Manually editing the CRUSH Map
==============================

.. note:: Manually editing the CRUSH map is an advanced administrator
   operation. For the majority of installations, CRUSH changes can be
   implemented via the Ceph CLI and do not require manual CRUSH map edits. If
   you have identified a use case where manual edits *are* necessary with a
   recent Ceph release, consider contacting the Ceph developers at dev@ceph.io
   so that future versions of Ceph do not have this problem.

To edit an existing CRUSH map, carry out the following procedure:

#. `Get the CRUSH map`_.
#. `Decompile`_ the CRUSH map.
#.  Edit at least one of the following sections: `Devices`_, `Buckets`_, and
    `Rules`_. Use a text editor for this task.
#. `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 the CRUSH Map
-----------------

To get the CRUSH map for your cluster, run a command of the following form:

.. prompt:: bash $

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

Ceph outputs (``-o``) a compiled CRUSH map to the filename that you have
specified. Because the CRUSH map is in a compiled form, you must first
decompile it before you can edit it.

.. _decompilecrushmap:

Decompile the CRUSH Map
-----------------------

To decompile the CRUSH map, run a command of the following form:

.. prompt:: bash $

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

.. _compilecrushmap:

Recompile the CRUSH Map
-----------------------

To compile the CRUSH map, run a command of the following form:

.. prompt:: bash $

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

.. _setcrushmap:

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

To set the CRUSH map for your cluster, run a command of the following form:

.. prompt:: bash $

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

Ceph loads (``-i``) a compiled CRUSH map from the filename that you have
specified.

Sections
--------

A CRUSH map has six main sections:

#. **tunables:** The preamble at the top of the map describes any *tunables*
   that are not a part of legacy CRUSH behavior. These tunables correct for old
   bugs, optimizations, or other changes that have been made over the years to
   improve CRUSH's behavior.

#. **devices:** Devices are individual OSDs that store data.

#. **types**: Bucket ``types`` define the types of buckets that are used in
   your CRUSH hierarchy. 

#. **buckets:** Buckets consist of a hierarchical aggregation of storage
   locations (for example, rows, racks, chassis, hosts) and their assigned
   weights. After the bucket ``types`` have been defined, the CRUSH map defines
   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 in order to optimize data placement. A
   single ``choose_args`` map can be used for the entire cluster, or a number
   of ``choose_args`` maps can be created such that each map is crafted for a
   particular pool.


.. _crushmapdevices:

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

Devices are individual OSDs that store data. In this section, there is usually
one device defined for each OSD daemon in your cluster. Devices are identified
by an ``id`` (a non-negative integer) and a ``name`` (usually ``osd.N``, where
``N`` is the device's ``id``).


.. _crush-map-device-class:

A device can also have a *device class* associated with it: for example,
``hdd`` or ``ssd``. Device classes make it possible for devices to be targeted
by CRUSH rules. This means that device classes allow CRUSH rules to select only
OSDs that match certain characteristics. For example, you might want an RBD
pool associated only with SSDs and a different RBD pool associated only with
HDDs.

To see a list of devices, run the following command:

.. prompt:: bash #

   ceph device ls 

The output of this command takes the following form:

::

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

For example:

.. prompt:: bash #

    ceph device ls

::

    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 corresponding ``ceph-osd`` daemon. This
daemon might map to 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 or a partition of a larger storage device.


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

The second list in the CRUSH map defines 'bucket' types. Buckets facilitate a
hierarchy of nodes and leaves. Node buckets (also known as 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:: In the context of CRUSH, the term "bucket" is used to refer to
   a node in the hierarchy (that is, to a location or a piece of physical
   hardware). In the context of RADOS Gateway APIs, however, the term
   "bucket" has a different meaning.

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

    # 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. The CRUSH map represents the available storage devices
and the logical elements that contain them.

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

We recommend adapting your CRUSH map to your preferred hardware-naming
conventions and using bucket names that clearly reflect the physical
hardware. Clear naming practice can make it easier to administer the cluster
and easier to troubleshoot problems when OSDs malfunction (or other hardware
malfunctions) and the administrator needs access to physical hardware.


In the following example, the CRUSH hierarchy has ``osd`` leaf buckets
and two node-level buckets at the ``host`` level that are in turn children
of a ``rack`` bucket :

.. ditaa::
                           +-----------+
                           | {o}rack11 |
                           +-----+-----+
                                 |
                 +---------------+---------------+
                 |                               |
           +-----+-----+                   +-----+-----+
           | {o}host1  |                   | {o}host2  |
           +-----+-----+                   +-----+-----+
                 |                               |
         +-------+-------+               +-------+-------+
         |               |               |               |
   +-----+-----+   +-----+-----+   +-----+-----+   +-----+-----+
   |    osd0   |   |    osd1   |   |    osd2   |   |    osd3   |
   +-----------+   +-----------+   +-----------+   +-----------+

.. note:: The higher type-number ``rack`` bucket aggregates the
   lower type-number ``host`` buckets, which in turn aggregate the basal
   type 0 ``osd`` buckets.

