Pools

Pools are logical partitions that are used to store objects.

Pools provide:

  • Resilience: It is possible to set the number of OSDs that are allowed to fail without any data being lost. If your cluster uses replicated pools, the number of OSDs that can fail without data loss is equal to the number of replicas.

    For example: a typical configuration stores an object and two replicas (copies) of each RADOS object (that is: size = 3), but you can configure the number of replicas on a per-pool basis. For erasure-coded pools, resilience is defined as the number of coding chunks (for example, m = 2 in the default erasure code profile).

  • Placement Groups: You can set the number of placement groups (PGs) for the pool. In a typical configuration, the target number of PGs is approximately one hundred PGs per OSD. This provides reasonable balancing without consuming excessive computing resources. When setting up multiple pools, be careful to set an appropriate number of PGs for each pool and for the cluster as a whole. Each PG belongs to a specific pool: when multiple pools use the same OSDs, make sure that the sum of PG replicas per OSD is in the desired PG-per-OSD target range. To calculate an appropriate number of PGs for your pools, use the pgcalc tool.

  • CRUSH Rules: When data is stored in a pool, the placement of the object and its replicas (or chunks, in the case of erasure-coded pools) in your cluster is governed by CRUSH rules. Custom CRUSH rules can be created for a pool if the default rule does not fit your use case.

  • Snapshots: The command ceph osd pool mksnap creates a snapshot of a pool.

Pool Names

Pool names beginning with . are reserved for use by Ceph’s internal operations. Do not create or manipulate pools with these names.

List Pools

To list your cluster’s pools, run the following command:

ceph osd lspools

Creating a Pool

Before creating a pool, consult Pool, PG and CRUSH Config Reference. The Ceph central configuration database in the monitor cluster contains a setting (namely, pg_num) that determines the number of PGs per pool when a pool has been created and no per-pool value has been specified. It is possible to change this value from its default. For more on the subject of setting the number of PGs per pool, see setting the number of placement groups.

Note

In Luminous and later releases, each pool must be associated with the application that will be using the pool. For more information, see Associating a Pool with an Application below.

To create a pool, run one of the following commands:

ceph osd pool create {pool-name} [{pg-num} [{pgp-num}]] [replicated] \
         [crush-rule-name] [expected-num-objects]

or:

ceph osd pool create {pool-name} [{pg-num} [{pgp-num}]]   erasure \
         [erasure-code-profile] [crush-rule-name] [expected_num_objects] [--autoscale-mode=<on,off,warn>]

For a brief description of the elements of the above commands, consult the following:

{pool-name}

The name of the pool. It must be unique.

Type

String

Required

Yes.

{pg-num}

The total number of PGs in the pool. For details on calculating an appropriate number, see Placement Groups. The default value 8 is NOT suitable for most systems.

Type

Integer

Required

Yes.

Default

8

{pgp-num}

The total number of PGs for placement purposes. This should be equal to the total number of PGs, except briefly while pg_num is being increased or decreased.

Type

Integer

Required

Yes. If no value has been specified in the command, then the default value is used (unless a different value has been set in Ceph configuration).

Default

8

{replicated|erasure}

The pool type. This can be either replicated (to recover from lost OSDs by keeping multiple copies of the objects) or erasure (to achieve a kind of generalized parity RAID capability). The replicated pools require more raw storage but can implement all Ceph operations. The erasure pools require less raw storage but can perform only some Ceph tasks and may provide decreased performance.

Type

String

Required

No.

Default

replicated

[crush-rule-name]

The name of the CRUSH rule to use for this pool. The specified rule must exist; otherwise the command will fail.

Type

String

Required

No.

Default

For replicated pools, it is the rule specified by the osd_pool_default_crush_rule configuration variable. This rule must exist. For erasure pools, it is the erasure-code rule if the default erasure code profile is used or the {pool-name} rule if not. This rule will be created implicitly if it doesn’t already exist.

[erasure-code-profile=profile]

For erasure pools only. Instructs Ceph to use the specified erasure code profile. This profile must be an existing profile as defined by osd erasure-code-profile set.

Type

String

Required

No.

--autoscale-mode=<on,off,warn>
  • on: the Ceph cluster will autotune or recommend changes to the number of PGs in your pool based on actual usage.

  • warn: the Ceph cluster will autotune or recommend changes to the number of PGs in your pool based on actual usage.

  • off: refer to Placement Groups for more information.

