Notice

This document is for a development version of Ceph.

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 interval and 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:

journal_dio

Enables direct i/o to the journal. Requires journal block align set to true.

type

bool

default

true

journal_aio

Enables using libaio for asynchronous writes to the journal. Requires journal dio set to true. Version 0.61 and later, true. Version 0.60 and earlier, false.

type

bool

default

true

journal_block_align

Block aligns write operations. Required for dio and aio.

type

bool

default

true

journal_max_write_bytes

The maximum number of bytes the journal will write at any one time.

type

size

default

10Mi

journal_max_write_entries

The maximum number of entries the journal will write at any one time.

type

int

default

100

journal_align_min_size

Align data payloads greater than the specified minimum.

type

size

default

64Ki

journal_zero_on_create

Causes the file store to overwrite the entire journal with 0’s during mkfs.

type

bool

default

false