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Ceph Storage is a free and open source software-defined, distributed storage solution designed to be massively scalable for modern data analytics, artificial intelligence(AI), machine learning (ML), data analytics and emerging mission critical workloads. In this article, we will talk about how you can create Ceph Pool with a custom number of placement groups(PGs).

In Ceph terms, Placement groups (PGs) are shards or fragments of a logical object pool that place objects as a group into OSDs. Placement groups reduce the amount of per-object metadata when Ceph stores the data in OSDs.

A larger number of placement groups (e.g., 100 per OSD) leads to better balancing. The Ceph client will calculate which placement group an object should be in. It does this by hashing the object ID and applying an operation based on the number of PGs in the defined pool and the ID of the pool. See Mapping PGs to OSDs for details.

Calculating total number of Placement Groups.

             (OSDs * 100)
Total PGs =  ------------
              pool size

For example, let’s say your cluster has 9 OSDs, and default pool size of 3. So your PGs will be

             9 * 100
Total PGs =  ------------ = 300
              3

Create a Pool

To syntax for creating a pool is:

ceph osd pool create {pool-name} {pg-num}

Where:

  • {pool-name} – The name of the pool. It must be unique.
  • {pg-num} – The total number of placement groups for the pool.

I’ll create a new pool named k8s-uat with placement groups count of 100.

$ sudo ceph osd pool create k8s-uat 100
pool 'k8s-uat' created

Now list available pools to confirm it was created.

$ sudo ceph osd lspools
1 .rgw.root
2 default.rgw.control
3 default.rgw.meta
4 default.rgw.log
5 k8s-uat

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.

--- Ceph Filesystem ---
$ sudo ceph osd pool application enable <pool-name> cephfs

--- Ceph Block Device ---
$ sudo ceph osd pool application enable <pool-name> rbd

--- Ceph Object Gateway ---
 $ sudo ceph osd pool application enable <pool-name> rgw

Example:

$ sudo ceph osd pool application enable k8s-uat-rbd rbd
enabled application 'rbd' on pool 'k8s-uat-rbd'

Pools that are intended for use with RBD should be initialized using the rbd tool:

sudo rbd pool init k8s-uat-rbd

To disable app, use:

ceph osd pool application disable <poolname> <app> {--yes-i-really-mean-it}

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

$ sudo ceph osd pool stats [{pool-name}]

Doing it from Ceph Dashboard

Login to your Ceph Management Dashboard and create a new Pool – Pools > Create

Delete a Pool

To delete a pool, execute:

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

More articles on Ceph will be published in few weeks to come. Stay connected.

Consumption guides:

Ceph Persistent Storage for Kubernetes with Cephfs

Persistent Storage for Kubernetes with Ceph RBD

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