crush-map-edits.rst

Manually editing a CRUSH Map
============================

.. note:: Manually editing the CRUSH map is considered an advanced
	  administrator operation.  All CRUSH changes that are
	  necessary for the overwhelming majority of installations are
	  possible via the standard ceph CLI and do not require manual
	  CRUSH map edits.  If you have identified a use case where
	  manual edits *are* necessary, consider contacting the Ceph
	  developers so that future versions of Ceph can make this
	  unnecessary.

To edit an existing CRUSH map:

#. `Get the CRUSH map`_.
#. `Decompile`_ the CRUSH map.
#. Edit at least one of `Devices`_, `Buckets`_ and `Rules`_.
#. `Recompile`_ the CRUSH map.
#. `Set the CRUSH map`_.

For details on setting the CRUSH map rule for a specific pool, see `Set
Pool Values`_.

.. _Get the CRUSH map: #getcrushmap
.. _Decompile: #decompilecrushmap
.. _Devices: #crushmapdevices
.. _Buckets: #crushmapbuckets
.. _Rules: #crushmaprules
.. _Recompile: #compilecrushmap
.. _Set the CRUSH map: #setcrushmap
.. _Set Pool Values: ../pools#setpoolvalues

.. _getcrushmap:

Get a CRUSH Map
---------------

To get the CRUSH map for your cluster, execute the following::

	ceph osd getcrushmap -o {compiled-crushmap-filename}

Ceph will output (-o) a compiled CRUSH map to the filename you specified. Since
the CRUSH map is in a compiled form, you must decompile it first before you can
edit it.

.. _decompilecrushmap:

Decompile a CRUSH Map
---------------------

To decompile a CRUSH map, execute the following::

	crushtool -d {compiled-crushmap-filename} -o {decompiled-crushmap-filename}

.. _compilecrushmap:

Recompile a CRUSH Map
---------------------

To compile a CRUSH map, execute the following::

	crushtool -c {decompiled-crushmap-filename} -o {compiled-crushmap-filename}

.. _setcrushmap:

Set the CRUSH Map
-----------------

To set the CRUSH map for your cluster, execute the following::

	ceph osd setcrushmap -i {compiled-crushmap-filename}

Ceph will load (-i) a compiled CRUSH map from the filename you specified.

Sections
--------

There are six main sections to a CRUSH Map.

#. **tunables:** The preamble at the top of the map described any *tunables*
   for CRUSH behavior that vary from the historical/legacy CRUSH behavior. These
   correct for old bugs, optimizations, or other changes in behavior that have
   been made over the years to improve CRUSH's behavior.

#. **devices:** Devices are individual ``ceph-osd`` daemons that can
   store data.

#. **types**: Bucket ``types`` define the types of buckets used in
   your CRUSH hierarchy. Buckets consist of a hierarchical aggregation
   of storage locations (e.g., rows, racks, chassis, hosts, etc.) and
   their assigned weights.

#. **buckets:** Once you define bucket types, you must define each node
   in the hierarchy, its type, and which devices or other nodes it
   contains.

#. **rules:** Rules define policy about how data is distributed across
   devices in the hierarchy.

#. **choose_args:** Choose_args are alternative weights associated with
   the hierarchy that have been adjusted to optimize data placement.  A single
   choose_args map can be used for the entire cluster, or one can be
   created for each individual pool.


.. _crushmapdevices:

CRUSH Map Devices
-----------------

Devices are individual ``ceph-osd`` daemons that can store data.  You
will normally have one defined here for each OSD daemon in your
cluster.  Devices are identified by an id (a non-negative integer) and
a name, normally ``osd.N`` where ``N`` is the device id.

.. _crush-map-device-class:

Devices may also have a *device class* associated with them (e.g.,
``hdd`` or ``ssd``), allowing them to be conveniently targeted by a
crush rule.

::

	# devices
	device {num} {osd.name} [class {class}]

For example::

	# devices
	device 0 osd.0 class ssd
	device 1 osd.1 class hdd
	device 2 osd.2
	device 3 osd.3

In most cases, each device maps to a single ``ceph-osd`` daemon.  This
is normally a single storage device, a pair of devices (for example,
one for data and one for a journal or metadata), or in some cases a
small RAID device.





