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==========================
Journal Config Reference
==========================
.. warning:: Filestore has been deprecated in the Reef release and is no longer supported.
.. index:: journal; journal configuration
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:
.. confval:: journal_dio
.. confval:: journal_aio
.. confval:: journal_block_align
.. confval:: journal_max_write_bytes
.. confval:: journal_max_write_entries
.. confval:: journal_align_min_size
.. confval:: journal_zero_on_create
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