[virt-tools-list] Default use of caching on raw volumes

Thomas Fjellstrom tfjellstrom at shaw.ca
Fri Aug 20 12:11:54 UTC 2010

On August 20, 2010, Nikolai K. Bochev wrote:
> Could you try :
> 1. Share a folder from winxp
> 2. From another box start copying a large file to the winxp share
> 3. Observe loadavg on the kvm host
> Both with default settings and cache=none
> I was getting loadavg ~35 ( on a 2xXeon E5520 ) with the default setting
> and read/write speeds around 20Mb/s . With cache=none the loadavg dropped
> to ~3 and the speeds increased to 40Mb/s.
> The host is ubuntu 10.04.
> The guest is ubuntu 10.04 with raw image as disk.

I've since done some more testing, and while read access did seem to be 
better, write access is absolutely horrible with the defaults. with cache=none 
I at least get 40-50MB/s write, as well as read. so I'm happy enough.

> From: "Thomas Fjellstrom" <tfjellstrom at shaw.ca>
> To: virt-tools-list at redhat.com
> Sent: Friday, August 20, 2010 12:00:41 PM
> Subject: Re: [virt-tools-list] Default use of caching on raw volumes
> After seeing this thread I tried to see what would happen with a windows xp
> guest using lvm+virtio and changing the cache option to none. Performance
> drops about 10-20MB/s. Whatever the default is, I get an average of 55MB/s
> read according to hd-tune. With cache=none I get more like 35-40MB/s.

Thomas Fjellstrom
tfjellstrom at shaw.ca

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