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

Jamin W. Collins jcollins at asgardsrealm.net
Wed Aug 18 14:59:43 UTC 2010

On 08/18/2010 08:35 AM, Cole Robinson wrote:
> I really think this default should be changed at the qemu level. IIRC qemu
> devs were okay with the idea of an adaptive cache default depending on the
> underlying storage or image format. qemu/kvm devs are in the most
> knowledgeable position to decide these defaults, it shouldn't be pushed up to
> libvirt/virt-manager. And if the experts aren't willing to change the default
> for block devices, I don't know why virt-manager or libvirt should be expected
> to either.
> So I would recommend you take your case up with them.

I really don't understand this reasoning.  The "experts" are the ones 
making the recommendation that libvirt et al are currently ignoring. 
Read their website.  Caching should be disabled for raw volumes, it's as 
simple as that.  I'd even hazard to recommend that caching should be 
disabled completely unless explicitly selected, but that's only based on 
my personal experience thus far.  Disabling for raw volumes has been 
shown to improve performance significantly in numerous places (I 
reference a few in my bug reports).  I really don't get what appears to 
be a stubborn refusal to make a change that will provide a better 
end-user experience.

Jamin W. Collins

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