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virtio_blk: revert QUEUE_FLAG_VIRT addition

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It seems like the addition of QUEUE_FLAG_VIRT caueses major performance
regressions for Fedora users:

	https://bugzilla.redhat.com/show_bug.cgi?id=509383
	https://bugzilla.redhat.com/show_bug.cgi?id=505695

while I can't reproduce those extreme regressions myself I think the flag
is wrong.

Rationale:

  QUEUE_FLAG_VIRT expands to QUEUE_FLAG_NONROT which casus the queue
  unplugged immediately.  This is not a good behaviour for at least
  qemu and kvm where we do have significant overhead for every
  I/O operations.  Even with all the latested speeups (native AIO,
  MSI support, zero copy) we can only get native speed for up to 128kb
  I/O requests we already are down to 66% of native performance for 4kb
  requests even on my laptop running the Intel X25-M SSD for which the
  QUEUE_FLAG_NONROT was designed.
  If we ever get virtio-blk overhead low enough that this flag makes
  sense it should only be set based on a feature flag set by the host.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Christoph Hellwig 2009-09-04 22:44:42 +02:00 committed by Rusty Russell
parent 2fdc246aaf
commit f8b12e513b
1 changed files with 0 additions and 1 deletions
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drivers/block/virtio_blk.c
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@ -332,7 +332,6 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
}
vblk->disk->queue->queuedata = vblk;
queue_flag_set_unlocked(QUEUE_FLAG_VIRT, vblk->disk->queue);
if (index < 26) {
sprintf(vblk->disk->disk_name, "vd%c", 'a' + index % 26);