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

[Daniel P. Berrange]

On Wed, Aug 18, 2010 at 08:59:43AM -0600, Jamin W. Collins wrote:
> 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.

It is not as clearcut as you make it out to be. If cache=none was always
the best option, then QEMU/KVM would have that as their defaults. If there
is a reason for QEMU not to have this as the default, this applies equally
to libvirt, hence why libvirt does not override the default for this. The
default should be chosen at either the base layer implementing the feature,
or by the apps using the feature. libvirt is simply an API for accessing
the features, not a place to enforce policy decisions.

Daniel
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