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89 lines
3.1 KiB
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=======================
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librbd Cache Settings
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=======================
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See `Block Device`_ for additional details.
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.. sidebar:: Kernel Caching
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The kernel driver for Ceph block devices can use the Linux page cache to
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improve performance.
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The user space implementation of the Ceph block device (i.e., ``librbd``) cannot
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take advantage of the Linux page cache, so it includes its own in-memory
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caching, called "RBD caching." RBD caching behaves just like well-behaved hard
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disk caching. When the OS sends a barrier or a flush request, all dirty data is
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written to the OSDs. This means that using write-back caching is just as safe as
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using a well-behaved physical hard disk with a VM that properly sends flushes
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(i.e. Linux kernel >= 2.6.32). The cache uses a Least Recently Used (LRU)
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algorithm, and in write-back mode it can coalesce contiguous requests for
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better throughput.
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.. versionadded:: 0.46
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Ceph supports write-back caching for RBD. To enable it, add ``rbd cache =
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true`` to the ``[client]`` section of your ``ceph.conf`` file. By default
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``librbd`` does not perform any caching. Writes and reads go directly to the
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storage cluster, and writes return only when the data is on disk on all
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replicas. With caching enabled, writes return immediately, unless there are more
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than ``rbd cache max dirty`` unflushed bytes. In this case, the write triggers
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writeback and blocks until enough bytes are flushed.
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.. versionadded:: 0.47
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Ceph supports write-through caching for RBD. You can set the size of
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the cache, and you can set targets and limits to switch from
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write-back caching to write through caching. To enable write-through
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mode, set ``rbd cache max dirty`` to 0. This means writes return only
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when the data is on disk on all replicas, but reads may come from the
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cache. The cache is in memory on the client, and each RBD image has
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its own. Since the cache is local to the client, there's no coherency
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if there are others accesing the image. Running GFS or OCFS on top of
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RBD will not work with caching enabled.
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The ``ceph.conf`` file settings for RBD should be set in the ``[client]``
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section of your configuration file. The settings include:
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``rbd cache``
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:Description: Enable caching for RADOS Block Device (RBD).
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:Type: Boolean
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:Required: No
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:Default: ``false``
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``rbd cache size``
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:Description: The RBD cache size in bytes.
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:Type: 64-bit Integer
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:Required: No
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:Default: ``32 MiB``
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``rbd cache max dirty``
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:Description: The ``dirty`` limit in bytes at which the cache triggers write-back. If ``0``, uses write-through caching.
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:Type: 64-bit Integer
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:Required: No
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:Constraint: Must be less than ``rbd cache size``.
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:Default: ``24 MiB``
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``rbd cache target dirty``
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:Description: The ``dirty target`` before the cache begins writing data to the data storage. Does not block writes to the cache.
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:Type: 64-bit Integer
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:Required: No
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:Constraint: Must be less than ``rbd cache max dirty``.
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:Default: ``16 MiB``
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``rbd cache max dirty age``
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:Description: The number of seconds dirty data is in the cache before writeback starts.
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:Type: Float
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:Required: No
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:Default: ``1.0``
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.. _Block Device: ../../rbd/rbd/ |