Red Hat Training

A Red Hat training course is available for Red Hat Ceph Storage

Chapter 4. Pools

Ceph clients store data in pools. When you create pools, you are creating an I/O interface for clients to store data. From the perspective of a Ceph client (i.e., block device, gateway, etc.), interacting with the Ceph storage cluster is remarkably simple: create a cluster handle and connect to the cluster; then, create an I/O context for reading and writing objects and their extended attributes.

Create a Cluster Handle and Connect to the Cluster

To connect to the Ceph storage cluster, the Ceph client needs the cluster name (usually ceph by default) and an initial monitor address. Ceph clients usually retrieve these parameters using the default path for the Ceph configuration file and then read it from the file, but a user may also specify the parameters on the command line too. The Ceph client also provides a user name and secret key (authentication is on by default). Then, the client contacts the Ceph monitor cluster and retrieves a recent copy of the cluster map, including its monitors, OSDs and pools.

Create Handle

Create a Pool I/O Context

To read and write data, the Ceph client creates an i/o context to a specific pool in the Ceph storage cluster. If the specified user has permissions for the pool, the Ceph client can read from and write to the specified pool.

I/O Context

Ceph’s architecture enables the storage cluster to provide this remarkably simple interface to Ceph clients so that clients may select one of the sophisticated storage strategies you define simply by specifying a pool name and creating an I/O context. Storage strategies are invisible to the Ceph client in all but capacity and performance. Similarly, the complexities of Ceph clients (mapping objects into a block device representation, providing an S3/Swift RESTful service) are invisible to the Ceph storage cluster.

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 50-100 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, a CRUSH rule mapped to the pool enables CRUSH to identify the rule for the placement of each object and its replicas (or chunks for erasure coded pools) in your cluster. You can create a custom CRUSH rule for your pool.
  • Snapshots: When you create snapshots with ceph osd pool mksnap, you effectively take a snapshot of a particular pool.
  • Quotas: When you set quotas on a pool with ceph osd pool set-quota you may limit the maximum number of objects or the maximum number of bytes stored in the specified pool.

4.1. Pools and Storage Strategies

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

4.2. List Pools

To list your cluster’s pools, execute:

ceph osd lspools

4.3. Create a Pool

Before creating pools, see the Pool, PG and CRUSH Configuration Reference chapter in the Configuration Guide for Red Hat Ceph Storage 3.

Note

In Red Hat Ceph Storage 3 and later releases, system administrators must expressly enable a pool to receive I/O operations from Ceph clients. See Enable Application for details. Failure to enable a pool will result in a HEALTH_WARN status.

It is better to adjust the default value for the number of placement groups in the Ceph configuration file, as the default value does not have to suit your needs. For example:

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

To create a replicated pool, execute:

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

To create an erasure-coded pool, execute:

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. If not specified, it is set to the value listed in the Ceph configuration file or to the default value.
Default
ceph
pg-num
Description
The total number of placement groups for the pool. See the Placement Groups section and the Ceph Placement Groups (PGs) per Pool Calculator 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 value must be equal to the total number of placement groups, except for placement group splitting scenarios.
Type
Integer
Required
Yes. If not specified it is set to the value listed in the Ceph configuration file or to the default value.
Default
8
replicated or erasure
Description
The pool type which can be either 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-coded 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 the crush rule for the pool. The rule MUST exist. For replicated pools, the name is the rule specified by the osd_pool_default_crush_rule configuration setting. For erasure-coded pools the name is erasure-code if you specify the default erasure code profile or {pool-name} otherwise. Ceph creates this rule with the specified name implicitly if the rule doesn’t already exist.
Type
String
Required
No
Default
Uses erasure-code for an erasure-coded pool. For replicated pools, it uses the value of the osd_pool_default_crush_rule variable from the Ceph configuration.
expected-num-objects
Description
The expected number of objects for the pool. By setting this value together with a negative filestore_merge_threshold variable, Ceph splits the placement groups at pool creation time to avoid the latency impact to perform runtime directory splitting.
Type
Integer
Required
No
Default
0, no splitting at the pool creation time
erasure-code-profile
Description
For erasure-coded pools only. Use the erasure code profile. It must be an existing profile as defined by the osd erasure-code-profile set variable in the Ceph configuration file. For further information, see the Erasure Code Profiles section.
Type
String
Required
No

When you create a pool, set the number of placement groups to a reasonable value (for example to 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, for example, 50 pools with 100 placement groups each. The point of diminishing returns depends upon the power of the OSD host.

See the Placement Groups section and Ceph Placement Groups (PGs) per Pool Calculator for details on calculating an appropriate number of placement groups for your pool.

