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

[Thomas Fjellstrom]

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