By default, OpenStack doesn’t use any caching. However, you might want to enable the RBD caching.

As you may recall, the current implementation of the RBD caching is in-memory caching solution. Although, at the last Ceph Developer Summit (last week), was discussed the refactoring of the current implementation was discussed in order to bring a better caching solution. For more information, please refer to the official blueprint here.

I. Ceph configuration

Edit your ceph.conf with the following:

[client]
    rbd cache = true
    rbd cache writethrough until flush = true
    admin socket = /var/run/ceph/rbd-client-$pid.asok

R Note: the socket will only appear during an operation (something that is requesting the librbd cache for instance, can be a virtual machine).

Available flags:

rbd cache size =
rbd cache max dirty =
rbd cache target dirty =
rbd cache max dirty age =

For more detailed information check the official documentation.

II. Nova configuration

Edit your nova.conf with:

disk_cachemodes="network=writeback"

Several options can be used depending on the disk type:

• file
• block
• network
• mount

Caching methods available:

• none,
• writethrough,
• writeback,
• directsync,
• writethrough,
• unsafe,

That’s all for now. Please note that things changed in the version of libvirt above 1.2