Adding upstream version 14.2.21.upstream/14.2.21upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+========================
+SHEC erasure code plugin
+========================
+
+The *shec* plugin encapsulates the `multiple SHEC
+<http://tracker.ceph.com/projects/ceph/wiki/Shingled_Erasure_Code_(SHEC)>`_
+library. It allows ceph to recover data more efficiently than Reed Solomon codes.
+
+Create an SHEC profile
+======================
+
+To create a new *shec* erasure code profile::
+
+        ceph osd erasure-code-profile set {name} \
+             plugin=shec \
+             [k={data-chunks}] \
+             [m={coding-chunks}] \
+             [c={durability-estimator}] \
+             [crush-root={root}] \
+             [crush-failure-domain={bucket-type}] \
+             [crush-device-class={device-class}] \
+             [directory={directory}] \
+             [--force]
+
+Where:
+
+``k={data-chunks}``
+
+:Description: Each object is split in **data-chunks** parts,
+              each stored on a different OSD.
+
+:Type: Integer
+:Required: No.
+:Default: 4
+
+``m={coding-chunks}``
+
+:Description: Compute **coding-chunks** for each object and store them on
+              different OSDs. The number of **coding-chunks** does not necessarily
+              equal the number of OSDs that can be down without losing data.
+
+:Type: Integer
+:Required: No.
+:Default: 3
+
+``c={durability-estimator}``
+
+:Description: The number of parity chunks each of which includes each data chunk in its
+              calculation range. The number is used as a **durability estimator**.
+              For instance, if c=2, 2 OSDs can be down without losing data.
+
+:Type: Integer
+:Required: No.
+:Default: 2
+
+``crush-root={root}``
+
+:Description: The name of the crush bucket used for the first step of
+              the CRUSH rule. For instance **step take default**.
+
+:Type: String
+:Required: No.
+:Default: default
+
+``crush-failure-domain={bucket-type}``
+
+:Description: Ensure that no two chunks are in a bucket with the same
+              failure domain. For instance, if the failure domain is
+              **host** no two chunks will be stored on the same
+              host. It is used to create a CRUSH rule step such as **step
+              chooseleaf host**.
+
+:Type: String
+:Required: No.
+:Default: host
+
+``crush-device-class={device-class}``
+
+:Description: Restrict placement to devices of a specific class (e.g.,
+              ``ssd`` or ``hdd``), using the crush device class names
+              in the CRUSH map.
+
+:Type: String
+:Required: No.
+:Default:
+
+``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
+
+``--force``
+
+:Description: Override an existing profile by the same name.
+
+:Type: String
+:Required: No.
+
+Brief description of SHEC's layouts
+===================================
+
+Space Efficiency
+----------------
+
+Space efficiency is a ratio of data chunks to all ones in a object and
+represented as k/(k+m).
+In order to improve space efficiency, you should increase k or decrease m.
+
+::
+
+        space efficiency of SHEC(4,3,2) = 4/(4+3) = 0.57
+        SHEC(5,3,2) or SHEC(4,2,2) improves SHEC(4,3,2)'s space efficiency
+
+Durability
+----------
+
+The third parameter of SHEC (=c) is a durability estimator, which approximates
+the number of OSDs that can be down without losing data.
+
+``durability estimator of SHEC(4,3,2) = 2``
+
+Recovery Efficiency
+-------------------
+
+Describing calculation of recovery efficiency is beyond the scope of this document,
+but at least increasing m without increasing c achieves improvement of recovery efficiency.
+(However, we must pay attention to the sacrifice of space efficiency in this case.)
+
+``SHEC(4,2,2) -> SHEC(4,3,2) : achieves improvement of recovery efficiency``
+
+Erasure code profile examples
+=============================
+
+::
+
+        $ ceph osd erasure-code-profile set SHECprofile \
+             plugin=shec \
+             k=8 m=4 c=3 \
+             crush-failure-domain=host
+        $ ceph osd pool create shecpool 256 256 erasure SHECprofile