convert.
> > > > > > 
> > > > > > 
> > > [...]
> > > 
> > > > > Does this patch work with compressed images?  Especially the
> > > > > out-of-order write mode may be problematic with a compressed qcow2
> > > > > image.
> > > > It does, but you are right, out-of-order writes and compression should
> > > > be mutually exclusive.
> > > 
> > > Sorry for being late, but can you please explain to me, why?
> > There are image format-specific limitations on compressed writes.  For
> > some reason I thought they were append-only in qcow2, but I was wrong.
> > 
> > Stefan
> 
> And what are limitations for compressed writes in qcow2?

Writes must be cluster-aligned and the size must be 1 cluster (except
for the last cluster in an image).

qemu-img convert honors this, so it's not a problem.

> We can't write asynchronously? Why?

Async compressed writes are supported nowadays.

I think my original comment was wrong.  It should be fine to use
out-of-order compressed writes.

Stefan

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