mirror of https://github.com/ceph/ceph
143 lines
3.9 KiB
ReStructuredText
143 lines
3.9 KiB
ReStructuredText
========================
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SHEC erasure code plugin
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========================
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The *shec* plugin encapsulates the `multiple SHEC
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<http://tracker.ceph.com/projects/ceph/wiki/Shingled_Erasure_Code_(SHEC)>`_
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library. It allows ceph to recover data more efficiently than Reed Solomon codes.
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Create an SHEC profile
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======================
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To create a new *shec* erasure code profile::
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ceph osd erasure-code-profile set {name} \
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plugin=shec \
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[k={data-chunks}] \
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[m={coding-chunks}] \
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[c={durability-estimator}] \
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[crush-root={root}] \
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[crush-failure-domain={bucket-type}] \
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[crush-device-class={device-class}] \
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[directory={directory}] \
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[--force]
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Where:
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``k={data-chunks}``
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:Description: Each object is split in **data-chunks** parts,
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each stored on a different OSD.
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:Type: Integer
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:Required: No.
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:Default: 4
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``m={coding-chunks}``
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:Description: Compute **coding-chunks** for each object and store them on
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different OSDs. The number of **coding-chunks** does not necessarily
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equal the number of OSDs that can be down without losing data.
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:Type: Integer
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:Required: No.
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:Default: 3
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``c={durability-estimator}``
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:Description: The number of parity chunks each of which includes each data chunk in its
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calculation range. The number is used as a **durability estimator**.
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For instance, if c=2, 2 OSDs can be down without losing data.
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:Type: Integer
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:Required: No.
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:Default: 2
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``crush-root={root}``
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:Description: The name of the crush bucket used for the first step of
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the CRUSH rule. For instance **step take default**.
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:Type: String
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:Required: No.
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:Default: default
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``crush-failure-domain={bucket-type}``
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:Description: Ensure that no two chunks are in a bucket with the same
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failure domain. For instance, if the failure domain is
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**host** no two chunks will be stored on the same
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host. It is used to create a CRUSH rule step such as **step
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chooseleaf host**.
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:Type: String
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:Required: No.
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:Default: host
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``crush-device-class={device-class}``
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:Description: Restrict placement to devices of a specific class (e.g.,
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``ssd`` or ``hdd``), using the crush device class names
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in the CRUSH map.
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:Type: String
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:Required: No.
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:Default:
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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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``--force``
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:Description: Override an existing profile by the same name.
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:Type: String
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:Required: No.
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Brief description of SHEC's layouts
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===================================
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Space Efficiency
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----------------
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Space efficiency is a ratio of data chunks to all ones in a object and
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represented as k/(k+m).
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In order to improve space efficiency, you should increase k or decrease m:
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space efficiency of SHEC(4,3,2) = :math:`\frac{4}{4+3}` = 0.57
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SHEC(5,3,2) or SHEC(4,2,2) improves SHEC(4,3,2)'s space efficiency
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Durability
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----------
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The third parameter of SHEC (=c) is a durability estimator, which approximates
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the number of OSDs that can be down without losing data.
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``durability estimator of SHEC(4,3,2) = 2``
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Recovery Efficiency
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-------------------
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Describing calculation of recovery efficiency is beyond the scope of this document,
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but at least increasing m without increasing c achieves improvement of recovery efficiency.
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(However, we must pay attention to the sacrifice of space efficiency in this case.)
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``SHEC(4,2,2) -> SHEC(4,3,2) : achieves improvement of recovery efficiency``
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Erasure code profile examples
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=============================
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::
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$ ceph osd erasure-code-profile set SHECprofile \
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plugin=shec \
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k=8 m=4 c=3 \
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crush-failure-domain=host
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$ ceph osd pool create shecpool erasure SHECprofile
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