# 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

Description

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

Type

Boolean

Required

Yes when using aio.

Default

true

journal_aio

Changed in version 0.61: Cuttlefish

Description

Enables using libaio for asynchronous writes to the journal. Requires journal dio set to true.

Type

Boolean

Required

No.

Default

Version 0.61 and later, true. Version 0.60 and earlier, false.

journal_block_align

Description

Block aligns write operations. Required for dio and aio.

Type

Boolean

Required

Yes when using dio and aio.

Default

true

journal_max_write_bytes

Description

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

Type

Integer

Required

No

Default

10 << 20

journal_max_write_entries

Description

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

Type

Integer

Required

No

Default

100

journal_queue_max_ops

Description

The maximum number of operations allowed in the queue at any one time.

Type

Integer

Required

No

Default

500

journal_queue_max_bytes

Description

The maximum number of bytes allowed in the queue at any one time.

Type

Integer

Required

No

Default

10 << 20

journal_align_min_size

Description

Align data payloads greater than the specified minimum.

Type

Integer

Required

No

Default

64 << 10

journal_zero_on_create

Description

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

Type

Boolean

Required

No

Default

false