How to enable RBD caching in a Red Hat Ceph Storage cluster?

Solution Verified - Updated

Environment

  • Red Hat Ceph Storage 1.2.3
  • Red Hat Ceph Storage 1.3 and above

Issue

  • Does RBD support caching?

  • How can the caching in RBD enabled?

Resolution

  • RBD is unable to use the kernel page cache in linux, and so has an in-memory cache option which is disabled in Ceph Firefly and earlier.

  • To enable RBD cache, add the following under [client] section of /etc/ceph/ceph.conf, in the node using the RBD device.

rbd cache = true

  • This tunable will enable write-back cache by default, which will have writes acknowledged immediately unless there are more than 'rbd cache max dirty' unflushed bytes.

  • Some users may wish to only acknowledge writes when the data has been safely stored on the cluster; to do so the following should also be added to the ceph.conf. This will force caching to act as write-through instead of write-back.

rbd cache max dirty = 0
  • If using latest version of Ceph for example hammer release then new option rbd cache writethrough until flush can be used with value true then changing value of rbd cache max dirty to zero is not needed.
rbd cache writethrough until flush = true
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