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From: Virtbie <virtbie@shiftmail.org>
Subject: Re: Virtual drives performance
Date: Wed, 07 Sep 2011 16:38:44 +0200
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On 09/07/11 00:25, TooMeeK wrote:
>
> Next, I've tried following combinations with virt-manager 0.8.4 (from
> XML of VM):
> 1.on Debian VM with virtio drivers for both storage and NIC:
> <disk type='block' device='disk'>
> <source dev='/dev/md3'/>
> <target dev='vdb' bus='virtio'/>
> partition type used in guest: EXT4
> result: poor performance, 9-10MB/s sequentional copy via SMB
> 2.on Debian VM with virtio drivers:
> <disk type='block' device='disk' cache='writeback'>
> <source dev='/dev/md3'/>
> <target dev='vdb' bus='virtio'/>
> partition type used: EXT4
> result: poor performance, 10-15MB/s sequentional copy via SMB
> 3.Direct attached partition to FreeBSD VM without virtio support
> (e1000 NIC and SCSI disk):
> <disk type='block' device='disk' cache='writeback'>

Shouldn't have you used cache=none when using virtio?
http://www.linux-kvm.org/page/Tuning_KVM
how is that performance with cache none?

Also note that when you don't specify it, I think the default is not 
none. Maybe it is writethrough, I don't remember.