Pools

When you first deploy a cluster without creating a pool, Ceph uses the default pools for storing data. A pool provides you with:

  • Resilience: You can set how many OSD are allowed to fail without losing data. For replicated pools, it is the desired number of copies/replicas of an object. A typical configuration stores an object and one additional copy (i.e., size = 2), but you can determine the number of copies/replicas. For erasure coded pools, it is the number of coding chunks (i.e. m=2 in the erasure code profile)

  • Placement Groups: You can set the number of placement groups for the pool. A typical configuration uses approximately 100 placement groups per OSD to provide optimal balancing without using up too many computing resources. When setting up multiple pools, be careful to ensure you set a reasonable number of placement groups for both the pool and the cluster as a whole.

  • CRUSH Rules: When you store data in a pool, placement of the object and its replicas (or chunks for erasure coded pools) in your cluster is governed by CRUSH rules. You can create a custom CRUSH rule for your pool if the default rule is not appropriate for your use case.

  • Snapshots: When you create snapshots with ceph osd pool mksnap, you effectively take a snapshot of a particular pool.

To organize data into pools, you can list, create, and remove pools. You can also view the utilization statistics for each pool.

List Pools

To list your cluster’s pools, execute:

ceph osd lspools

Create a Pool

Before creating pools, refer to the Pool, PG and CRUSH Config Reference. Ideally, you should override the default value for the number of placement groups in your Ceph configuration file, as the default is NOT ideal. For details on placement group numbers refer to setting the number of placement groups

Note

Starting with Luminous, all pools need to be associated to the application using the pool. See Associate Pool to Application below for more information.

For example:

osd pool default pg num = 100
osd pool default pgp num = 100

To create a pool, execute:

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

Where:

{pool-name}

Description

The name of the pool. It must be unique.

Type

String

Required

Yes.

{pg-num}

Description

The total number of placement groups for the pool. See Placement Groups for details on calculating a suitable number. The default value 8 is NOT suitable for most systems.

Type

Integer

Required

Yes.

Default

8

{pgp-num}

Description

The total number of placement groups for placement purposes. This should be equal to the total number of placement groups, except for placement group splitting scenarios.

Type

Integer

Required

Yes. Picks up default or Ceph configuration value if not specified.

Default

8

{replicated|erasure}

Description

The pool type which may either be replicated to recover from lost OSDs by keeping multiple copies of the objects or erasure to get a kind of generalized RAID5 capability. The replicated pools require more raw storage but implement all Ceph operations. The erasure pools require less raw storage but only implement a subset of the available operations.

Type

String

Required

No.

Default

replicated

[crush-rule-name]

Description

The name of a CRUSH rule to use for this pool. The specified rule must exist.

Type

String

Required

No.

Default

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

[erasure-code-profile=profile]

Description

For erasure pools only. Use the erasure code profile. It must be an existing profile as defined by osd erasure-code-profile set.

Type

String

Required

No.

When you create a pool, set the number of placement groups to a reasonable value (e.g., 100). Consider the total number of placement groups per OSD too. Placement groups are computationally expensive, so performance will degrade when you have many pools with many placement groups (e.g., 50 pools with 100 placement groups each). The point of diminishing returns depends upon the power of the OSD host.

See Placement Groups for details on calculating an appropriate number of placement groups for your pool.

[expected-num-objects]

Description

The expected number of objects for this pool. By setting this value ( together with a negative filestore merge threshold), the PG folder splitting would happen at the pool creation time, to avoid the latency impact to do a runtime folder splitting.

Type

Integer

Required

No.

Default

0, no splitting at the pool creation time.

Associate Pool to Application

Pools need to be associated with an application before use. Pools that will be used with CephFS or pools that are automatically created by RGW are automatically associated. Pools that are intended for use with RBD should be initialized using 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.:

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.

Set Pool Quotas

You can set pool quotas for the maximum number of bytes and/or the maximum number of objects per pool.

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.

Delete a Pool

To delete a pool, execute:

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

To remove a pool the mon_allow_pool_delete flag must be set to true in the Monitor’s configuration. Otherwise they will refuse to remove a pool.

See Monitor Configuration for more information.

If you created your own rules for a pool you created, you should consider removing them when you no longer need your pool:

ceph osd pool get {pool-name} crush_rule

If the rule was “123”, for example, you can check the other pools like so:

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

If no other pools use that custom rule, then it’s safe to delete that rule from the cluster.

If you created users with permissions strictly for a pool that no longer exists, you should consider deleting those users too:

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

Rename a Pool

To rename a pool, execute:

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

If you rename a pool and you have per-pool capabilities for an authenticated user, you must update the user’s capabilities (i.e., caps) with the new pool name.

Show Pool Statistics

To show a pool’s utilization statistics, execute:

rados df

Additionally, to obtain I/O information for a specific pool or all, execute:

ceph osd pool stats [{pool-name}]

Make a Snapshot of a Pool

To make a snapshot of a pool, execute:

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

Remove a Snapshot of a Pool

To remove a snapshot of a pool, execute:

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

Set Pool Values

To set a value to a pool, execute the following:

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

You may set values for the following keys:

compression_algorithm

Description

Sets inline compression algorithm to use for underlying BlueStore. This setting overrides the global setting of bluestore compression algorithm.

Type

String

Valid Settings

lz4, snappy, zlib, zstd

compression_mode

Description

Sets the policy for the inline compression algorithm for underlying BlueStore. This setting overrides the global setting of bluestore compression mode.

Type

String

Valid Settings

none, passive, aggressive, force

compression_min_blob_size

Description

Chunks smaller than this are never compressed. This setting overrides the global setting of bluestore compression min blob *.