Type

String

Required

No.

Default

The default behavior is determined by the osd_pool_default_pg_autoscale_mode option.

[expected-num-objects]

The expected number of RADOS objects for this pool. By setting this value and assigning a negative value to filestore merge threshold, you arrange for the PG folder splitting to occur at the time of pool creation and avoid the latency impact that accompanies runtime folder splitting.

Type

Integer

Required

No.

Default

0, no splitting at the time of pool creation.

Associating a Pool with an Application

Pools need to be associated with an application before they can be used. Pools that are intended for use with CephFS and pools that are created automatically by RGW are associated automatically. Pools that are intended for use with RBD should be initialized with the rbd tool (see Block Device Commands for more information).

For other cases, you can manually associate a free-form application name to a pool by running the following command.:

ceph osd pool application enable {pool-name} {application-name}

Note

CephFS uses the application name cephfs, RBD uses the application name rbd, and RGW uses the application name rgw.

Setting Pool Quotas

To set pool quotas for the maximum number of bytes and/or the maximum number of RADOS objects per pool, run the following command:

ceph osd pool set-quota {pool-name} [max_objects {obj-count}] [max_bytes {bytes}]

For example:

ceph osd pool set-quota data max_objects 10000

To remove a quota, set its value to 0.

Deleting a Pool

To delete a pool, run a command of the following form:

ceph osd pool delete {pool-name} [{pool-name} --yes-i-really-really-mean-it]

To remove a pool, you must set the mon_allow_pool_delete flag to true in the monitor’s configuration. Otherwise, monitors will refuse to remove pools.

For more information, see Monitor Configuration.

If there are custom rules for a pool that is no longer needed, consider deleting those rules.

ceph osd pool get {pool-name} crush_rule

For example, if the custom rule is “123”, check all pools to see whether they use the rule by running the following command:

ceph osd dump | grep "^pool" | grep "crush_rule 123"

If no pools use this custom rule, then it is safe to delete the rule from the cluster.

Similarly, if there are users with permissions restricted to a pool that no longer exists, consider deleting those users by running commands of the following forms:

ceph auth ls | grep -C 5 {pool-name}
ceph auth del {user}

Renaming a Pool

To rename a pool, run a command of the following form:

ceph osd pool rename {current-pool-name} {new-pool-name}

If you rename a pool for which an authenticated user has per-pool capabilities, you must update the user’s capabilities (“caps”) to refer to the new pool name.

Showing Pool Statistics

To show a pool’s utilization statistics, run the following command:

rados df

To obtain I/O information for a specific pool or for all pools, run a command of the following form:

ceph osd pool stats [{pool-name}]

Making a Snapshot of a Pool

To make a snapshot of a pool, run a command of the following form:

ceph osd pool mksnap {pool-name} {snap-name}

Removing a Snapshot of a Pool

To remove a snapshot of a pool, run a command of the following form:

ceph osd pool rmsnap {pool-name} {snap-name}

Setting Pool Values

To assign values to a pool’s configuration keys, run a command of the following form:

ceph osd pool set {pool-name} {key} {value}

You may set values for the following keys:

compression_algorithm
Description

Sets the inline compression algorithm used in storing data on the underlying BlueStore back end. This key’s setting overrides the global setting bluestore_compression_algorithm.

Type

String

Valid Settings

lz4, snappy, zlib, zstd

compression_mode
Description

Sets the policy for the inline compression algorithm used in storing data on the underlying BlueStore back end. This key’s setting overrides the global setting bluestore_compression_mode.

Type

String

Valid Settings

none, passive, aggressive, force

compression_min_blob_size
Description

Sets the minimum size for the compression of chunks: that is, chunks smaller than this are not compressed. This key’s setting overrides the following global settings:

Type

Unsigned Integer

compression_max_blob_size
Description

Sets the maximum size for chunks: that is, chunks larger than this are broken into smaller blobs of this size before compression is performed.

Type

Unsigned Integer

size
Description

Sets the number of replicas for objects in the pool. For further details, see Setting the Number of RADOS Object Replicas. Replicated pools only.

Type

Integer

min_size
Description

Sets the minimum number of replicas required for I/O. For further details, see Setting the Number of RADOS Object Replicas. For erasure-coded pools, this should be set to a value greater than ‘k’. If I/O is allowed at the value ‘k’, then there is no redundancy and data will be lost in the event of a permanent OSD failure. For more information, see Erasure Code

Type

Integer

Version

0.54 and above

pg_num
Description

Sets the effective number of PGs to use when calculating data placement.

