Am Thu, 08 Apr 2010 06:09:05 +0000 schrieb Thomas Mueller: > Am Thu, 08 Apr 2010 10:05:09 +0400 schrieb Michael Tokarev: > >> 08.04.2010 09:07, Thomas Mueller wrote: [] >>> This helped alot: >>> >>> I enabled "deadline" block scheduler instead of the default "cfq" on >>> the host system. tested with: Host Debian with scheduler deadline, >>> Guest Win2008 with Virtio and cache=none. (26MB/s to 50MB/s boost >>> measured) Maybe this is also true for Linux/Linux. >>> >>> I expect that scheduler "noop" for linux guests would be good. >> >> Hmm. I wonder why it helped. In theory, host scheduler should not >> change anything for cache=none case, at least for raw partitions of LVM >> volumes. This is because with cache=none, the virtual disk image is >> opened with O_DIRECT flag, which means all I/O bypasses host scheduler >> and buffer cache. >> >> I tried a few quick tests here, -- with LVM volumes it makes no >> measurable difference. But if the guest disk images are on plain files >> (also raw), scheduler makes some difference, and indeed deadline works >> better. Maybe you were testing with plain files instead of block >> devices? > > ah yes, qcow2 images.
... but does the scheduler really now about O_DIRECT? isn't O_DIRECT meant to bypass only buffers (aka "return write not before it really hit the disk")? my understanding is that the scheduler is layer down the stack. but only guessing - i'm not a kernel hacker. :) - Thomas -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
