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96 lines
2.7 KiB
ReStructuredText
96 lines
2.7 KiB
ReStructuredText
==========================
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Journal Config Reference
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==========================
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Ceph OSDs use a journal for two reasons: speed and consistency.
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- **Speed:** The journal enables the OSD to commit small writes quickly.
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Ceph writes small, random i/o to the journal sequentially, which tends to
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speed up bursty workloads by allowing the backing filesystem more time to
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coalesce writes. The OSD journal, however, can lead to spiky performance
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with short spurts of high-speed writes followed by periods without any
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write progress as the filesystem catches up to the journal.
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- **Consistency:** Ceph OSDs require a filesystem interface that guarantees
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atomic compound operations. Ceph OSDs write a description of the operation
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to the journal and apply the operation to the filesystem. This enables
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atomic updates to an object (for example, placement group metadata). Every
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few seconds--between ``filestore max sync interval`` and
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``filestore min sync interval``--the OSD stops writes and synchronizes the
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journal with the filesystem, allowing OSDs to trim operations from the
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journal and reuse the space. On failure, OSDs replay the journal starting
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after the last synchronization operation.
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Ceph OSDs support the following journal settings:
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``journal dio``
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:Description: Enables direct i/o to the journal. Requires ``journal block align`` set to ``true``.
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:Type: Boolean
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:Required: Yes when using ``aio``.
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:Default: ``true``
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``journal aio``
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:Description: Enables using ``libaio`` for asynchronous writes to the journal. Requires ``journal dio`` set to ``true``.
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:Type: Boolean
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:Required: No.
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:Default: ``false``
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``journal block align``
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:Description: Block aligns writes. Required for ``dio`` and ``aio``.
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:Type: Boolean
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:Required: Yes when using ``dio`` and ``aio``.
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:Default: ``true``
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``journal max write bytes``
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:Description: The maximum number of bytes the journal will write at any one time.
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:Type: Integer
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:Required: No
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:Default: ``10 << 20``
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``journal max write entries``
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:Description: The maximum number of entries the journal will write at any one time.
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:Type: Integer
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:Required: No
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:Default: ``100``
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``journal queue max ops``
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:Description: The maximum number of operations allowed in the queue at any one time.
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:Type: Integer
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:Required: No
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:Default: ``500``
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``journal queue max bytes``
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:Description: The maximum number of bytes allowed in the queue at any one time.
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:Type: Integer
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:Required: No
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:Default: ``10 << 20``
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``journal align min size``
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:Description: Align data payloads greater than the specified minimum.
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:Type: Integer
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:Required: No
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:Default: ``64 << 10``
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``journal zero on create``
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:Description: Causes the file store to overwrite the entire journal with ``0``'s during ``mkfs``.
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:Type: Boolean
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:Required: No
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:Default: ``false``
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