Comment 0 for bug 1412856

Ceph v0.87 Giant release or later enables "rbd cache" and "rbd cache writethrough until flush" by default[1].
In Ubuntu repositories, vivid+ or cloud-archive:kilo+.

It would be nice if nova-compute charm has a config to enable "rbd cache" and "rbd cache writethrough until flush" as an option. Regarding rbd cache settings, please refer [2].

[1] http://ceph.com/docs/master/release-notes/#upgrading-from-v0-80x-firefly
> The client-side caching for librbd is now enabled by default (rbd
> cache = true). A safety option (rbd cache writethrough until flush =
> true) is also enabled so that writeback caching is not used until the
> library observes a ‘flush’ command, indicating that the librbd
> users is passing that operation through from the guest VM. This
> avoids potential data loss when used with older versions of qemu that
> do not support flush.

[2] http://ceph.com/docs/master/rbd/rbd-config-ref/
> The user space implementation of the Ceph block device (i.e., librbd)
> cannot take advantage of the Linux page cache, so it includes its own
> in-memory caching, called “RBD caching.” RBD caching behaves just
> like well-behaved hard disk caching.