.. _ecpool:

==============
 Erasure code
==============

By default, Ceph `pools <../pools>`_ are created with the type "replicated". In
replicated-type pools, every object is copied to multiple disks. This
multiple copying is the method of data protection known as "replication".

By contrast, `erasure-coded <https://en.wikipedia.org/wiki/Erasure_code>`_
pools use a method of data protection that is different from replication. In
erasure coding, data is broken into fragments of two kinds: data blocks and
parity blocks. If a drive fails or becomes corrupted, the parity blocks are
used to rebuild the data. At scale, erasure coding saves space relative to
replication.

In this documentation, data blocks are referred to as "data chunks"
and parity blocks are referred to as "coding chunks".

Erasure codes are also called "forward error correction codes". The
first forward error correction code was developed in 1950 by Richard
Hamming at Bell Laboratories.


Creating a sample erasure-coded pool
------------------------------------

The simplest erasure-coded pool is similar to `RAID5
<https://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5>`_ and
requires at least three hosts:

.. prompt:: bash $

   ceph osd pool create ecpool erasure

::

   pool 'ecpool' created

.. prompt:: bash $

   echo ABCDEFGHI | rados --pool ecpool put NYAN -
   rados --pool ecpool get NYAN -
   
::

   ABCDEFGHI

Erasure-code profiles
---------------------

The default erasure-code profile can sustain the overlapping loss of two OSDs
without losing data. This erasure-code profile is equivalent to a replicated
pool of size three, but with different storage requirements: instead of
requiring 3TB to store 1TB, it requires only 2TB to store 1TB. The default
profile can be displayed with this command:

.. prompt:: bash $

   ceph osd erasure-code-profile get default
   
::

   k=2
   m=2
   plugin=jerasure
   crush-failure-domain=host
   technique=reed_sol_van

.. note::
  The profile just displayed is for the *default* erasure-coded pool, not the
  *simplest* erasure-coded pool. These two pools are not the same:

   The default erasure-coded pool has two data chunks (K) and two coding chunks
   (M). The profile of the default erasure-coded pool is "k=2 m=2".

   The simplest erasure-coded pool has two data chunks (K) and one coding chunk
   (M). The profile of the simplest erasure-coded pool is "k=2 m=1".

Choosing the right profile is important because the profile cannot be modified
after the pool is created. If you find that you need an erasure-coded pool with
a profile different than the one you have created, you must create a new pool
with a different (and presumably more carefully considered) profile. When the
new pool is created, all objects from the wrongly configured pool must be moved
to the newly created pool. There is no way to alter the profile of a pool after
the pool has been created.

The most important parameters of the profile are *K*, *M*, and
*crush-failure-domain* because they define the storage overhead and
the data durability. For example, if the desired architecture must
sustain the loss of two racks with a storage overhead of 67%,
the following profile can be defined:

.. prompt:: bash $

   ceph osd erasure-code-profile set myprofile \
       k=3 \
       m=2 \
       crush-failure-domain=rack
   ceph osd pool create ecpool erasure myprofile
   echo ABCDEFGHI | rados --pool ecpool put NYAN -
   rados --pool ecpool get NYAN -

::

    ABCDEFGHI

The *NYAN* object will be divided in three (*K=3*) and two additional
*chunks* will be created (*M=2*). The value of *M* defines how many
OSDs can be lost simultaneously without losing any data. The
*crush-failure-domain=rack* will create a CRUSH rule that ensures
no two *chunks* are stored in the same rack.

.. ditaa::
                            +-------------------+
                       name |       NYAN        |
                            +-------------------+
                    content |     ABCDEFGHI     |
                            +--------+----------+
                                     |
                                     |
                                     v
                              +------+------+
              +---------------+ encode(3,2) +-----------+
              |               +--+--+---+---+           |
              |                  |  |   |               |
              |          +-------+  |   +-----+         |
              |          |          |         |         |
           +--v---+   +--v---+   +--v---+  +--v---+  +--v---+
     name  | NYAN |   | NYAN |   | NYAN |  | NYAN |  | NYAN |
           +------+   +------+   +------+  +------+  +------+
    shard  |  1   |   |  2   |   |  3   |  |  4   |  |  5   |
           +------+   +------+   +------+  +------+  +------+
  content  | ABC  |   | DEF  |   | GHI  |  | YXY  |  | QGC  |
           +--+---+   +--+---+   +--+---+  +--+---+  +--+---+
              |          |          |         |         |
              |          |          v         |         |
              |          |       +--+---+     |         |
              |          |       | OSD1 |     |         |
              |          |       +------+     |         |
              |          |                    |         |
              |          |       +------+     |         |
              |          +------>| OSD2 |     |         |
              |                  +------+     |         |
              |                               |         |
              |                  +------+     |         |
              |                  | OSD3 |<----+         |
              |                  +------+               |
              |                                         |
              |                  +------+               |
              |                  | OSD4 |<--------------+
              |                  +------+
              |
              |                  +------+
              +----------------->| OSD5 |
                                 +------+