CRUSH Map Bucket Types
----------------------

The second list in the CRUSH map defines 'bucket' types. Buckets facilitate
a hierarchy of nodes and leaves. Node (or non-leaf) buckets typically represent
physical locations in a hierarchy. Nodes aggregate other nodes or leaves.
Leaf buckets represent ``ceph-osd`` daemons and their corresponding storage
media.

.. tip:: The term "bucket" used in the context of CRUSH means a node in
   the hierarchy, i.e. a location or a piece of physical hardware. It
   is a different concept from the term "bucket" when used in the
   context of RADOS Gateway APIs.

To add a bucket type to the CRUSH map, create a new line under your list of
bucket types. Enter ``type`` followed by a unique numeric ID and a bucket name.
By convention, there is one leaf bucket and it is ``type 0``;  however, you may
give it any name you like (e.g., osd, disk, drive, storage, etc.)::

	#types
	type {num} {bucket-name}

For example::

	# types
	type 0 osd
	type 1 host
	type 2 chassis
	type 3 rack
	type 4 row
	type 5 pdu
	type 6 pod
	type 7 room
	type 8 datacenter
	type 9 zone
	type 10 region
	type 11 root



.. _crushmapbuckets:

CRUSH Map Bucket Hierarchy
--------------------------

The CRUSH algorithm distributes data objects among storage devices according
to a per-device weight value, approximating a uniform probability distribution.
CRUSH distributes objects and their replicas according to the hierarchical
cluster map you define. Your CRUSH map represents the available storage
devices and the logical elements that contain them.

To map placement groups to OSDs across failure domains, a CRUSH map defines a
hierarchical list of bucket types (i.e., under ``#types`` in the generated CRUSH
map). The purpose of creating a bucket hierarchy is to segregate the
leaf nodes by their failure domains, such as hosts, chassis, racks, power
distribution units, pods, rows, rooms, and data centers. With the exception of
the leaf nodes representing OSDs, the rest of the hierarchy is arbitrary, and
you may define it according to your own needs.

We recommend adapting your CRUSH map to your firms's hardware naming conventions
and using instances names that reflect the physical hardware. Your naming
practice can make it easier to administer the cluster and troubleshoot
problems when an OSD and/or other hardware malfunctions and the administrator
need access to physical hardware.

In the following example, the bucket hierarchy has a leaf bucket named ``osd``,
and two node buckets named ``host`` and ``rack`` respectively.

.. ditaa::
                           +-----------+
                           | {o}rack   |
                           |   Bucket  |
                           +-----+-----+
                                 |
                 +---------------+---------------+
                 |                               |
           +-----+-----+                   +-----+-----+
           | {o}host   |                   | {o}host   |
           |   Bucket  |                   |   Bucket  |
           +-----+-----+                   +-----+-----+
                 |                               |
         +-------+-------+               +-------+-------+
         |               |               |               |
   +-----+-----+   +-----+-----+   +-----+-----+   +-----+-----+
   |    osd    |   |    osd    |   |    osd    |   |    osd    |
   |   Bucket  |   |   Bucket  |   |   Bucket  |   |   Bucket  |
   +-----------+   +-----------+   +-----------+   +-----------+

.. note:: The higher numbered ``rack`` bucket type aggregates the lower
   numbered ``host`` bucket type.

Since leaf nodes reflect storage devices declared under the ``#devices`` list
at the beginning of the CRUSH map, you do not need to declare them as bucket
instances. The second lowest bucket type in your hierarchy usually aggregates
the devices (i.e., it's usually the computer containing the storage media, and
uses whatever term you prefer to describe it, such as  "node", "computer",
"server," "host", "machine", etc.). In high density environments, it is
increasingly common to see multiple hosts/nodes per chassis. You should account
for chassis failure too--e.g., the need to pull a chassis if a node fails may
result in bringing down numerous hosts/nodes and their OSDs.

When declaring a bucket instance, you must specify its type, give it a unique
name (string), assign it a unique ID expressed as a negative integer (optional),
specify a weight relative to the total capacity/capability of its item(s),
specify the bucket algorithm (usually ``straw2``), and the hash (usually ``0``,
reflecting hash algorithm ``rjenkins1``). A bucket may have one or more items.
The items may consist of node buckets or leaves. Items may have a weight that
reflects the relative weight of the item.

