This document is for a development version of Ceph.

Experimental Features

CephFS includes a number of experimental features which are not fully stabilized or qualified for users to turn on in real deployments. We generally do our best to clearly demarcate these and fence them off so they cannot be used by mistake.

Some of these features are closer to being done than others, though. We describe each of them with an approximation of how risky they are and briefly describe what is required to enable them. Note that doing so will irrevocably flag maps in the monitor as having once enabled this flag to improve debugging and support processes.

Inline data

By default, all CephFS file data is stored in RADOS objects. The inline data feature enables small files (generally <2KB) to be stored in the inode and served out of the MDS. This may improve small-file performance but increases load on the MDS. It is not sufficiently tested to support at this time, although failures within it are unlikely to make non-inlined data inaccessible

Inline data has always been off by default and requires setting the inline_data flag.

Inline data has been declared deprecated for the Octopus release, and will likely be removed altogether in the Q release.

Mantle: Programmable Metadata Load Balancer

Mantle is a programmable metadata balancer built into the MDS. The idea is to protect the mechanisms for balancing load (migration, replication, fragmentation) but stub out the balancing policies using Lua. For details, see Mantle.


LazyIO relaxes POSIX semantics. Buffered reads/writes are allowed even when a file is opened by multiple applications on multiple clients. Applications are responsible for managing cache coherency themselves.