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.. _erasure-code-profiles:
=====================
Erasure code profiles
=====================
Erasure code is defined by a **profile** and is used when creating an
erasure coded pool and the associated CRUSH rule.
The **default** erasure code profile (which is created when the Ceph
cluster is initialized) will split the data into 2 equal-sized chunks,
and have 2 parity chunks of the same size. It will take as much space
in the cluster as a 2-replica pool but can sustain the data loss of 2
chunks out of 4. It is described as a profile with **k=2** and **m=2**,
meaning the information is spread over four OSD (k+m == 4) and two of
them can be lost.
To improve redundancy without increasing raw storage requirements, a
new profile can be created. For instance, a profile with **k=10** and
**m=4** can sustain the loss of four (**m=4**) OSDs by distributing an
object on fourteen (k+m=14) OSDs. The object is first divided in
**10** chunks (if the object is 10MB, each chunk is 1MB) and **4**
coding chunks are computed, for recovery (each coding chunk has the
same size as the data chunk, i.e. 1MB). The raw space overhead is only
40% and the object will not be lost even if four OSDs break at the
same time.
.. _list of available plugins:
.. toctree::
:maxdepth: 1
erasure-code-jerasure
erasure-code-isa
erasure-code-lrc
erasure-code-shec
erasure-code-clay
osd erasure-code-profile set
============================
To create a new erasure code profile::
ceph osd erasure-code-profile set {name} \
[{directory=directory}] \
[{plugin=plugin}] \
[{stripe_unit=stripe_unit}] \
[{key=value} ...] \
[--force]
Where:
``{directory=directory}``
:Description: Set the **directory** name from which the erasure code
plugin is loaded.
:Type: String
:Required: No.
:Default: /usr/lib/ceph/erasure-code
``{plugin=plugin}``
:Description: Use the erasure code **plugin** to compute coding chunks
and recover missing chunks. See the `list of available
plugins`_ for more information.
:Type: String
:Required: No.
:Default: jerasure
``{stripe_unit=stripe_unit}``
:Description: The amount of data in a data chunk, per stripe. For
example, a profile with 2 data chunks and stripe_unit=4K
would put the range 0-4K in chunk 0, 4K-8K in chunk 1,
then 8K-12K in chunk 0 again. This should be a multiple
of 4K for best performance. The default value is taken
from the monitor config option
``osd_pool_erasure_code_stripe_unit`` when a pool is
created. The stripe_width of a pool using this profile
will be the number of data chunks multiplied by this
stripe_unit.
:Type: String
:Required: No.
``{key=value}``
:Description: The semantic of the remaining key/value pairs is defined
by the erasure code plugin.
:Type: String
:Required: No.
``--force``
:Description: Override an existing profile by the same name, and allow
setting a non-4K-aligned stripe_unit.
:Type: String
:Required: No.
osd erasure-code-profile rm
============================
To remove an erasure code profile::
ceph osd erasure-code-profile rm {name}
If the profile is referenced by a pool, the deletion will fail.
osd erasure-code-profile get
============================
To display an erasure code profile::
ceph osd erasure-code-profile get {name}
osd erasure-code-profile ls
===========================
To list the names of all erasure code profiles::
ceph osd erasure-code-profile ls
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