Handling a full Ceph file system
================================

When a RADOS cluster reaches its ``mon_osd_full_ratio`` (default
95%) capacity, it is marked with the OSD full flag.  This flag causes
most normal RADOS clients to pause all operations until it is resolved
(for example by adding more capacity to the cluster).

The file system has some special handling of the full flag, explained below.

Hammer and later
----------------

Since the hammer release, a full file system will lead to ENOSPC
results from:

 * Data writes on the client
 * Metadata operations other than deletes and truncates

Because the full condition may not be encountered until
data is flushed to disk (sometime after a ``write`` call has already
returned 0), the ENOSPC error may not be seen until the application
calls ``fsync`` or ``fclose`` (or equivalent) on the file handle.

Calling ``fsync`` is guaranteed to reliably indicate whether the data
made it to disk, and will return an error if it doesn't.  ``fclose`` will
only return an error if buffered data happened to be flushed since
the last write -- a successful ``fclose`` does not guarantee that the
data made it to disk, and in a full-space situation, buffered data
may be discarded after an ``fclose`` if no space is available to persist it.

.. warning::
    If an application appears to be misbehaving on a full file system,
    check that it is performing ``fsync()`` calls as necessary to ensure
    data is on disk before proceeding.

Data writes may be cancelled by the client if they are in flight at the
time the OSD full flag is sent.  Clients update the ``osd_epoch_barrier``
when releasing capabilities on files affected by cancelled operations, in
order to ensure that these cancelled operations do not interfere with
subsequent access to the data objects by the MDS or other clients.  For
more on the epoch barrier mechanism, see :ref:`background_blacklisting_and_osd_epoch_barrier`.

Legacy (pre-hammer) behavior
----------------------------

In versions of Ceph earlier than hammer, the MDS would ignore
the full status of the RADOS cluster, and any data writes from
clients would stall until the cluster ceased to be full.

There are two dangerous conditions to watch for with this behaviour:

* If a client had pending writes to a file, then it was not possible
  for the client to release the file to the MDS for deletion: this could
  lead to difficulty clearing space on a full file system
* If clients continued to create a large number of empty files, the
  resulting metadata writes from the MDS could lead to total exhaustion
  of space on the OSDs such that no further deletions could be performed.