4.4. Set Pool Quotas

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

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.

Note

In-flight write operations may overrun pool quotas for a short time until Ceph propagates the pool usage across the cluster. This is normal behavior. Enforcing pool quotas on in-flight write operations would impose significant performance penalties.

4.5. Delete a Pool

To delete a pool, execute:

ceph osd pool delete <pool-name> [<pool-name> --yes-i-really-really-mean-it]
Important

To protect data, in RHCS 3 and later releases, administrators cannot delete pools by default. Set the mon_allow_pool_delete configuration option before deleting pools.

If a pool has its own rule, consider removing it after deleting the pool. If a pool has users strictly for its own use, consider deleting those users after deleting the pool.

4.6. 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.

4.7. Show Pool Statistics

To show a pool’s utilization statistics, execute:

rados df

4.8. Make a Snapshot of a Pool

To make a snapshot of a pool, execute:

ceph osd pool mksnap <pool-name> <snap-name>
Warning

If you create a pool snapshot, you will never be able to take RBD image snapshots within the pool and it will be irreversible.

4.9. Remove a Snapshot of a Pool

To remove a snapshot of a pool, execute:

ceph osd pool rmsnap <pool-name> <snap-name>

4.10. Set Pool Values

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

ceph osd pool set <pool-name> <key> <value>

The Pool Values section lists all key-values pairs that you can set.

4.11. Get Pool Values

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

ceph osd pool get <pool-name> <key>

The Pool Values section lists all key-values pairs that you can get.

4.12. Enable Application

RHCS 3 and later releases provide additional protection for pools to prevent unauthorized types of clients from writing data to the pool. This means that system administrators must expressly enable pools to receive I/O operations from Ceph Block Device, Ceph Object Gateway, Ceph Filesystem or for a custom application.

To enable a client application to conduct I/O operations on a pool, execute the following:

[root@host ~]# ceph osd pool application enable <poolname> <app> {--yes-i-really-mean-it}

Where <app> is:

  • cephfs for the Ceph Filesystem.
  • rbd for the Ceph Block Device
  • rgw for the Ceph Object Gateway
Note

Specify a different <app> value for a custom application.

Important

A pool that is not enabled will generate a HEALTH_WARN status. In that scenario, the output for ceph health detail -f json-pretty will output the following:

{
    "checks": {
        "POOL_APP_NOT_ENABLED": {
            "severity": "HEALTH_WARN",
            "summary": {
                "message": "application not enabled on 1 pool(s)"
            },
            "detail": [
                {
                    "message": "application not enabled on pool '<pool-name>'"
                },
                {
                    "message": "use 'ceph osd pool application enable <pool-name> <app-name>', where <app-name> is 'cephfs', 'rbd', 'rgw', or freeform for custom applications."
                }
            ]
        }
    },
    "status": "HEALTH_WARN",
    "overall_status": "HEALTH_WARN",
    "detail": [
        "'ceph health' JSON format has changed in luminous. If you see this your monitoring system is scraping the wrong fields. Disable this with 'mon health preluminous compat warning = false'"
    ]
}
NOTE
Initialize pools for the Ceph Block Device with rbd pool init <pool-name>.

4.13. Disable Application

To disable a client application from conducting I/O operations on a pool, execute the following:

[root@host ~]# ceph osd pool application disable <poolname> <app> {--yes-i-really-mean-it}

Where <app> is:

  • cephfs for the Ceph Filesystem.
  • rbd for the Ceph Block Device
  • rgw for the Ceph Object Gateway
Note

Specify a different <app> value for a custom application.

4.14. Set Application Metadata

RHCS 3 and later releases provide functionality to set key-value pairs describing attributes of the client application.

To set client application metadata on a pool, execute the following:

[root@host ~]# ceph osd pool application set <poolname> <app> <key> <value>

Where <app> is:

  • cephfs for the Ceph Filesystem.
  • rbd for the Ceph Block Device
  • rgw for the Ceph Object Gateway
Note

Specify a different <app> value for a custom application.

4.15. Remove Application Metadata

To remove client application metadata on a pool, execute the following:

[root@host ~]# ceph osd pool application set <poolname> <app> <key>

Where <app> is:

  • cephfs for the Ceph Filesystem.
  • rbd for the Ceph Block Device
  • rgw for the Ceph Object Gateway
Note

Specify a different <app> value for a custom application.

4.16. Set the Number of Object Replicas

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

ceph osd pool set <poolname> size <num-replicas>
Important

The <num-replicas> parameter includes the object itself. If you want to include 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 can execute this command for each pool.

