[libvirt-users] [ceph-users] Setting RBD cache parameters for libvirt+qemu

[Josh Durgin]

On 06/07/2013 02:41 PM, John Nielsen wrote:
> I am running some qemu-kvm virtual machines via libvirt using Ceph RBD as the back-end storage. Today I was testing an update to libvirt-1.0.6 on one of my hosts and discovered that it includes this change:
> 	[libvirt] [PATCH] Forbid use of ':' in RBD pool names
> 		...People are known to be abusing the lack of escaping in current libvirt to pass arbitrary args to QEMU.
>
> I am one of those people--I have been storing colon-separated arguments in my disk source 'name' attribute to control the RBD cache settings on a per-volume basis. Under libvirt-1.0.6 such VM's refuse to start:
> error: Failed to start domain foo
> error: unsupported configuration: ':' not allowed in RBD source volume name 'mypool/foo:rbd_cache_size=268435456:rbd_cache_max_dirty=134217728:rbd_cache_target_dirty=33554432:rbd_cache_max_dirty_age=5'
>
> So my question is how do I control the cache settings now?

You can set the cache mode with the cache attribute on the <driver>
element inside a <disk> [1]. This works for rbd with QEMU 1.2 or later.

QEMU will try to read /etc/ceph/ceph.conf by default as well, so you
can put more specific settings in a [global] or [client.foo] section
there. Note that if you enable writeback caching here, you need to
include cache='writeback' in the xml as well, or qemu will not send
flushes.

Josh

[1] http://libvirt.org/formatdomain.html#elementsDisks