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-=======================
- librbd Settings
-=======================
-
-See `Block Device`_ for additional details.
-
-Cache Settings
-=======================
-
-.. 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 accessing 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: ``true``
-
-
-``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: ``true``
-
-.. _Block Device: ../../rbd
-
-
-Read-ahead Settings
-=======================
-
-.. versionadded:: 0.86
-
-RBD supports read-ahead/prefetching to optimize small, sequential reads.
-This should normally be handled by the guest OS in the case of a VM,
-but boot loaders may not issue efficient reads.
-Read-ahead is automatically disabled if caching is disabled.
-
-
-``rbd readahead trigger requests``
-
-:Description: Number of sequential read requests necessary to trigger read-ahead.
-:Type: Integer
-:Required: No
-:Default: ``10``
-
-
-``rbd readahead max bytes``
-
-:Description: Maximum size of a read-ahead request. If zero, read-ahead is disabled.
-:Type: 64-bit Integer
-:Required: No
-:Default: ``512 KiB``
-
-
-``rbd readahead disable after bytes``
-
-:Description: After this many bytes have been read from an RBD image, read-ahead is disabled for that image until it is closed. This allows the guest OS to take over read-ahead once it is booted. If zero, read-ahead stays enabled.
-:Type: 64-bit Integer
-:Required: No
-:Default: ``50 MiB``