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

Jamin W. Collins jcollins at asgardsrealm.net
Wed Aug 18 15:18:37 UTC 2010

On 08/18/2010 09:06 AM, Daniel P. Berrange wrote:
> It is not as clearcut as you make it out to be. If cache=none was always
> the best option,

Show me a single test case using raw volumes where it's not.  Every test 
I've done and those of others documented online indicate that disabling 
caching provides a better performance.  Qemu's own website indicates 
it'll provide a better performance.

 > 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.

This is a little more than a straw man (if at all).  QEMU/KVM provide 
little in the way of sensible defaults.  Upper level tools provide saner 
defaults all the time.  For example the guest VM templates used by 
virt-manager to default to different NIC or HD emulation or virtio.  The 
upper level tools provide alternate defaults that tend to provide a 
better end user experience through better performance.

> 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.

See above.  Is this not the list for virt-manager as well?  Isn't 
virt-manager already providing per guest defaults that differ from the 
"base layer".

Why insist on providing a sub-optimal end user experience?  Especially 
when the necessary change is known and relatively simple.

Jamin W. Collins

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