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diff --git a/doc/rados/configuration/journal-ref.rst b/doc/rados/configuration/journal-ref.rst new file mode 100644 index 000000000..5ce5a5e2d --- /dev/null +++ b/doc/rados/configuration/journal-ref.rst @@ -0,0 +1,39 @@ +========================== + 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 |