Type

Integer

Valid Range

0 to mon_max_pool_pg_num. If set to 0, the value of osd_pool_default_pg_num will be used.

pgp_num
Description

Sets the effective number of PGs to use when calculating data placement.

Type

Integer

Valid Range

Between 1 and the current value of pg_num.

crush_rule
Description

Sets the CRUSH rule that Ceph uses to map object placement within the pool.

Type

String

allow_ec_overwrites
Description

Determines whether writes to an erasure-coded pool are allowed to update only part of a RADOS object. This allows CephFS and RBD to use an EC (erasure-coded) pool for user data (but not for metadata). For more details, see Erasure Coding with Overwrites.

Type

Boolean

New in version 12.2.0.

hashpspool
Description

Sets and unsets the HASHPSPOOL flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

nodelete
Description

Sets and unsets the NODELETE flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

Version

Version FIXME

nopgchange
Description

Sets and unsets the NOPGCHANGE flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

Version

Version FIXME

nosizechange
Description

Sets and unsets the NOSIZECHANGE flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

Version

Version FIXME

bulk
Description

Sets and unsets the bulk flag on a given pool.

Type

Boolean

Valid Range

true/1 sets flag, false/0 unsets flag

write_fadvise_dontneed
Description

Sets and unsets the WRITE_FADVISE_DONTNEED flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

noscrub
Description

Sets and unsets the NOSCRUB flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

nodeep-scrub
Description

Sets and unsets the NODEEP_SCRUB flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

hit_set_type
Description

Enables HitSet tracking for cache pools. For additional information, see Bloom Filter.

Type

String

Valid Settings

bloom, explicit_hash, explicit_object

Default

bloom. Other values are for testing.

hit_set_count
Description

Determines the number of HitSets to store for cache pools. The higher the value, the more RAM is consumed by the ceph-osd daemon.

Type

Integer

Valid Range

1. Agent doesn’t handle > 1 yet.

hit_set_period
Description

Determines the duration of a HitSet period (in seconds) for cache pools. The higher the value, the more RAM is consumed by the ceph-osd daemon.

Type

Integer

Example

3600 (3600 seconds: one hour)

hit_set_fpp
Description

Determines the probability of false positives for the bloom HitSet type. For additional information, see Bloom Filter.

Type

Double

Valid Range

0.0 - 1.0

Default

0.05

cache_target_dirty_ratio
Description

Sets a flush threshold for the percentage of the cache pool containing modified (dirty) objects. When this threshold is reached, the cache-tiering agent will flush these objects to the backing storage pool.

Type

Double

Default

.4

cache_target_dirty_high_ratio
Description

Sets a flush threshold for the percentage of the cache pool containing modified (dirty) objects. When this threshold is reached, the cache-tiering agent will flush these objects to the backing storage pool with a higher speed (as compared with cache_target_dirty_ratio).

Type

Double

Default

.6

cache_target_full_ratio
Description

Sets an eviction threshold for the percentage of the cache pool containing unmodified (clean) objects. When this threshold is reached, the cache-tiering agent will evict these objects from the cache pool.

Type

Double

Default

.8

target_max_bytes
Description

Ceph will begin flushing or evicting objects when the max_bytes threshold is triggered.

Type

Integer

Example

1000000000000 #1-TB

target_max_objects
Description

Ceph will begin flushing or evicting objects when the max_objects threshold is triggered.

Type

Integer

Example

1000000 #1M objects

hit_set_grade_decay_rate
Description

Sets the temperature decay rate between two successive HitSets.

Type

Integer

Valid Range

0 - 100

Default

20

hit_set_search_last_n
Description

Count at most N appearances in HitSets. Used for temperature calculation.

Type

Integer

Valid Range

0 - hit_set_count

Default

1

cache_min_flush_age
Description

Sets the time (in seconds) before the cache-tiering agent flushes an object from the cache pool to the storage pool.

Type

Integer

Example

600 (600 seconds: ten minutes)

cache_min_evict_age
Description

Sets the time (in seconds) before the cache-tiering agent evicts an object from the cache pool.