Because leaf nodes reflect storage devices that have already been declared
under the ``#devices`` list at the beginning of the CRUSH map, there is no need
to declare them as bucket instances. The second-lowest bucket type in your
hierarchy is typically used to aggregate the devices: usually
the server that houses the storage drives.
In high-density environments, it is common to have multiple hosts
or nodes in a single chassis: blades or twins. It
is important to anticipate the potential consequences of chassis failure. For
example, during the replacement of a chassis in case of a node failure,
all of the
chassis's hosts and their associated OSDs will be in the ``down``
state and thus unavailable.  It is important to avoid placing multiple replicas
or shards of data within a single such chassis, which in this case is
a _failure domain_.

To declare a bucket instance, do the following: specify its type, give it a
unique name (an alphanumeric string), assign it a unique ID expressed as a
negative integer (this is optional), assign it a weight relative to the total
capacity and capability of the item(s) in the bucket, assign it a bucket
algorithm (usually ``straw2``), and specify the bucket algorithm's hash
(usually ``0``, a setting that reflects the 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.

To declare a node bucket, use 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, in the above diagram, two host buckets (referred to in the
declaration below as ``node1`` and ``node2``) and one rack bucket (referred to
in the declaration below as ``rack1``) are defined. 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 this example, the rack bucket does not contain any OSDs. Instead,
   it contains lower-level host buckets and includes the sum of their weight in
   the item entry.


.. topic:: Bucket Types

   Ceph supports five bucket types. Each bucket type provides a balance between
   performance and reorganization efficiency, and each is different from the
   others. If you are unsure of which bucket type to use, use the ``straw2``
   bucket. For a more technical discussion of bucket types than is offered
   here, see **Section 3.4** of `CRUSH - Controlled, Scalable, Decentralized
   Placement of Replicated Data`_. 
   
   The bucket types are as follows:

    #. **uniform**: Uniform buckets aggregate devices that have **exactly** 
       the same weight. For example, when hardware is commissioned or
       decommissioned, it is often done in sets of machines that have exactly
       the same physical configuration (this can be the case, for example,
       after 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. If your devices have
       non-uniform weights, you should not use the uniform bucket algorithm.

    #. **list**: List buckets aggregate their content as linked lists. The 
       behavior of list buckets is governed by the :abbr:`RUSH (Replication
       Under Scalable Hashing)`:sub:`P` algorithm. In the behavior of this
       bucket type, an object is either relocated to the newest device in
       accordance with an appropriate probability, or it remains on the older
       devices as before. This results in optimal data migration when items are
       added to the bucket. The removal of items from the middle or the tail of
       the list, however, can result in a significant amount of unnecessary
       data movement. This means that list buckets are most suitable for
       circumstances in which they **never shrink or very rarely shrink**.

    #. **tree**: Tree buckets use a binary search tree. They are more efficient
       at dealing with buckets that contain many items than are list buckets.
       The behavior of tree buckets is governed by the :abbr:`RUSH (Replication
       Under Scalable Hashing)`:sub:`R` algorithm. Tree buckets reduce the
       placement time to 0(log\ :sub:`n`). This means that tree buckets are
       suitable for managing large sets of devices or nested buckets.

    #. **straw**: Straw buckets allow all items in the bucket to "compete" 
       against each other for replica placement through a process analogous to
       drawing straws. This is different from the behavior of list buckets and
       tree buckets, which use a divide-and-conquer strategy that either gives
       certain items precedence (for example, those at the beginning of a list)
       or obviates the need to consider entire subtrees of items. Such an
       approach 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, a removal, or the re-weighting of an
       item.

        * **straw2**: Straw2 buckets improve on Straw by correctly avoiding 
          any data movement between items when neighbor weights change. For
          example, if the weight of a given item changes (including during the
          operations of adding it to the cluster or removing it from the
          cluster), there will be data movement to or from only that item.
          Neighbor weights are not taken into account.


.. topic:: Hash

   Each bucket uses a hash algorithm. As of Reef, Ceph supports the
   ``rjenkins1`` algorithm. To select ``rjenkins1`` as the hash algorithm,
   enter ``0`` as your hash setting.

