ceph/doc/rados/operations/erasure-code-lrc.rst
Nathan Cutler aea9fa01ae doc: globally change CRUSH ruleset to CRUSH rule
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>
2017-12-11 17:15:23 +01:00

372 lines
12 KiB
ReStructuredText

======================================
Locally repairable erasure code plugin
======================================
With the *jerasure* plugin, when an erasure coded object is stored on
multiple OSDs, recovering from the loss of one OSD requires reading
from all the others. For instance if *jerasure* is configured with
*k=8* and *m=4*, losing one OSD requires reading from the eleven
others to repair.
The *lrc* erasure code plugin creates local parity chunks to be able
to recover using less OSDs. For instance if *lrc* is configured with
*k=8*, *m=4* and *l=4*, it will create an additional parity chunk for
every four OSDs. When a single OSD is lost, it can be recovered with
only four OSDs instead of eleven.
Erasure code profile examples
=============================
Reduce recovery bandwidth between hosts
---------------------------------------
Although it is probably not an interesting use case when all hosts are
connected to the same switch, reduced bandwidth usage can actually be
observed.::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
k=4 m=2 l=3 \
crush-failure-domain=host
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
Reduce recovery bandwidth between racks
---------------------------------------
In Firefly the reduced bandwidth will only be observed if the primary
OSD is in the same rack as the lost chunk.::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
k=4 m=2 l=3 \
crush-locality=rack \
crush-failure-domain=host
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
Create an lrc profile
=====================
To create a new lrc erasure code profile::
ceph osd erasure-code-profile set {name} \
plugin=lrc \
k={data-chunks} \
m={coding-chunks} \
l={locality} \
[crush-root={root}] \
[crush-locality={bucket-type}] \
[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: Yes.
:Example: 4
``m={coding-chunks}``
:Description: Compute **coding chunks** for each object and store them
on different OSDs. The number of coding chunks is also
the number of OSDs that can be down without losing data.
:Type: Integer
:Required: Yes.
:Example: 2
``l={locality}``
:Description: Group the coding and data chunks into sets of size
**locality**. For instance, for **k=4** and **m=2**,
when **locality=3** two groups of three are created.
Each set can be recovered without reading chunks
from another set.
:Type: Integer
:Required: Yes.
:Example: 3
``crush-root={root}``
:Description: The name of the crush bucket used for the first step of
the CRUSH rule. For intance **step take default**.
:Type: String
:Required: No.
:Default: default
``crush-locality={bucket-type}``
:Description: The type of the crush bucket in which each set of chunks
defined by **l** will be stored. For instance, if it is
set to **rack**, each group of **l** chunks will be
placed in a different rack. It is used to create a
CRUSH rule step such as **step choose rack**. If it is not
set, no such grouping is done.
:Type: String
:Required: No.
``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.
Low level plugin configuration
==============================
The sum of **k** and **m** must be a multiple of the **l** parameter.
The low level configuration parameters do not impose such a
restriction and it may be more convienient to use it for specific
purposes. It is for instance possible to define two groups, one with 4
chunks and another with 3 chunks. It is also possible to recursively
define locality sets, for instance datacenters and racks into
datacenters. The **k/m/l** are implemented by generating a low level
configuration.
The *lrc* erasure code plugin recursively applies erasure code
techniques so that recovering from the loss of some chunks only
requires a subset of the available chunks, most of the time.
For instance, when three coding steps are described as::
chunk nr 01234567
step 1 _cDD_cDD
step 2 cDDD____
step 3 ____cDDD
where *c* are coding chunks calculated from the data chunks *D*, the
loss of chunk *7* can be recovered with the last four chunks. And the
loss of chunk *2* chunk can be recovered with the first four
chunks.
