Device class

The Ceph CRUSH map provides a lot of flexibility in controlling data placement.

The flexibility in controlling data placement is one of Ceph’s greatest strengths. Early Ceph deployments used hard disk drives almost exclusively. Today, Ceph clusters are frequently built with multiple types of storage devices: HDD, SSD, NVMe, or even various classes of the foregoing. For example, it is common in Ceph Object Gateway deployments to have storage policies where clients can store data on slower HDDs and other storage policies for storing data on fast SSDs. Ceph Object Gateway deployments might even have a pool backed by fast SSDs for bucket indices. Additionally, OSD nodes also frequently have SSDs used exclusively for journals or write-ahead logs that do NOT appear in the CRUSH map. These complex hardware scenarios historically required manually editing the CRUSH map, which can be time-consuming and tedious. It is not required to have different CRUSH hierarchies for different classes of storage devices.

CRUSH rules work in terms of the CRUSH hierarchy. However, if different classes of storage devices reside in the same hosts, the process becomes more complicated—requiring users to create multiple CRUSH hierarchies for each class of device, and then disable the osd crush update on start option that automates much of the CRUSH hierarchy management. Device classes eliminate this tediousness by telling the CRUSH rule what class of device to use, dramatically simplifying CRUSH management tasks.
Note: The ceph osd tree command has a column reflecting a device class.