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Re: [Qemu-block] [PATCH] mirror: Improve zero-write and discard with fra
Re: [Qemu-block] [PATCH] mirror: Improve zero-write and discard with fragmented image
Mon, 9 Nov 2015 17:29:34 +0100
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
> > * 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.