From: Christoph Hellwig <hch at lst.de>
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 at lst.de>
Signed-off-by: Rusty Russell <rusty at rustcorp.com.au>
---
drivers/block/virtio_blk.c | 1 -
1 file changed, 1 deletion(-)
Index: linux-2.6/drivers/block/virtio_blk.c
==================================================================---
linux-2.6.orig/drivers/block/virtio_blk.c 2009-09-04 17:33:48.802523987 -0300
+++ linux-2.6/drivers/block/virtio_blk.c 2009-09-04 17:33:56.186522158 -0300
@@ -314,7 +314,6 @@ static int __devinit virtblk_probe(struc
}
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);