Type

Integer

Example

1800 (1800 seconds: thirty minutes)

fast_read
Description

For erasure-coded pools, if this flag is turned on, the read request issues “sub reads” to all shards, and then waits until it receives enough shards to decode before it serves the client. If jerasure or isa erasure plugins are in use, then after the first K replies have returned, the client’s request is served immediately using the data decoded from these replies. This approach sacrifices resources in exchange for better performance. This flag is supported only for erasure-coded pools.

Type

Boolean

Defaults

0

scrub_min_interval
Description

Sets the minimum interval (in seconds) for successive scrubs of the pool’s PGs when the load is low. If the default value of 0 is in effect, then the value of osd_scrub_min_interval from central config is used.

Type

Double

Default

0

scrub_max_interval
Description

Sets the maximum interval (in seconds) for scrubs of the pool’s PGs regardless of cluster load. If the value of scrub_max_interval is 0, then the value osd_scrub_max_interval from central config is used.

Type

Double

Default

0

deep_scrub_interval
Description

Sets the interval (in seconds) for pool “deep” scrubs of the pool’s PGs. If the value of deep_scrub_interval is 0, the value osd_deep_scrub_interval from central config is used.

Type

Double

Default

0

recovery_priority
Description

Setting this value adjusts a pool’s computed reservation priority. This value must be in the range -10 to 10. Any pool assigned a negative value will be given a lower priority than any new pools, so users are directed to assign negative values to low-priority pools.

Type

Integer

Default

0

recovery_op_priority
Description

Sets the recovery operation priority for a specific pool’s PGs. This overrides the general priority determined by osd_recovery_op_priority.

Type

Integer

Default

0

Getting Pool Values

To get a value from a pool’s key, run a command of the following form:

ceph osd pool get {pool-name} {key}

You may get values from the following keys:

size

Description

See size.

Type

Integer

min_size

Description

See min_size.

Type

Integer

Version

0.54 and above

pg_num

Description

See pg_num.

Type

Integer

pgp_num

Description

See pgp_num.

Type

Integer

Valid Range

Equal to or less than pg_num.

crush_rule

Description

See crush_rule.

hit_set_type

Description

See hit_set_type.

Type

String

Valid Settings

bloom, explicit_hash, explicit_object

hit_set_count

Description

See hit_set_count.

Type

Integer

hit_set_period

Description

See hit_set_period.

Type

Integer

hit_set_fpp

Description

See hit_set_fpp.

Type

Double

cache_target_dirty_ratio

Description

See cache_target_dirty_ratio.

Type

Double

cache_target_dirty_high_ratio

Description

See cache_target_dirty_high_ratio.

Type

Double

cache_target_full_ratio

Description

See cache_target_full_ratio.

Type

Double

target_max_bytes

Description

See target_max_bytes.

Type

Integer

target_max_objects

Description

See target_max_objects.

Type

Integer

cache_min_flush_age

Description

See cache_min_flush_age.

Type

Integer

cache_min_evict_age

Description

See cache_min_evict_age.

Type

Integer

fast_read

Description

See fast_read.

Type

Boolean

scrub_min_interval

Description

See scrub_min_interval.

Type

Double

scrub_max_interval

Description

See scrub_max_interval.

Type

Double

deep_scrub_interval

Description

See deep_scrub_interval.

Type

Double

allow_ec_overwrites

Description

See allow_ec_overwrites.

Type

Boolean

recovery_priority

Description

See recovery_priority.

Type

Integer

recovery_op_priority

Description

See recovery_op_priority.

Type

Integer

Setting the Number of RADOS Object Replicas

To set the number of data replicas on a replicated pool, run a command of the following form:

ceph osd pool set {poolname} size {num-replicas}

Important

The {num-replicas} argument includes the primary object itself. For example, if you want there to be two replicas of the object in addition to the original object (for a total of three instances of the object) specify 3 by running the following command:

ceph osd pool set data size 3

You may run the above command for each pool.

Note

An object might accept I/Os in degraded mode with fewer than pool size replicas. To set a minimum number of replicas required for I/O, you should use the min_size setting. For example, you might run the following command:

ceph osd pool set data min_size 2

This command ensures that no object in the data pool will receive I/O if it has fewer than min_size (in this case, two) replicas.

Getting the Number of Object Replicas

To get the number of object replicas, run the following command:

ceph osd dump | grep 'replicated size'

Ceph will list pools and highlight the replicated size attribute. By default, Ceph creates two replicas of an object (a total of three copies, for a size of 3).

Managing pools that are flagged with --bulk

See Managing pools that are flagged with bulk.