You may declare a node bucket with the following syntax::

	[bucket-type] [bucket-name] {
		id [a unique negative numeric ID]
		weight [the relative capacity/capability of the item(s)]
		alg [the bucket type: uniform | list | tree | straw | straw2 ]
		hash [the hash type: 0 by default]
		item [item-name] weight [weight]
	}

For example, using the diagram above, we would define two host buckets
and one rack bucket. The OSDs are declared as items within the host buckets::

	host node1 {
		id -1
		alg straw2
		hash 0
		item osd.0 weight 1.00
		item osd.1 weight 1.00
	}

	host node2 {
		id -2
		alg straw2
		hash 0
		item osd.2 weight 1.00
		item osd.3 weight 1.00
	}

	rack rack1 {
		id -3
		alg straw2
		hash 0
		item node1 weight 2.00
		item node2 weight 2.00
	}

.. note:: In the foregoing example, note that the rack bucket does not contain
   any OSDs. Rather it contains lower level host buckets, and includes the
   sum total of their weight in the item entry.

.. topic:: Bucket Types

   Ceph supports five bucket types, each representing a tradeoff between
   performance and reorganization efficiency. If you are unsure of which bucket
   type to use, we recommend using a ``straw2`` bucket.  For a detailed
   discussion of bucket types, refer to
   `CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data`_,
   and more specifically to **Section 3.4**. The bucket types are:

	#. **uniform**: Uniform buckets aggregate devices with **exactly** the same
	   weight. For example, when firms commission or decommission hardware, they
	   typically do so with many machines that have exactly the same physical
	   configuration (e.g., bulk purchases). When storage devices have exactly
	   the same weight, you may use the ``uniform`` bucket type, which allows
	   CRUSH to map replicas into uniform buckets in constant time. With
	   non-uniform weights, you should use another bucket algorithm.

	#. **list**: List buckets aggregate their content as linked lists. Based on
	   the :abbr:`RUSH (Replication Under Scalable Hashing)` :sub:`P` algorithm,
	   a list is a natural and intuitive choice for an **expanding cluster**:
	   either an object is relocated to the newest device with some appropriate
	   probability, or it remains on the older devices as before. The result is
	   optimal data migration when items are added to the bucket. Items removed
	   from the middle or tail of the list, however, can result in a significant
	   amount of unnecessary movement, making list buckets most suitable for
	   circumstances in which they **never (or very rarely) shrink**.

	#. **tree**: Tree buckets use a binary search tree. They are more efficient
	   than list buckets when a bucket contains a larger set of items. Based on
	   the :abbr:`RUSH (Replication Under Scalable Hashing)` :sub:`R` algorithm,
	   tree buckets reduce the placement time to O(log :sub:`n`), making them
	   suitable for managing much larger sets of devices or nested buckets.

	#. **straw**: List and Tree buckets use a divide and conquer strategy
	   in a way that either gives certain items precedence (e.g., those
	   at the beginning of a list) or obviates the need to consider entire
	   subtrees of items at all. That improves the performance of the replica
	   placement process, but can also introduce suboptimal reorganization
	   behavior when the contents of a bucket change due an addition, removal,
	   or re-weighting of an item. The straw bucket type allows all items to
	   fairly “compete” against each other for replica placement through a
	   process analogous to a draw of straws.

        #. **straw2**: Straw2 buckets improve Straw to correctly avoid any data
           movement between items when neighbor weights change.

           For example the weight of item A including adding it anew or removing
           it completely, there will be data movement only to or from item A.

.. topic:: Hash

   Each bucket uses a hash algorithm. Currently, Ceph supports ``rjenkins1``.
   Enter ``0`` as your hash setting to select ``rjenkins1``.


.. _weightingbucketitems:

.. topic:: Weighting Bucket Items

   Ceph expresses bucket weights as doubles, which allows for fine
   weighting. A weight is the relative difference between device capacities. We
   recommend using ``1.00`` as the relative weight for a 1TB storage device.
   In such a scenario, a weight of ``0.5`` would represent approximately 500GB,
   and a weight of ``3.00`` would represent approximately 3TB. Higher level
   buckets have a weight that is the sum total of the leaf items aggregated by
   the bucket.

