On 05.08.19 15:00, Vladimir Sementsov-Ogievskiy wrote: > 05.08.2019 14:49, Max Reitz wrote: >> In write-blocking mode, all writes to the top node directly go to the >> target. We must only mirror chunks of data that are aligned to the >> job's granularity, because that is how the dirty bitmap works. >> Therefore, the request alignment for writes must be the job's >> granularity (in write-blocking mode). >> >> Unfortunately, this forces all reads and writes to have the same >> granularity (we only need this alignment for writes to the target, not >> the source), but that is something to be fixed another time. >> >> Signed-off-by: Max Reitz <[email protected]> >> --- >> This is an alternative to Vladimir's "util/hbitmap: fix unaligned reset" >> patch. I don't mind much either way, both of pros and cons. Comparing >> this patch to Vladimir's: >> >> + Makes copy-mode=write-blocking really work (unless I'm mistaken) >> - Lowers performance with copy-mode=write-blocking unnecessarily >> --- >> block/mirror.c | 12 ++++++++++++ >> 1 file changed, 12 insertions(+) >> >> diff --git a/block/mirror.c b/block/mirror.c >> index 8cb75fb409..3f9c5a178a 100644 >> --- a/block/mirror.c >> +++ b/block/mirror.c >> @@ -1481,6 +1481,15 @@ static void >> bdrv_mirror_top_child_perm(BlockDriverState *bs, BdrvChild *c, >> *nshared = BLK_PERM_ALL; >> } >> >> +static void bdrv_mirror_top_refresh_limits(BlockDriverState *bs, Error >> **errp) >> +{ >> + MirrorBDSOpaque *s = bs->opaque; >> + >> + if (s && s->job && s->job->copy_mode == >> MIRROR_COPY_MODE_WRITE_BLOCKING) { >> + bs->bl.request_alignment = s->job->granularity; >> + } >> +} >> + >> /* Dummy node that provides consistent read to its users without requiring >> it >> * from its backing file and that allows writes on the backing file chain. >> */ >> static BlockDriver bdrv_mirror_top = { >> @@ -1493,6 +1502,7 @@ static BlockDriver bdrv_mirror_top = { >> .bdrv_co_block_status = bdrv_co_block_status_from_backing, >> .bdrv_refresh_filename = bdrv_mirror_top_refresh_filename, >> .bdrv_child_perm = bdrv_mirror_top_child_perm, >> + .bdrv_refresh_limits = bdrv_mirror_top_refresh_limits, >> }; >> >> static BlockJob *mirror_start_job( >> @@ -1678,6 +1688,8 @@ static BlockJob *mirror_start_job( >> >> QTAILQ_INIT(&s->ops_in_flight); >> >> + bdrv_refresh_limits(mirror_top_bs, &error_abort); >> + >> trace_mirror_start(bs, s, opaque); >> job_start(&s->common.job); >> >> > > Am I right that the fact that no guest request will skip this limit is > guaranteed by > aio_context_acquire/release around blockdev_mirror_common?
Hm. As long as we don’t drain or release the context. Unfortunately, block_job_add_bdrv() does release the context (if the job runs in a foreign context). The reason we need this call is because the automatic calls invoke the function when either bs->opaque or bs->opaque->job are still NULL, or when bs->opaque->job->copy_mode is not yet set. So I could just call it when copy_mode has been set (which is still before the dirty bitmap is created; and if we have I/O before that point, we have bigger problems than this.) Max > Not sure how much it lowers performance, but it should work.. > Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]> >
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