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Re: [Qemu-devel] [PATCH] mirror: Only mirror granularity-aligned chunks
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH] mirror: Only mirror granularity-aligned chunks |
Date: |
Mon, 5 Aug 2019 15:49:26 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
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 <address@hidden>
>> ---
>> 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 <address@hidden>
>
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