.. _weightingbucketitems:

.. topic:: Weighting Bucket Items

   Ceph expresses bucket weights as doubles, which allows for fine-grained
   weighting. A weight is the relative difference between device capacities. We
   recommend using ``1.00`` as the relative weight for a 1 TB storage device.
   In such a scenario, a weight of ``0.50`` would represent approximately 500
   GB, and a weight of ``3.00`` would represent approximately 3 TB. Buckets
   higher in the CRUSH hierarchy have a weight that is the sum of the weight of
   the leaf items aggregated by the bucket.


.. _crushmaprules:

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

CRUSH maps have rules that include data placement for a pool: these are
called "CRUSH rules". The default CRUSH map has one rule for each pool. If you
are running a large cluster, you might create many pools and each of those
pools might have its own non-default CRUSH rule.


.. note:: In most cases, there is no need to modify the default rule. When a
   new pool is created, by default the rule will be set to the value ``0``
   (which indicates the default CRUSH rule, which has the numeric ID ``0``).

CRUSH rules define policy that governs how data is distributed across the devices in
the hierarchy. The rules define placement as well as replication strategies or
distribution policies that allow you to specify exactly how CRUSH places data
replicas. For example, you might create one rule selecting a pair of targets for
two-way mirroring, another rule for selecting three targets in two different data
centers for three-way replication, and yet another rule for erasure coding across
six storage devices. For a detailed discussion of CRUSH rules, see **Section 3.2**
of `CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data`_.

A rule takes the following form::

    rule <rulename> {

        id [a unique integer ID]
        type [replicated|erasure]
        step take <bucket-name> [class <device-class>]
        step [choose|chooseleaf] [firstn|indep] <N> type <bucket-type>
        step emit
    }


``id``
   :Description: A unique integer that identifies the rule.
   :Purpose: A component of the rule mask.
   :Type: Integer
   :Required: Yes
   :Default: 0


``type``
   :Description: Denotes the type of replication strategy to be enforced by the
                 rule.
   :Purpose: A component of the rule mask.
   :Type: String
   :Required: Yes
   :Default: ``replicated``
   :Valid Values: ``replicated`` or ``erasure``


``step take <bucket-name> [class <device-class>]``
   :Description: Takes a bucket name and iterates down the tree. If
                 the ``device-class`` argument is specified, the argument must
                 match a class assigned to OSDs within the cluster. Only
                 devices belonging to the class are included.
   :Purpose: A component of the rule.
   :Required: Yes
   :Example: ``step take data``



``step choose firstn {num} type {bucket-type}``
   :Description: Selects ``num`` buckets of the given type from within the
                 current bucket. ``{num}`` is usually the number of replicas in
                 the pool (in other words, the pool size).

                 - If ``{num} == 0``, choose ``pool-num-replicas`` buckets (as many buckets as are available).
                 - If ``pool-num-replicas > {num} > 0``, choose that many buckets.
                 - If ``{num} < 0``, choose ``pool-num-replicas - {num}`` buckets.

   :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 the given type and chooses a leaf
                 node (that is, an OSD) from the subtree of each bucket in that set of buckets. The
                 number of buckets in the set is usually the number of replicas in
                 the pool (in other words, the pool size).

                 - If ``{num} == 0``, choose ``pool-num-replicas`` buckets (as many buckets as are available).
                 - If ``pool-num-replicas > {num} > 0``, choose that many buckets.
                 - If ``{num} < 0``, choose ``pool-num-replicas - {num}`` buckets.
   :Purpose: A component of the rule. Using ``chooseleaf`` obviates the need to select a device in a separate step.
   :Prerequisite: Follows ``step take`` or ``step choose``.
   :Example: ``step chooseleaf firstn 0 type row``


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

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

.. important:: A single CRUSH rule can be assigned to multiple pools, but
   a single pool cannot have multiple CRUSH rules.

``firstn`` or ``indep``

   :Description: Determines which replacement strategy CRUSH uses when items (OSDs)
                 are marked ``down`` in the CRUSH map. When this rule is used
                 with replicated pools, ``firstn`` is used. When this rule is
                 used with erasure-coded pools, ``indep`` is used.

                 Suppose that a PG is stored on OSDs 1, 2, 3, 4, and 5 and then
                 OSD 3 goes down.

                 When in ``firstn`` mode, CRUSH simply adjusts its calculation
                 to select OSDs 1 and 2, then selects 3 and discovers that 3 is
                 down, retries and selects 4 and 5, and finally goes on to
                 select a new OSD: OSD 6. The final CRUSH mapping
                 transformation is therefore 1, 2, 3, 4, 5 → 1, 2, 4, 5, 6.

                 However, if you were storing an erasure-coded pool, the above
                 sequence would have changed the data that is mapped to OSDs 4,
                 5, and 6. The ``indep`` mode attempts to avoid this unwanted
                 consequence. When in ``indep`` mode, CRUSH can be expected to
                 select 3, discover that 3 is down, retry, and select 6. The
                 final CRUSH mapping transformation is therefore 1, 2, 3, 4, 5
                 → 1, 2, 6, 4, 5.

