Re: Shouldn't cache=none be the default for drives?

[Michael Tokarev <mjt@tls.msk.ru>]

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?

Thanks!

/mjt