Journal Config Reference
Filestore OSDs use a journal for two reasons: speed and consistency. Note that since Luminous, the BlueStore OSD back end has been preferred and default. This information is provided for pre-existing OSDs and for rare situations where Filestore is preferred for new deployments.
Speed: The journal enables the Ceph OSD Daemon to commit small writes quickly. Ceph writes small, random i/o to the journal sequentially, which tends to speed up bursty workloads by allowing the backing file system more time to coalesce writes. The Ceph OSD Daemon's journal, however, can lead to spiky performance with short spurts of high-speed writes followed by periods without any write progress as the file system catches up to the journal.
Consistency: Ceph OSD Daemons require a file system interface that guarantees atomic compound operations. Ceph OSD Daemons write a description of the operation to the journal and apply the operation to the file system. This enables atomic updates to an object (for example, placement group metadata). Every few seconds--between
filestore max sync intervaland
filestore min sync interval--the Ceph OSD Daemon stops writes and synchronizes the journal with the file system, allowing Ceph OSD Daemons to trim operations from the journal and reuse the space. On failure, Ceph OSD Daemons replay the journal starting after the last synchronization operation.
Ceph OSD Daemons recognize the following journal settings:
Enables direct i/o to the journal. Requires
journal block alignset to
libaiofor asynchronous writes to the journal. Requires
journal dioset to
true. Version 0.61 and later,
true. Version 0.60 and earlier,
Block aligns write operations. Required for
The maximum number of bytes the journal will write at any one time.
The maximum number of entries the journal will write at any one time.
Align data payloads greater than the specified minimum.
Causes the file store to overwrite the entire journal with