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authorChristoph Hellwig <hch@lst.de>2009-09-04 16:44:42 -0400
committerRusty Russell <rusty@rustcorp.com.au>2009-10-22 02:09:26 -0400
commitf8b12e513b953aebf30f8ff7d2de9be7e024dbbe (patch)
treeec261949b674283b8ba214fd2715f3a7674da11c /include/linux/virtio_blk.h
parent2fdc246aaf9a7fa088451ad2a72e9119b5f7f029 (diff)
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>
Diffstat (limited to 'include/linux/virtio_blk.h')
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