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

Jamin W. Collins jcollins at asgardsrealm.net
Wed Aug 18 15:50:23 UTC 2010

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:


Here are general recommendations against the use of caching with 

> 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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