 
More information can be found in the `erasure-code profiles
<../erasure-code-profile>`_ documentation.


Erasure Coding with Overwrites
------------------------------

By default, erasure-coded pools work only with operations that
perform full object writes and appends (for example, RGW).

Since Luminous, partial writes for an erasure-coded pool may be
enabled with a per-pool setting. This lets RBD and CephFS store their
data in an erasure-coded pool:

.. prompt:: bash $

    ceph osd pool set ec_pool allow_ec_overwrites true

This can be enabled only on a pool residing on BlueStore OSDs, since
BlueStore's checksumming is used during deep scrubs to detect bitrot
or other corruption. Using Filestore with EC overwrites is not only
unsafe, but it also results in lower performance compared to BlueStore.

Erasure-coded pools do not support omap, so to use them with RBD and
CephFS you must instruct them to store their data in an EC pool and
their metadata in a replicated pool. For RBD, this means using the
erasure-coded pool as the ``--data-pool`` during image creation:

.. prompt:: bash $

    rbd create --size 1G --data-pool ec_pool replicated_pool/image_name

For CephFS, an erasure-coded pool can be set as the default data pool during
file system creation or via `file layouts <../../../cephfs/file-layouts>`_.


Erasure-coded pools and cache tiering
-------------------------------------

.. note:: Cache tiering is deprecated in Reef.

Erasure-coded pools require more resources than replicated pools and
lack some of the functionality supported by replicated pools (for example, omap).
To overcome these limitations, one can set up a `cache tier <../cache-tiering>`_
before setting up the erasure-coded pool.

For example, if the pool *hot-storage* is made of fast storage, the following commands
will place the *hot-storage* pool as a tier of *ecpool* in *writeback*
mode:

.. prompt:: bash $

   ceph osd tier add ecpool hot-storage
   ceph osd tier cache-mode hot-storage writeback
   ceph osd tier set-overlay ecpool hot-storage

The result is that every write and read to the *ecpool* actually uses
the *hot-storage* pool and benefits from its flexibility and speed.

More information can be found in the `cache tiering
<../cache-tiering>`_ documentation. Note, however, that cache tiering
is deprecated and may be removed completely in a future release.

Erasure-coded pool recovery
---------------------------
If an erasure-coded pool loses any data shards, it must recover them from others.
This recovery involves reading from the remaining shards, reconstructing the data, and
writing new shards.

In Octopus and later releases, erasure-coded pools can recover as long as there are at least *K* shards
available. (With fewer than *K* shards, you have actually lost data!)

Prior to Octopus, erasure-coded pools required that at least ``min_size`` shards be
available, even if ``min_size`` was greater than ``K``. This was a conservative
decision made out of an abundance of caution when designing the new pool
mode. As a result, however, pools with lost OSDs but without complete data loss were
unable to recover and go active without manual intervention to temporarily change
the ``min_size`` setting.

We recommend that ``min_size`` be ``K+1`` or greater to prevent loss of writes and
loss of data.



Glossary
--------

*chunk*
   When the encoding function is called, it returns chunks of the same size as each other. There are two
   kinds of chunks: (1) *data chunks*, which can be concatenated to reconstruct the original object, and
   (2) *coding chunks*, which can be used to rebuild a lost chunk.

*K*
   The number of data chunks into which an object is divided. For example, if *K* = 2, then a 10KB object
   is divided into two objects of 5KB each.

*M*
   The number of coding chunks computed by the encoding function. *M* is equal to the number of OSDs that can
   be missing from the cluster without the cluster suffering data loss. For example, if there are two coding
   chunks, then two OSDs can be missing without data loss.

Table of contents
-----------------

.. toctree::
    :maxdepth: 1

    erasure-code-profile
    erasure-code-jerasure
    erasure-code-isa
    erasure-code-lrc
    erasure-code-shec
    erasure-code-clay
