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xen/blkfront: use tagged queuing for barriers 

When barriers are supported, then use QUEUE_ORDERED_TAG to tell the block
subsystem that it doesn't need to do anything else with the barriers.
Previously we used ORDERED_DRAIN which caused the block subsystem to
drain all pending IO before submitting the barrier, which would be
very expensive.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
This commit is contained in:
Jeremy Fitzhardinge 2010-07-22 14:17:00 -07:00 committed by Jens Axboe
parent e96f6abe02
commit 4dab46ff26
1 changed files with 13 additions and 4 deletions
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17
drivers/block/xen-blkfront.c
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@ -420,9 +420,19 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 sector_size)
static int xlvbd_barrier(struct blkfront_info *info)
{
int err;
unsigned ordered = QUEUE_ORDERED_NONE;
err = blk_queue_ordered(info->rq,
info->feature_barrier ? QUEUE_ORDERED_DRAIN : QUEUE_ORDERED_NONE);
/*
* If we don't have barrier support, then there's really no
* way to guarantee write ordering, so we really just have to
* send writes to the backend and hope for the best. If
* barriers are supported then we can treat them as proper
* ordering tags.
*/
if (info->feature_barrier)
ordered = QUEUE_ORDERED_TAG;
err = blk_queue_ordered(info->rq, ordered);
if (err)
return err;
@ -509,8 +519,7 @@ static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
info->rq = gd->queue;
info->gd = gd;
if (info->feature_barrier)
xlvbd_barrier(info);
xlvbd_barrier(info);
if (vdisk_info & VDISK_READONLY)
set_disk_ro(gd, 1);