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

On 08/18/2010 09:23 AM, Daniel P. Berrange wrote:

Previously when anyone has suggested setting cache=none for QEMU,
other people have appeared with test results showing that other

Care to provide references? I've yet to find a single report where caching provides better performance with VMs stored on raw volumes except for the use of writeback which is not recommended for anything where data integrity is concerned. My suggested change improves performance without risking data integrity.

cache settings perform better for their workloads. There is no
perfect default setting for everyone. Changing the defaults just
shifts the non-optimal performance problem from one set  people
to a different set of people.

Again, please provide references.

Here is a quick (incomplete) list of references where changing the cache setting to "none" with raw volumes has increased the performance significantly. As far as I can tell there have been no responses indicating that someone has actually performed and documented tests to the contrary:

http://www.mail-archive.com/kvm vger kernel org/msg30425.html

Here are general recommendations against the use of caching with virtualization:

If you think QEMU's defaults are wrong then raise the question of
best default cache settings on QEMU lists again. Maybe you'll find
everyone agrees this time around, but there hasn't been agreement on
previous occassions.

QEMU's defaults aren't best in other areas either, but that hasn't stopped virt-manager from providing different defaults. The default NIC unless one is specified is e1000 (based on the current man page information). Yet, virt-manager (via virtinst) overrides this and explicitly sets the NIC based on guest specification. The same is true for the guest HD.

Jamin W. Collins

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