Journal

Ceph OSDs use the journal to achieve fast speed and consistency.

• Speed: The journal enables OSDs to quickly submit small write data blocks. Ceph writes small data blocks and random I/Os to the journal. In this way, the back-end file system can merge write operations and improve the concurrent bearing capacity. Therefore, the OSD journal can provide excellent burst write performance. Actually, data has not been written to an OSD because the file system captures the data in the journal.
• Consistency: Ceph OSDs require a file system interface that can ensure atomic operations. An OSD writes the description of an operation to the journal and then applies the operation to the file system. This requires the atomic update of an object (for example, the metadata of a PG). The OSD stops writing data and synchronizes the journal to the file system at intervals. In this way, the OSD can modify operations in the journal and reuse space. If the synchronization fails, the OSD replays the journal from the previous synchronization point.