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

[Thomas Mueller <thomas@chaschperli.ch>]

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. 

- Thomas