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

Cole Robinson crobinso at redhat.com
Wed Aug 18 14:35:46 UTC 2010

On 08/17/2010 09:22 AM, Jamin W. Collins wrote:
> On 08/17/2010 07:17 AM, Nikolai K. Bochev wrote:
>> Well i just tried this ( i never really did read the documents on that
>> part ) and amazingly my loadavg on the kvm host dropped by a factor of
>> 10.
>> All the load was coming out of a heavily loaded VM with pgsql in it that
>> was writing all the time ( zabbix monitoring etc. ).
>> I guess this should really be set as default, not sure if it's up to
>> virt-manager or libvirt.
> This is precisely my point.  With defaults the way they are right now, I
> suspect quite a few people will get the wrong impression of KVM's
> performance.  I've even found a significant performance increase with
> file backed VMs by disabling caching.

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.


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