Re: [Qemu-block] [PATCH] mirror: Only mirror granularity-aligned chunks

[Max Reitz]

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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