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Since kraken, Ceph enforces a 1:1 correspondence between CRUSH ruleset and CRUSH rule, so effectively ruleset and rule are the same thing, although the term "ruleset" still survives - notably in the CRUSH rule itself, where it effectively denotes the number of the rule. This commit updates the documentation to more faithfully reflect the current state of the code. Fixes: http://tracker.ceph.com/issues/20559 Signed-off-by: Nathan Cutler <ncutler@suse.com>
124 lines
3.4 KiB
ReStructuredText
124 lines
3.4 KiB
ReStructuredText
.. _erasure-code-profiles:
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=====================
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Erasure code profiles
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=====================
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Erasure code is defined by a **profile** and is used when creating an
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erasure coded pool and the associated CRUSH rule.
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The **default** erasure code profile (which is created when the Ceph
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cluster is initialized) provides the same level of redundancy as two
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copies but requires 25% less disk space. It is described as a profile
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with **k=2** and **m=1**, meaning the information is spread over three
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OSD (k+m == 3) and one of them can be lost.
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To improve redundancy without increasing raw storage requirements, a
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new profile can be created. For instance, a profile with **k=10** and
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**m=4** can sustain the loss of four (**m=4**) OSDs by distributing an
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object on fourteen (k+m=14) OSDs. The object is first divided in
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**10** chunks (if the object is 10MB, each chunk is 1MB) and **4**
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coding chunks are computed, for recovery (each coding chunk has the
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same size as the data chunk, i.e. 1MB). The raw space overhead is only
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40% and the object will not be lost even if four OSDs break at the
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same time.
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.. _list of available plugins:
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.. toctree::
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:maxdepth: 1
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erasure-code-jerasure
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erasure-code-isa
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erasure-code-lrc
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erasure-code-shec
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osd erasure-code-profile set
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============================
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To create a new erasure code profile::
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ceph osd erasure-code-profile set {name} \
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[{directory=directory}] \
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[{plugin=plugin}] \
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[{stripe_unit=stripe_unit}] \
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[{key=value} ...] \
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[--force]
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Where:
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``{directory=directory}``
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:Description: Set the **directory** name from which the erasure code
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plugin is loaded.
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:Type: String
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:Required: No.
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:Default: /usr/lib/ceph/erasure-code
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``{plugin=plugin}``
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:Description: Use the erasure code **plugin** to compute coding chunks
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and recover missing chunks. See the `list of available
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plugins`_ for more information.
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:Type: String
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:Required: No.
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:Default: jerasure
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``{stripe_unit=stripe_unit}``
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:Description: The amount of data in a data chunk, per stripe. For
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example, a profile with 2 data chunks and stripe_unit=4K
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would put the range 0-4K in chunk 0, 4K-8K in chunk 1,
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then 8K-12K in chunk 0 again. This should be a multiple
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of 4K for best performance. The default value is taken
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from the monitor config option
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``osd_pool_erasure_code_stripe_unit`` when a pool is
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created. The stripe_width of a pool using this profile
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will be the number of data chunks multiplied by this
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stripe_unit.
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:Type: String
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:Required: No.
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``{key=value}``
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:Description: The semantic of the remaining key/value pairs is defined
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by the erasure code plugin.
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:Type: String
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:Required: No.
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``--force``
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:Description: Override an existing profile by the same name, and allow
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setting a non-4K-aligned stripe_unit.
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:Type: String
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:Required: No.
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osd erasure-code-profile rm
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============================
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To remove an erasure code profile::
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ceph osd erasure-code-profile rm {name}
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If the profile is referenced by a pool, the deletion will fail.
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osd erasure-code-profile get
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============================
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To display an erasure code profile::
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ceph osd erasure-code-profile get {name}
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osd erasure-code-profile ls
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===========================
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To list the names of all erasure code profiles::
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ceph osd erasure-code-profile ls
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