Write intensive SSDs are the most expensive type; they will often offer write endurances up to and over 10 DWPD. They should be used for journals for spinning disks in Ceph clusters or also for SSD-only OSDs if very heavy write workloads are planned.

Currently, Ceph uses filestore as its method of storing objects on disks. The details of how and why filestore works is covered later in Chapter 3, BlueStore. For now, it's important to understand that due to the limitations in normal POSIX filesystems to be able to provide atomic transactions to the several pieces of data Ceph needs to write a journal is used. If no separate SSD is used for the journal, a separate partition is created for it. Every write that the OSD handles will first be written to the journal and then flushed to the main storage area on the disk. This is the main reason why using SSD for a journal for spinning disks is advised. The double write severely impacts spinning disk performance, which is mainly caused by the random nature of the disk heads moving between the journal and data areas.

Likewise, SSD OSD still requires a journal, and so it will experience approximately double the number of writes and thus provide half the client performance expected.

As can now be seen, not all models of SSDs are equal, and Ceph's requirements can make choosing the correct one a tough process. Fortunately, a quick test can be carried out to establish SSD's potential for use as a Ceph journal.