As touched upon in Chapter 3, Hardware and Network Selection, when using FileStore each OSD requires a journal. For the remainder of this section when discussing journals we assume that the FileStore back end is being used, as BlueStore does not employ journals as such. Ceph's FileStore employs journals for two reasons: performance and data integrity. The write performance of small random client requests is improved by combining them into fewer, more efficient sequential journal writes. Ceph returns writes to clients after a requisite number of copies are written to OSD journals, which often are faster than the OSD filesystem proper. If hardware fails before data is persisted to an OSD filesystem it can be recovered from the journal. ...

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