.. _crush-reclassify:

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

Prior to the Luminous release's introduction of the *device class* feature, in
order to write rules that applied to a specialized device type (for example,
SSD), it was necessary to manually edit the CRUSH map and maintain a parallel
hierarchy for each device type. The device class feature provides a more
transparent way to achieve this end.

However, if your cluster is migrated from an existing manually-customized
per-device map to new device class-based rules, all data in the system will be
reshuffled.

The ``crushtool`` utility has several commands that can transform a legacy rule
and hierarchy and allow you to start using the new device class rules. There
are three possible types of transformation:

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

   This command examines everything under ``root-name`` in the hierarchy and
   rewrites any rules that reference the specified root and that have the
   form ``take <root-name>`` so that they instead have the
   form ``take <root-name> class <device-class>``. The command also renumbers
   the buckets in such a way that the old IDs are used for the specified
   class's "shadow tree" and as a result no data movement takes place.

   For example, suppose you have the following as an existing rule::

     rule replicated_rule {
        id 0
        type replicated
        step take default
        step chooseleaf firstn 0 type rack
        step emit
     }

   If the root ``default`` is reclassified as class ``hdd``, the new rule will
   be as follows::

     rule replicated_rule {
        id 0
        type replicated
        step take default class hdd
        step chooseleaf firstn 0 type rack
        step emit
     }

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

   This command marks every device in the subtree that is rooted at *bucket-name*
   with the specified device class.

   This command is typically used in conjunction with the ``--reclassify-root`` option
   in order to ensure that all devices in that root are labeled with the
   correct class. In certain circumstances, however, some of those devices
   are correctly labeled with a different class and must not be relabeled. To
   manage this difficulty, one can exclude the ``--set-subtree-class``
   option. The remapping process will not be perfect, because the previous rule
   had an effect on devices of multiple classes but the adjusted rules will map
   only to devices of the specified device class. However, when there are not many
   outlier devices, the resulting level of data movement is often within tolerable
   limits.


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

   This command allows you to merge a parallel type-specific hierarchy with the
   normal hierarchy. For example, many users have maps that resemble the
   following::

     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 command reclassifies each bucket that matches a certain
   pattern. The pattern can be of the form ``%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 (corresponding to 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 (for example, ``node12-ssd`` exists but ``node12`` does
   not), then it is created and linked under the specified
   *default parent* bucket. In each case, care is taken to preserve
   the old bucket IDs for the new shadow buckets in order to prevent data
   movement. Any rules with ``take`` steps that reference the old
   buckets are adjusted accordingly.


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

   The same command can also be used without a wildcard in order 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 that consists of the above fragments
   resembles the following:

   .. prompt:: bash $

      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

``--compare`` flag
------------------

A ``--compare`` flag is available to make sure that the conversion performed in
:ref:`Migrating from a legacy SSD rule to device classes <crush-reclassify>` is
correct. This flag tests a large sample of inputs against the CRUSH map and
checks that the expected result is output. The options that control these
inputs are the same as the options that apply to the ``--test`` command. For an
illustration of how this ``--compare`` command applies to the above example,
see the following:

.. prompt:: bash $

   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 the command finds any differences, the ratio of remapped inputs is reported
in the parentheses.

When you are satisfied with the adjusted map, apply it to the cluster by
running the following command:

.. prompt:: bash $

   ceph osd setcrushmap -i adjusted

Manually Tuning CRUSH
---------------------

If you have verified that all clients are running recent code, you can adjust
the CRUSH tunables by extracting the CRUSH map, modifying the values, and
reinjecting the map into the cluster. The procedure is carried out as follows:

#. Extract the latest CRUSH map:

   .. prompt:: bash $

      ceph osd getcrushmap -o /tmp/crush

#. Adjust tunables. In our tests, the following values appear to result in the 
   best behavior for both large and small clusters. The procedure requires that
   you specify the ``--enable-unsafe-tunables`` flag in the ``crushtool``
   command. Use this option with **extreme care**:

   .. prompt:: bash $

      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 the modified map:

   .. prompt:: bash $

     ceph osd setcrushmap -i /tmp/crush.new

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

To set the legacy values of the CRUSH tunables, run the following command:

.. prompt:: bash $

   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

The special ``--enable-unsafe-tunables`` flag is required. Be careful when
running old versions of the ``ceph-osd`` daemon after reverting to legacy
values, because the feature bit is not perfectly enforced.

.. _CRUSH - Controlled, Scalable, Decentralized Placement of Replicated Data: https://ceph.io/assets/pdfs/weil-crush-sc06.pdf
