Re: [Qemu-block] [Qemu-devel] [PATCH 7/7] qcow2: Merge the writing of the COW regions with the guest data

[Kevin Wolf]

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.

Kevin