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+==========================
+ Journal Config Reference
+==========================
+
+.. index:: journal; journal configuration
+
+Ceph OSDs use a journal for two reasons: speed and consistency.
+
+- **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 filesystem 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 filesystem catches up to the
+ journal.
+
+- **Consistency:** Ceph OSD Daemons require a filesystem interface that
+ guarantees atomic compound operations. Ceph OSD Daemons write a description
+ of the operation to the journal and apply the operation to the filesystem.
+ 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 filesystem, 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 support 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``
+
+.. versionchanged:: 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``