   A bucket item weight is one dimensional, but you may also calculate your
   item weights to reflect the performance of the storage drive. For example,
   if you have many 1TB drives where some have relatively low data transfer
   rate and the others have a relatively high data transfer rate, you may
   weight them differently, even though they have the same capacity (e.g.,
   a weight of 0.80 for the first set of drives with lower total throughput,
   and 1.20 for the second set of drives with higher total throughput).


.. _crushmaprules:

CRUSH Map Rules
---------------

CRUSH maps support the notion of 'CRUSH rules', which are the rules that
determine data placement for a pool. The default CRUSH map has a rule for each
pool. For large clusters, you will likely create many pools where each pool may
have its own non-default CRUSH rule.

.. note:: In most cases, you will not need to modify the default rule. When
   you create a new pool, by default the rule will be set to ``0``.


CRUSH rules define placement and replication strategies or distribution policies
that allow you to specify exactly how CRUSH places object replicas. For
example, you might create a rule selecting a pair of targets for 2-way
mirroring, another rule for selecting three targets in two different data
centers for 3-way mirroring, and yet another rule for erasure coding over six
storage devices. For a detailed discussion of CRUSH rules, refer to
`CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data`_,
and more specifically to **Section 3.2**.

A rule takes the following form::

	rule <rulename> {

		id [a unique whole numeric ID]
		type [ replicated | erasure ]
		min_size <min-size>
		max_size <max-size>
		step take <bucket-name> [class <device-class>]
		step [choose|chooseleaf] [firstn|indep] <N> type <bucket-type>
		step emit
	}


``id``

:Description: A unique whole number for identifying the rule.

:Purpose: A component of the rule mask.
:Type: Integer
:Required: Yes
:Default: 0


``type``

:Description: Describes a rule for either a storage drive (replicated)
              or a RAID.

:Purpose: A component of the rule mask.
:Type: String
:Required: Yes
:Default: ``replicated``
:Valid Values: Currently only ``replicated`` and ``erasure``

``min_size``

:Description: If a pool makes fewer replicas than this number, CRUSH will
              **NOT** select this rule.

:Type: Integer
:Purpose: A component of the rule mask.
:Required: Yes
:Default: ``1``

``max_size``

:Description: If a pool makes more replicas than this number, CRUSH will
              **NOT** select this rule.

:Type: Integer
:Purpose: A component of the rule mask.
:Required: Yes
:Default: 10


``step take <bucket-name> [class <device-class>]``

:Description: Takes a bucket name, and begins iterating down the tree.
              If the ``device-class`` is specified, it must match
              a class previously used when defining a device. All
              devices that do not belong to the class are excluded.
:Purpose: A component of the rule.
:Required: Yes
:Example: ``step take data``


``step choose firstn {num} type {bucket-type}``

:Description: Selects the number of buckets of the given type from within the
	      current bucket. The number is usually the number of replicas in
	      the pool (i.e., pool size).

              - If ``{num} == 0``, choose ``pool-num-replicas`` buckets (all available).
              - If ``{num} > 0 && < pool-num-replicas``, choose that many buckets.
              - If ``{num} < 0``, it means ``pool-num-replicas - {num}``.

:Purpose: A component of the rule.
:Prerequisite: Follows ``step take`` or ``step choose``.
:Example: ``step choose firstn 1 type row``


``step chooseleaf firstn {num} type {bucket-type}``

:Description: Selects a set of buckets of ``{bucket-type}`` and chooses a leaf
              node (that is, an OSD) from the subtree of each bucket in the set of buckets.
              The number of buckets in the set is usually the number of replicas in
              the pool (i.e., pool size).

              - If ``{num} == 0``, choose ``pool-num-replicas`` buckets (all available).
              - If ``{num} > 0 && < pool-num-replicas``, choose that many buckets.
              - If ``{num} < 0``, it means ``pool-num-replicas - {num}``.

