Re: [patch]block: add plug for blkdev_issue_discard

From: Mike Snitzer
Date: Mon Oct 08 2012 - 21:56:56 EST


On Mon, Oct 8, 2012 at 9:11 PM, Shaohua Li <shli@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> ---
> 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)
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