Re: [patch]block: add plug for blkdev_issue_discard

[Jens Axboe <axboe@kernel.dk>]

On 2012-10-08 19:56, Mike Snitzer wrote:
> On Mon, Oct 8, 2012 at 9:11 PM, Shaohua Li <shli@kernel.org> wrote:
>> Now discard merge works, add plug for blkdev_issue_discard. This will help
>> discard request merge especially for raid0 case. In raid0, a big discard
>> request is split to small requests, and if correct plug is added, such small
>> requests can be merged in low layer.
>>
>> Signed-off-by: Shaohua Li <shli@fusionio.com>
>> ---
>>  block/blk-lib.c |    3 +++
>>  1 file changed, 3 insertions(+)
>>
>> Index: linux/block/blk-lib.c
>> ===================================================================
>> --- linux.orig/block/blk-lib.c  2012-09-28 15:09:38.655327319 +0800
>> +++ linux/block/blk-lib.c       2012-10-09 08:45:04.358598888 +0800
>> @@ -48,6 +48,7 @@ int blkdev_issue_discard(struct block_de
>>         struct bio_batch bb;
>>         struct bio *bio;
>>         int ret = 0;
>> +       struct blk_plug plug;
>>
>>         if (!q)
>>                 return -ENXIO;
>> @@ -81,6 +82,7 @@ int blkdev_issue_discard(struct block_de
>>         bb.flags = 1 << BIO_UPTODATE;
>>         bb.wait = &wait;
>>
>> +       blk_start_plug(&plug);
>>         while (nr_sects) {
>>                 unsigned int req_sects;
>>                 sector_t end_sect;
>> @@ -117,6 +119,7 @@ int blkdev_issue_discard(struct block_de
>>                 atomic_inc(&bb.done);
>>                 submit_bio(type, bio);
>>         }
>> +       blk_finish_plug(&plug);
>>
>>         /* Wait for bios in-flight */
>>         if (!atomic_dec_and_test(&bb.done))
> 
> That plug looks rather coarse-grained.  Any adverse affects when mkfs
> discards an entire (large) raid0 device?  (I suspect not but figured
> I'd ask)

Block core auto-unplugs after a certain depth is reached. So it's better
to make it coarse than try to second guess when to unplug.

-- 
Jens Axboe