:Purpose: A component of the rule. Usage removes the need to select a device using two steps.
:Prerequisite: Follows ``step take`` or ``step choose``.
:Example: ``step chooseleaf firstn 0 type row``


``step emit``

:Description: Outputs the current value and empties the stack. Typically used
              at the end of a rule, but may also be used to pick from different
              trees in the same rule.

:Purpose: A component of the rule.
:Prerequisite: Follows ``step choose``.
:Example: ``step emit``

.. important:: A given CRUSH rule may be assigned to multiple pools, but it
   is not possible for a single pool to have multiple CRUSH rules.

``firstn`` versus ``indep``

:Description: Controls the replacement strategy CRUSH uses when items (OSDs)
	      are marked down in the CRUSH map. If this rule is to be used with
	      replicated pools it should be ``firstn`` and if it's for
	      erasure-coded pools it should be ``indep``.

	      The reason has to do with how they behave when a
	      previously-selected device fails. Let's say you have a PG stored
	      on OSDs 1, 2, 3, 4, 5. Then 3 goes down.
	      
	      With the "firstn" mode, CRUSH simply adjusts its calculation to
	      select 1 and 2, then selects 3 but discovers it's down, so it
	      retries and selects 4 and 5, and then goes on to select a new
	      OSD 6. So the final CRUSH mapping change is
	      1, 2, 3, 4, 5 -> 1, 2, 4, 5, 6.

	      But if you're storing an EC pool, that means you just changed the
	      data mapped to OSDs 4, 5, and 6! So the "indep" mode attempts to
	      not do that. You can instead expect it, when it selects the failed
	      OSD 3, to try again and pick out 6, for a final transformation of:
	      1, 2, 3, 4, 5 -> 1, 2, 6, 4, 5
	      
.. _crush-reclassify:

Migrating from a legacy SSD rule to device classes
--------------------------------------------------

It used to be necessary to manually edit your CRUSH map and maintain a
parallel hierarchy for each specialized device type (e.g., SSD) in order to
write rules that apply to those devices.  Since the Luminous release,
the *device class* feature has enabled this transparently.

However, migrating from an existing, manually customized per-device map to
the new device class rules in the trivial way will cause all data in the
system to be reshuffled.

The ``crushtool`` has a few commands that can transform a legacy rule
and hierarchy so that you can start using the new class-based rules.
There are three types of transformations possible:

#. ``--reclassify-root <root-name> <device-class>``

   This will take everything in the hierarchy beneath root-name and
   adjust any rules that reference that root via a ``take
   <root-name>`` to instead ``take <root-name> class <device-class>``.
   It renumbers the buckets in such a way that the old IDs are instead
   used for the specified class's "shadow tree" so that no data
   movement takes place.

   For example, imagine you have an existing rule like::

     rule replicated_ruleset {
        id 0
        type replicated
        min_size 1
        max_size 10
        step take default
        step chooseleaf firstn 0 type rack
        step emit
     }

   If you reclassify the root `default` as class `hdd`, the rule will
   become::

     rule replicated_ruleset {
        id 0
        type replicated
        min_size 1
        max_size 10
        step take default class hdd
        step chooseleaf firstn 0 type rack
        step emit
     }

#. ``--set-subtree-class <bucket-name> <device-class>``

   This will mark every device in the subtree rooted at *bucket-name*
   with the specified device class.

   This is normally used in conjunction with the ``--reclassify-root``
   option to ensure that all devices in that root are labeled with the
   correct class.  In some situations, however, some of those devices
   (correctly) have a different class and we do not want to relabel
   them.  In such cases, one can exclude the ``--set-subtree-class``
   option.  This means that the remapping process will not be perfect,
   since the previous rule distributed across devices of multiple
   classes but the adjusted rules will only map to devices of the
   specified *device-class*, but that often is an accepted level of
   data movement when the number of outlier devices is small.

