Re: virtio disk slower than IDE?

[Dor Laor <dlaor@redhat.com>]

On 11/16/2009 08:11 PM, Charles Duffy wrote:
> Gordan Bobic wrote:
>>> Lastly, do you use cache=wb on qemu? it's just a fun mode, we use
>>> cache=off only.
>>
>> I don't see the option being set in the logs, so I'd guess it's
>> whatever qemu-kvm defaults to.
>
> You can set this through libvirt by putting an element such as the
> following within your <disk> element:
>
> <driver name='qemu' type='qcow2' cache='none'/>

It's not needed on rhel5.4 qemu - we have cache=none as a default

>
> (Setting the type is preferred to avoid security issues wherein a guest
> writes an arbitrary qcow2 header to the beginning of a raw disk, reboots
> and allows qemu's autodetection to decide that this formerly-raw disk
> should now be treated as a delta against a file they otherwise might not
> have access to read; as such, it's particularly important if you intend
> that the type be raw).
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