ceph/doc/rbd/rbd-config-ref.rst

=======================
 librbd Cache Settings
=======================

See `Block Device`_ for additional details.

.. sidebar:: Kernel Caching

	The kernel driver for Ceph block devices can use the Linux page cache to 
	improve performance.

The user space implementation of the Ceph block device (i.e., ``librbd``) cannot
take advantage of the Linux page cache, so it includes its own in-memory
caching, called "RBD caching." RBD caching behaves just like well-behaved hard
disk caching.  When the OS sends a barrier or a flush request, all dirty data is
written to the OSDs. This means that using write-back caching is just as safe as
using a well-behaved physical hard disk with a VM that properly sends flushes
(i.e. Linux kernel >= 2.6.32). The cache uses a Least Recently Used (LRU)
algorithm, and in write-back mode it  can coalesce contiguous requests for
better throughput.

.. versionadded:: 0.46

Ceph supports write-back caching for RBD. To enable it, add  ``rbd cache =
true`` to the ``[client]`` section of your ``ceph.conf`` file. By default
``librbd`` does not perform any caching. Writes and reads go directly to the
storage cluster, and writes return only when the data is on disk on all
replicas. With caching enabled, writes return immediately, unless there are more
than ``rbd cache max dirty`` unflushed bytes. In this case, the write triggers
writeback and blocks until enough bytes are flushed.

.. versionadded:: 0.47

Ceph supports write-through caching for RBD. You can set the size of
the cache, and you can set targets and limits to switch from
write-back caching to write through caching. To enable write-through
mode, set ``rbd cache max dirty`` to 0. This means writes return only
when the data is on disk on all replicas, but reads may come from the
cache. The cache is in memory on the client, and each RBD image has
its own.  Since the cache is local to the client, there's no coherency
if there are others accesing the image. Running GFS or OCFS on top of
RBD will not work with caching enabled.

The ``ceph.conf`` file settings for RBD should be set in the ``[client]``
section of your configuration file. The settings include: 


``rbd cache``

:Description: Enable caching for RADOS Block Device (RBD).
:Type: Boolean
:Required: No
:Default: ``false``


``rbd cache size``

:Description: The RBD cache size in bytes.
:Type: 64-bit Integer
:Required: No
:Default: ``32 MiB``


``rbd cache max dirty``

:Description: The ``dirty`` limit in bytes at which the cache triggers write-back.  If ``0``, uses write-through caching.
:Type: 64-bit Integer
:Required: No
:Constraint: Must be less than ``rbd cache size``.
:Default: ``24 MiB``


``rbd cache target dirty``

:Description: The ``dirty target`` before the cache begins writing data to the data storage. Does not block writes to the cache.
:Type: 64-bit Integer
:Required: No
:Constraint: Must be less than ``rbd cache max dirty``.
:Default: ``16 MiB``


``rbd cache max dirty age``

:Description: The number of seconds dirty data is in the cache before writeback starts. 
:Type: Float
:Required: No
:Default: ``1.0``

.. versionadded:: 0.60

``rbd cache writethrough until flush``

:Description: Start out in write-through mode, and switch to write-back after the first flush request is received. Enabling this is a conservative but safe setting in case VMs running on rbd are too old to send flushes, like the virtio driver in Linux before 2.6.32.
:Type: Boolean
:Required: No
:Default: ``false``

.. _Block Device: ../../rbd/rbd/