Chapter 9. Journal Configuration Reference

Ceph OSDs use a journal for the following reasons:

Speed: The journal enables the Ceph OSD Daemon to commit small write operations 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 write operations. 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 settings—the Ceph OSD stops write operations and synchronizes the journal with the file system, allowing Ceph OSDs to trim operations from the journal and reuse the space. On failure, Ceph OSDs replay the journal starting after the last synchronization operation.

9.1. Settings
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Ceph OSD Daemons support the following journal settings:

journal_dio

Description: Enables direct I/O to the journal. Requires the journal block align option set to true.
Type: Boolean
Required: Yes when using aio.
Default: true

journal_aio

Description: Enables using libaio for asynchronous writes to the journal. Requires the journal dio option set to true.
Type: Boolean
Required: No.
Default: true.

journal_block_align

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

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