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Re: [libvirt] [PATCH] set cache=off on -disk line in qemu driver if <shareable/> (but not <readonly/>) is set.



On Mon, Nov 03, 2008 at 06:37:45PM +0200, Dor Laor wrote:
> Daniel Veillard wrote:
>> On Sat, Nov 01, 2008 at 06:20:27PM -0500, Charles Duffy wrote:
>>   
>>> As cache=off is necessary for clustering filesystems such as GFS (and 
>>>  such is the point of shareable, yes?), I believe this is correct  
>>> behavior.
>>>
>>> Comments?
>>>     
>>
>>   Yes that sounds right, if shared over network we should not cache
>> locally in the OS,
>>
>>     applied and commited,
>>
>>       thanks !
>>
>> Daniel
> cache=off should be the default case (or the similar O_DSYNC case).
> cache=on should only be used for temporal usage and not really  
> production data.
> It's just unsafe to use caching if you data is important to you.
> It's a hot issue in the qemu mailing list.

  My POV at the moment is that whatever safe default need to be should
be set in QEmu, and in libvirt we should allow to override the default.
I'm still waiting for the battle to end in QEmu, unfortunately I don't
really see a consensus, but I lost track a couple of weeks ago :-)
The patch looked okay because the fs is labelled as shared in the XML
config, so really that's user provided informations that a default
check in QEmu can't guess, hence overriding the default QEmu options
makes sense IMHO.

Daniel

-- 
Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel veillard com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/


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