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Re: [Qemu-block] [Qemu-devel] [PATCH 7/7] qcow2: Merge the writing of th

From: Kevin Wolf
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 7/7] qcow2: Merge the writing of the COW regions with the guest data
Date: Fri, 26 May 2017 12:12:05 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 24.05.2017 um 21:05 hat Alberto Garcia geschrieben:
> On Wed 24 May 2017 06:43:31 PM CEST, Anton Nefedov wrote:
> >> +    if (m->data_qiov) {
> >> +        qemu_iovec_reset(&qiov);
> >> +        qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
> >> +        qemu_iovec_concat(&qiov, m->data_qiov, 0, data_bytes);
> >> +        qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
> >
> > Can it be a problem if (m->data_qiov->niov == IOV_MAX)?
> > We had to add merge-iovecs code for the case (maybe there's better
> > solution?)
> You're right, good catch! I'll add a check for that. To be honest I
> don't think that's likely to happen in practice, so if it does we can
> simply fall back to the old behavior (separate writes).
> > Also, will this work if allocation is split into several l2metas?
> > e.g. one has cow_start.nb_bytes and another has cow_end.nb_bytes
> The guest request will be merged with the first l2meta that has to copy
> at least one of the two regions. It doesn't matter if the other one has
> nb_bytes == 0.

I think we can improve this later so that we merge everything together
(multiple l2metas and guest data) into a single request, but what this
seris implements is still a very good first step, so we shouldn't let
this stop us from taking the good rather than waiting for the perfect.


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