Christoph Hellwig <hch@xxxxxx> writes: > 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. I agree with that rationale. Acked-by: Jeff Moyer <jmoyer@xxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