Note

An object might accept I/O operations in degraded mode with fewer replicas than specified by the pool size setting. To set a minimum number of required replicas for I/O, 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 replicas than specified by the min_size setting.

4.17. Get the Number of Object Replicas

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

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, that is a total of three copies, or a size of 3.

4.18. Pool Values

The following list contains key-values pairs that you can set or get. For further information, see the Set Pool Values and Get Pool Values sections.

size
Description
Specifies the number of replicas for objects in the pool. See the Set the Number of Object Replicas section for further details. Applicable for the replicated pools only.
Type
Integer
min_size
Description
Specifies the minimum number of replicas required for I/O. See the Set the Number of Object Replicas section for further details. Applicable for the replicated pools only.
Type
Integer
crash_replay_interval
Description
Specifies the number of seconds to allow clients to replay acknowledged, but uncommitted requests.
Type
Integer
pg-num
Description
The total number of placement groups for the pool. See the Pool, PG and CRUSH Configuration Reference section in the Red Hat Ceph Storage 3 Configuration Guide 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
Valid Range
Equal to or less than what specified by the pg_num variable.
crush_rule
Description
The rule to use for mapping object placement in the cluster.
Type
String
hashpspool
Description
Enable or disable the HASHPSPOOL flag on a given pool. With this option enabled, pool hashing and placement group mapping are changed to improve the way pools and placement groups overlap.
Type
Integer
Valid Range
1 enables the flag, 0 disables the flag.
Important

Do not enable this option on production pools of a cluster with a large amount of OSDs and data. All placement groups in the pool would have to be remapped causing too much data movement.

fast_read
Description
On a pool that uses erasure coding, if this flag is enabled, the read request issues subsequent reads to all shards, and wait until it receives enough shards to decode to serve the client. In the case of the jerasure and isa erasure plug-ins, once the first K replies return, client’s request is served immediately using the data decoded from these replies. This helps to allocate some resources for better performance. Currently this flag is only supported for erasure coding pools.
Type
Boolean
Defaults
0
allow_ec_overwrites
Description
Whether writes to an erasure coded pool can update part of an object, so the Ceph Filesystem and Ceph Block Device can use it.
Type
Boolean
Version
RHCS 3 and later.
compression_algorithm
Description
Sets inline compression algorithm to use with the BlueStore storage backend. This setting overrides the bluestore_compression_algorithm configuration setting.
Type
String
Valid Settings
lz4, snappy, zlib, zstd
compression_mode
Description
Sets the policy for the inline compression algorithm for the BlueStore storage backend. This setting overrides the bluestore_compression_mode configuration setting.
Type
String
Valid Settings
none, passive, aggressive, force
compression_min_blob_size
Description
BlueStore will not compress chunks smaller than this size. This setting overrides the bluestore_compression_min_blob_size configuration setting.
Type
Unsigned Integer
compression_max_blob_size
Description
BlueStore will break chunks larger than this size into smaller blobs of compression_max_blob_size before compressing the data.
Type
Unsigned Integer
nodelete
Description
Set or unset the NODELETE flag on a given pool.
Type
Integer
Valid Range
1 sets flag. 0 unsets flag.
nopgchange
Description
Set or unset the NOPGCHANGE flag on a given pool.
Type
Integer
Valid Range
1 sets the flag. 0 unsets the flag.
nosizechange
Description
Set or unset the NOSIZECHANGE flag on a given pool.
Type
Integer
Valid Range
1 sets the flag. 0 unsets the flag.
write_fadvise_dontneed
Description
Set or unset the WRITE_FADVISE_DONTNEED flag on a given pool.
Type
Integer
Valid Range
1 sets the flag. 0 unsets the flag.
noscrub
Description
Set or unset the NOSCRUB flag on a given pool.
Type
Integer
Valid Range
1 sets the flag. 0 unsets the flag.

nodeep-scrub

Description
Set or unset the NODEEP_SCRUB flag on a given pool.
Type
Integer
Valid Range

1 sets the flag. 0 unsets the flag.

scrub_min_interval
Description
The minimum interval in seconds for pool scrubbing when load is low. If it is 0, Ceph uses the osd_scrub_min_interval configuration setting.
Type
Double
Default

0

scrub_max_interval
Description
The maximum interval in seconds for pool scrubbing irrespective of cluster load. If it is 0, Ceph uses the osd_scrub_max_interval configuration setting.
Type
Double
Default

0

deep_scrub_interval
Description
The interval in seconds for pool 'deep' scrubbing. If it is 0, Ceph uses the osd_deep_scrub_interval configuration setting.
Type
Double
Default
0