Re: [PATCH for-2.6.31] virtio_blk: revert QUEUE_FLAG_VIRT addition

From: Jeff Moyer
Date: Fri Sep 04 2009 - 17:19:28 EST

Christoph Hellwig <hch@xxxxxx> writes:

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

I agree with that rationale.

Acked-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
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