#. ``--reclassify-bucket <match-pattern> <device-class> <default-parent>``

   This will allow you to merge a parallel type-specific hierarchy with the normal hierarchy.  For example, many users have maps like::

     host node1 {
        id -2           # do not change unnecessarily
        # weight 109.152
        alg straw2
        hash 0  # rjenkins1
        item osd.0 weight 9.096
        item osd.1 weight 9.096
        item osd.2 weight 9.096
        item osd.3 weight 9.096
        item osd.4 weight 9.096
        item osd.5 weight 9.096
        ...
     }

     host node1-ssd {
        id -10          # do not change unnecessarily
        # weight 2.000
        alg straw2
        hash 0  # rjenkins1
        item osd.80 weight 2.000
	...
     }

     root default {
        id -1           # do not change unnecessarily
        alg straw2
        hash 0  # rjenkins1
        item node1 weight 110.967
        ...
     }

     root ssd {
        id -18          # do not change unnecessarily
        # weight 16.000
        alg straw2
        hash 0  # rjenkins1
        item node1-ssd weight 2.000
	...
     }

   This function will reclassify each bucket that matches a
   pattern.  The pattern can look like ``%suffix`` or ``prefix%``.
   For example, in the above example, we would use the pattern
   ``%-ssd``.  For each matched bucket, the remaining portion of the
   name (that matches the ``%`` wildcard) specifies the *base bucket*.
   All devices in the matched bucket are labeled with the specified
   device class and then moved to the base bucket.  If the base bucket
   does not exist (e.g., ``node12-ssd`` exists but ``node12`` does
   not), then it is created and linked underneath the specified
   *default parent* bucket.  In each case, we are careful to preserve
   the old bucket IDs for the new shadow buckets to prevent data
   movement.  Any rules with ``take`` steps referencing the old
   buckets are adjusted.

#. ``--reclassify-bucket <bucket-name> <device-class> <base-bucket>``

   The same command can also be used without a wildcard to map a
   single bucket.  For example, in the previous example, we want the
   ``ssd`` bucket to be mapped to the ``default`` bucket.

The final command to convert the map comprised of the above fragments would be something like::

  $ ceph osd getcrushmap -o original
  $ crushtool -i original --reclassify \
      --set-subtree-class default hdd \
      --reclassify-root default hdd \
      --reclassify-bucket %-ssd ssd default \
      --reclassify-bucket ssd ssd default \
      -o adjusted

In order to ensure that the conversion is correct, there is a ``--compare`` command that will test a large sample of inputs to the CRUSH map and ensure that the same result comes back out.  These inputs are controlled by the same options that apply to the ``--test`` command.  For the above example,::

  $ crushtool -i original --compare adjusted
  rule 0 had 0/10240 mismatched mappings (0)
  rule 1 had 0/10240 mismatched mappings (0)
  maps appear equivalent

If there were difference, you'd see what ratio of inputs are remapped
in the parentheses.

If you are satisfied with the adjusted map, you can apply it to the cluster with something like::

  ceph osd setcrushmap -i adjusted

Tuning CRUSH, the hard way
--------------------------

If you can ensure that all clients are running recent code, you can
adjust the tunables by extracting the CRUSH map, modifying the values,
and reinjecting it into the cluster.

* Extract the latest CRUSH map::

	ceph osd getcrushmap -o /tmp/crush

* Adjust tunables.  These values appear to offer the best behavior
  for both large and small clusters we tested with.  You will need to
  additionally specify the ``--enable-unsafe-tunables`` argument to
  ``crushtool`` for this to work.  Please use this option with
  extreme care.::

	crushtool -i /tmp/crush --set-choose-local-tries 0 --set-choose-local-fallback-tries 0 --set-choose-total-tries 50 -o /tmp/crush.new

* Reinject modified map::

	ceph osd setcrushmap -i /tmp/crush.new

Legacy values
-------------

For reference, the legacy values for the CRUSH tunables can be set
with::

   crushtool -i /tmp/crush --set-choose-local-tries 2 --set-choose-local-fallback-tries 5 --set-choose-total-tries 19 --set-chooseleaf-descend-once 0 --set-chooseleaf-vary-r 0 -o /tmp/crush.legacy

Again, the special ``--enable-unsafe-tunables`` option is required.
Further, as noted above, be careful running old versions of the
``ceph-osd`` daemon after reverting to legacy values as the feature
bit is not perfectly enforced.

.. _CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data: https://ceph.com/wp-content/uploads/2016/08/weil-crush-sc06.pdf