The following commit has been merged in the linux branch: commit f8b12e513b953aebf30f8ff7d2de9be7e024dbbe Author: Christoph Hellwig hch@lst.de Date: Fri Sep 4 22:44:42 2009 +0200
virtio_blk: revert QUEUE_FLAG_VIRT addition
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
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c index 43f1938..348befa 100644 --- a/drivers/block/virtio_blk.c +++ b/drivers/block/virtio_blk.c @@ -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);