mirror of
https://github.com/ceph/ceph
synced 2024-12-20 18:33:44 +00:00
dc7a2aaf7a
1) ruleset is an obsolete term, and
2) crush-{rule,failure-domain,...} is more descriptive.
Note that we are changing the names of the erasure code profile keys
from ruleset-* to crush-*. We will update this on upgrade when the
luminous flag is set, but that means that during mon upgrade you cannot
create EC pools that use these fields.
When the upgrade completes (users sets require_osd_release = luminous)
existing ec profiles are updated automatically.
Signed-off-by: Sage Weil <sage@redhat.com>
145 lines
3.9 KiB
ReStructuredText
145 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 ruleset. For intance **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 ruleset 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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::
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space efficiency of SHEC(4,3,2) = 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 256 256 erasure SHECprofile
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