Type

Unsigned Integer

compression_max_blob_size

Description

Chunks larger than this are broken into smaller blobs sizing compression_max_blob_size before being compressed.

Type

Unsigned Integer

size

Description

Sets the number of replicas for objects in the pool. See Set the Number of Object Replicas for further details. Replicated pools only.

Type

Integer

min_size

Description

Sets the minimum number of replicas required for I/O. See Set the Number of Object Replicas for further details. Replicated pools only.

Type

Integer

Version

0.54 and above

pg_num

Description

The effective number of placement groups to use when calculating data placement.

Type

Integer

Valid Range

Superior to pg_num current value.

pgp_num

Description

The effective number of placement groups for placement to use when calculating data placement.

Type

Integer

Valid Range

Equal to or less than pg_num.

crush_rule

Description

The rule to use for mapping object placement in the cluster.

Type

String

allow_ec_overwrites

Description

Whether writes to an erasure coded pool can update part of an object, so cephfs and rbd can use it. See Erasure Coding with Overwrites for more details.

Type

Boolean

Version

12.2.0 and above

hashpspool

Description

Set/Unset HASHPSPOOL flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

nodelete

Description

Set/Unset NODELETE flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

Version

Version FIXME

nopgchange

Description

Set/Unset NOPGCHANGE flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

Version

Version FIXME

nosizechange

Description

Set/Unset NOSIZECHANGE flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

Version

Version FIXME

write_fadvise_dontneed

Description

Set/Unset WRITE_FADVISE_DONTNEED flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

noscrub

Description

Set/Unset NOSCRUB flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

nodeep-scrub

Description

Set/Unset NODEEP_SCRUB flag on a given pool.

Type

Integer

Valid Range

1 sets flag, 0 unsets flag

hit_set_type

Description

Enables hit set tracking for cache pools. See Bloom Filter for additional information.

Type

String

Valid Settings

bloom, explicit_hash, explicit_object

Default

bloom. Other values are for testing.

hit_set_count

Description

The number of hit sets to store for cache pools. The higher the number, the more RAM consumed by the ceph-osd daemon.

Type

Integer

Valid Range

1. Agent doesn’t handle > 1 yet.

hit_set_period

Description

The duration of a hit set period in seconds for cache pools. The higher the number, the more RAM consumed by the ceph-osd daemon.

Type

Integer

Example

3600 1hr

hit_set_fpp

Description

The false positive probability for the bloom hit set type. See Bloom Filter for additional information.

Type

Double

Valid Range

0.0 - 1.0

Default

0.05

cache_target_dirty_ratio

Description

The percentage of the cache pool containing modified (dirty) objects before the cache tiering agent will flush them to the backing storage pool.

Type

Double

Default

.4

cache_target_dirty_high_ratio

Description

The percentage of the cache pool containing modified (dirty) objects before the cache tiering agent will flush them to the backing storage pool with a higher speed.

Type

Double

Default

.6

cache_target_full_ratio

Description

The percentage of the cache pool containing unmodified (clean) objects before the cache tiering agent will evict them 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

Temperature decay rate between two successive hit_sets

Type

Integer

Valid Range

0 - 100

Default

20

hit_set_search_last_n

Description

Count at most N appearance in hit_sets for temperature calculation

Type

Integer

Valid Range

0 - hit_set_count

Default

1

cache_min_flush_age

Description

The time (in seconds) before the cache tiering agent will flush an object from the cache pool to the storage pool.

Type

Integer

Example

600 10min

cache_min_evict_age

Description

The time (in seconds) before the cache tiering agent will evict an object from the cache pool.

Type

Integer

Example

1800 30min

fast_read

Description

On Erasure Coding pool, if this flag is turned on, the read request would issue sub reads to all shards, and waits until it receives enough shards to decode to serve the client. In the case of jerasure and isa erasure plugins, once the first K replies return, client’s request is served immediately using the data decoded from these replies. This helps to tradeoff some resources for better performance. Currently this flag is only supported for Erasure Coding pool.

Type

Boolean

Defaults

0

scrub_min_interval

Description

The minimum interval in seconds for pool scrubbing when load is low. If it is 0, the value osd_scrub_min_interval from config is used.

Type

Double

Default

0

scrub_max_interval

Description

The maximum interval in seconds for pool scrubbing irrespective of cluster load. If it is 0, the value osd_scrub_max_interval from config is used.

Type

Double

Default

0

deep_scrub_interval

Description

The interval in seconds for pool “deep” scrubbing. If it is 0, the value osd_deep_scrub_interval from config is used.

Type

Double

Default

0

recovery_priority

Description

When a value is set it will increase or decrease the computed reservation priority. This value must be in the range -10 to 10. Use a negative priority for less important pools so they have lower priority than any new pools.

Type

Integer

Default

0

recovery_op_priority

Description

Specify the recovery operation priority for this pool instead of osd_recovery_op_priority.

Type

Integer

Default

0

Get Pool Values

To get a value from a pool, execute the following:

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

You may get values for 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

Set the Number of Object Replicas

To set the number of object replicas on a replicated pool, execute the following:

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

Important

The {num-replicas} includes the object itself. If you want the object and two copies of the object for a total of three instances of the object, specify 3.

For example:

ceph osd pool set data size 3

You may execute this 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 required replicas for I/O, you should use the min_size setting. For example:

ceph osd pool set data min_size 2

This ensures that no object in the data pool will receive I/O with fewer than min_size replicas.

Get the Number of Object Replicas

To get the number of object replicas, execute the following:

ceph osd dump | grep 'replicated size'

Ceph will list the pools, with the replicated size attribute highlighted. By default, ceph creates two replicas of an object (a total of three copies, or a size of 3).