Chapter 2. Configuring Storage Strategies
Configuring storage strategies is about assigning Ceph OSDs to a CRUSH hierarchy, defining the number of placement groups for a pool, and creating a pool. The general steps are:
- Define a Storage Strategy: Storage strategies require you to analyze your use case, cost/benefit performance tradeoffs and data durability. Then, you create OSDs suitable for that use case. For example, you can create SSD-backed OSDs for a high performance pool or cache tier; SAS drive/SSD journal-backed OSDs for high-performance block device volumes and images; or, SATA-backed OSDs for low cost storage. Ideally, each OSD for a use case should have the same hardware configuration so that you have a consistent performance profile.
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Define a CRUSH Hierarchy: Ceph rules select a node (usually the
root) in a CRUSH hierarchy, and identify the appropriate OSDs for storing placement groups and the objects they contain. You must create a CRUSH hierarchy and a CRUSH rule for your storage strategy (cache tiers require one for the hot tier and one for the cold or backing tier). CRUSH hierarchies get assigned directly to a pool by the CRUSH ruleset setting; however, cache tiers are assigned to the backing tier, and the backing tier ruleset gets assigned to the pool. - Calculate Placement Groups: Ceph shards a pool into placement groups. You need to set an appropriate number of placement groups for your pool, and remain within a healthy maximum number of placement groups in the event that you assign multiple pools to the same CRUSH ruleset.
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Create a Pool: Finally, you must create a pool and determine whether it uses replicated or erasure-coded storage and whether it supports a cache tier. You must set the number of placement groups for the pool, the ruleset for the pool and the durability (size or
K+Mcoding chunks).
Remember, the pool is the Ceph client’s interface to the storage cluster, but the storage strategy is completely invisible to the Ceph client (except for capacity and performance).