Erasure code profile examples using low level configuration
===========================================================
Minimal testing
---------------
It is strictly equivalent to using the default erasure code profile. The *DD*
implies *K=2*, the *c* implies *M=1* and the *jerasure* plugin is used
by default.::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
mapping=DD_ \
layers='[ [ "DDc", "" ] ]'
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
Reduce recovery bandwidth between hosts
---------------------------------------
Although it is probably not an interesting use case when all hosts are
connected to the same switch, reduced bandwidth usage can actually be
observed. It is equivalent to **k=4**, **m=2** and **l=3** although
the layout of the chunks is different::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
mapping=__DD__DD \
layers='[
[ "_cDD_cDD", "" ],
[ "cDDD____", "" ],
[ "____cDDD", "" ],
]'
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
Reduce recovery bandwidth between racks
---------------------------------------
In Firefly the reduced bandwidth will only be observed if the primary
OSD is in the same rack as the lost chunk.::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
mapping=__DD__DD \
layers='[
[ "_cDD_cDD", "" ],
[ "cDDD____", "" ],
[ "____cDDD", "" ],
]' \
crush-steps='[
[ "choose", "rack", 2 ],
[ "chooseleaf", "host", 4 ],
]'
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
Testing with different Erasure Code backends
--------------------------------------------
LRC now uses jerasure as the default EC backend. It is possible to
specify the EC backend/algorithm on a per layer basis using the low
level configuration. The second argument in layers='[ [ "DDc", "" ] ]'
is actually an erasure code profile to be used for this level. The
example below specifies the ISA backend with the cauchy technique to
be used in the lrcpool.::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
mapping=DD_ \
layers='[ [ "DDc", "plugin=isa technique=cauchy" ] ]'
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
You could also use a different erasure code profile for for each
layer.::
$ ceph osd erasure-code-profile set LRCprofile \
plugin=lrc \
mapping=__DD__DD \
layers='[
[ "_cDD_cDD", "plugin=isa technique=cauchy" ],
[ "cDDD____", "plugin=isa" ],
[ "____cDDD", "plugin=jerasure" ],
]'
$ ceph osd pool create lrcpool 12 12 erasure LRCprofile
Erasure coding and decoding algorithm
=====================================
The steps found in the layers description::
chunk nr 01234567
step 1 _cDD_cDD
step 2 cDDD____
step 3 ____cDDD
are applied in order. For instance, if a 4K object is encoded, it will
first go thru *step 1* and be divided in four 1K chunks (the four
uppercase D). They are stored in the chunks 2, 3, 6 and 7, in
order. From these, two coding chunks are calculated (the two lowercase
c). The coding chunks are stored in the chunks 1 and 5, respectively.
The *step 2* re-uses the content created by *step 1* in a similar
fashion and stores a single coding chunk *c* at position 0. The last four
chunks, marked with an underscore (*_*) for readability, are ignored.
The *step 3* stores a single coding chunk *c* at position 4. The three
chunks created by *step 1* are used to compute this coding chunk,
i.e. the coding chunk from *step 1* becomes a data chunk in *step 3*.
If chunk *2* is lost::
chunk nr 01234567
step 1 _c D_cDD
step 2 cD D____
step 3 __ _cDDD
decoding will attempt to recover it by walking the steps in reverse
order: *step 3* then *step 2* and finally *step 1*.
The *step 3* knows nothing about chunk *2* (i.e. it is an underscore)
and is skipped.
The coding chunk from *step 2*, stored in chunk *0*, allows it to
recover the content of chunk *2*. There are no more chunks to recover
and the process stops, without considering *step 1*.
Recovering chunk *2* requires reading chunks *0, 1, 3* and writing
back chunk *2*.
If chunk *2, 3, 6* are lost::
chunk nr 01234567
step 1 _c _c D
step 2 cD __ _
step 3 __ cD D
The *step 3* can recover the content of chunk *6*::
chunk nr 01234567
step 1 _c _cDD
step 2 cD ____
step 3 __ cDDD
The *step 2* fails to recover and is skipped because there are two
chunks missing (*2, 3*) and it can only recover from one missing
chunk.
The coding chunk from *step 1*, stored in chunk *1, 5*, allows it to
recover the content of chunk *2, 3*::
chunk nr 01234567
step 1 _cDD_cDD
step 2 cDDD____
step 3 ____cDDD
Controlling CRUSH placement
===========================
The default CRUSH rule provides OSDs that are on different hosts. For instance::
chunk nr 01234567
step 1 _cDD_cDD
step 2 cDDD____
step 3 ____cDDD
needs exactly *8* OSDs, one for each chunk. If the hosts are in two
adjacent racks, the first four chunks can be placed in the first rack
and the last four in the second rack. So that recovering from the loss
of a single OSD does not require using bandwidth between the two
racks.
For instance::
crush-steps='[ [ "choose", "rack", 2 ], [ "chooseleaf", "host", 4 ] ]'
will create a rule that will select two crush buckets of type
*rack* and for each of them choose four OSDs, each of them located in
different buckets of type *host*.
The CRUSH rule can also be manually crafted for finer control.