Since the Firefly release of Ceph in 2014, there has been the ability to create a RADOS pool using erasure coding. There is one major thing that you should be aware of: the erasure coding support in RADOS does not allow an object to be partially updated. You can write to an object in an erasure pool, read it back, and even overwrite it whole, but you cannot update a partial section of it. This means that erasure-coded pools can't be used for RBD and CephFS workloads and are limited to providing pure object storage either via the RADOS gateway or applications written to use librados.

The solution at the time was to use the cache tiering ability which was released around the same time, to act as a layer above an erasure-coded pool so that RBD could be used. In theory this was a great idea; in practice, performance was extremely poor. Every time an object was required to be written to, the whole object first had to be promoted into the cache tier. This act of promotion probably also meant that another object somewhere in the cache pool was evicted. Finally, the object now in the cache tier could be written to. This whole process of constantly reading and writing data between the two pools meant that performance was unacceptable, unless a very high percentage of the data was idle.

During the development cycle of the Kraken release, an initial implementation for support for direct overwrites on an erasure-coded pool was introduced. As of the final Kraken release, support is marked as experimental and is expected to be marked as stable in the following release. Testing of this feature will be covered later in this chapter.