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

Nikolai K. Bochev n.bochev at grandstarco.com
Fri Aug 20 10:43:50 UTC 2010

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. 

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. 


Nikolai K. Bochev 
System Administrator 

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