mirror of
https://github.com/ceph/ceph
synced 2024-12-23 20:03:56 +00:00
89 lines
3.1 KiB
ReStructuredText
89 lines
3.1 KiB
ReStructuredText
=======================
|
|
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``
|
|
|
|
.. _Block Device: ../../rbd/rbd/ |