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

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

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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