Re: [Qemu-block] [PATCH] mirror: Improve zero-write and discard with fragmented image

[Kevin Wolf]

Am 09.11.2015 um 17:18 hat Paolo Bonzini geschrieben:
> 
> 
> On 09/11/2015 17:04, Kevin Wolf wrote:
> > Am 06.11.2015 um 11:22 hat Fam Zheng geschrieben:
> >> The "pnum < nb_sectors" condition in deciding whether to actually copy
> >> data is unnecessarily strict, and the qiov initialization is
> >> unnecessarily too, for both bdrv_aio_write_zeroes and bdrv_aio_discard
> >> branches.
> >>
> >> Reorganize mirror_iteration flow so that we:
> >>
> >>     1) Find the contiguous zero/discarded sectors with
> >>     bdrv_get_block_status_above() before deciding what to do. We query
> >>     s->buf_size sized blocks at a time.
> >>
> >>     2) If the sectors in question are zeroed/discarded and aligned to
> >>     target cluster, issue zero write or discard accordingly. It's done
> >>     in mirror_do_zero_or_discard, where we don't add buffer to qiov.
> >>
> >>     3) Otherwise, do the same loop as before in mirror_do_read.
> >>
> >> Signed-off-by: Fam Zheng <address@hidden>
> > 
> > I'm not sure where in the patch to comment on this, so I'll just do it
> > here right in the beginning.
> > 
> > I'm concerned that we need to be more careful about races in this patch,
> > in particular regarding the bitmaps. I think the conditions for the two
> > bitmaps are:
> > 
> > * Dirty bitmap: We must clear the bit after finding the next piece of
> >   data to be mirrored, but before we yield after getting information
> >   that we use for the decision which kind of operation we need.
> > 
> >   In other words, we need to clear the dirty bitmap bit before calling
> >   bdrv_get_block_status_above(), because that's both the function that
> >   retrieves information about the next chunk and also a function that
> >   can yield.
> > 
> >   If after this point the data is written to, we need to mirror it
> >   again.
> 
> With Fam's patch, that's not trivial for two reasons:
> 
> 1) bdrv_get_block_status_above() can return a smaller amount than what
> is asked.
> 
> 2) the "read and write" case can handle s->granularity sectors per
> iteration (many of them can be coalesced, but still that's how the
> iteration works).
> 
> The simplest solution is to perform the query with s->granularity size
> rather than s->buf_size.

Then we end up with many small operations, that's not what we want.

Why can't we mark up to s->buf_size dirty clusters as clean first, then
query the status, and mark all of those that we can't handle dirty
again?

Kevin

> > * In-flight bitmap: We need to make sure that we never mirror the same
> >   data twice at the same time as older data could overwrite newer data
> >   then.
> > 
> >   Strictly speaking, it looks to me as if this meant we can delay
> >   setting the bit until before we issue an AIO operation. It might be
> >   more obviously correct to set it at the same time as the dirty bitmap
> >   is cleared.
> 
>