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Re: [Qemu-devel] Default cache mode


From: Alexander Graf
Subject: Re: [Qemu-devel] Default cache mode
Date: Wed, 29 Jun 2011 16:05:22 +0200

On 29.06.2011, at 15:57, Stefan Hajnoczi wrote:

> On Wed, Jun 29, 2011 at 1:21 PM, Alexander Graf <address@hidden> wrote:
>> On 29.06.2011, at 13:59, Kevin Wolf wrote:
>>> I'm not entirely sure if I should suggest writeback or none as the new
>>> default, but I think it could make sense to change it.
>> 
>> None as default would be a bad choice, as not all underlying file systems 
>> support it. Try cache=none on tmpfs or nfs. It will just fail on you.
>> What I'd personally like to see is some sort of detection that can determine 
>> which caching mode would perform best. So it would do cache=none when it 
>> makes sense (raw block device), otherwise cache=writeback.
> 
> O_DIRECT works on the Linux NFS client.  The performance currently
> isn't great with vectored I/O (which QEMU does) but there is work
> ongoing to improve it.

It does? One of the things I stumbled over when doing autotest was that 
cache=none was the default - and that simply failed on my nfs-root system.

> Your point makes sense though.  The default (no explicit cache=)
> should mean "the most appropriate caching mode".

Yeah, that's what defaults should be like in most cases. The same reason people 
are for going with -cpu host as default :).


Alex




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