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58 lines
2.5 KiB
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58 lines
2.5 KiB
ReStructuredText
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MDS Journaling
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==============
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CephFS Metadata Pool
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--------------------
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CephFS uses a separate (metadata) pool for managing file metadata (inodes and
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dentries) in a Ceph File System. The metadata pool has all the information about
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files in a Ceph File System including the File System hierarchy. Additionally,
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CephFS maintains meta information related to other entities in a file system
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such as file system journals, open file table, session map, etc.
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This document describes how Ceph Metadata Servers use and rely on journaling.
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CephFS MDS Journaling
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---------------------
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CephFS metadata servers stream a journal of metadata events into RADOS in the metadata
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pool prior to executing a file system operation. Active MDS daemon(s) manage metadata
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for files and directories in CephFS.
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CephFS uses journaling for couple of reasons:
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#. Consistency: On an MDS failover, the journal events can be replayed to reach a
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consistent file system state. Also, metadata operations that require multiple
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updates to the backing store need to be journaled for crash consistency (along
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with other consistency mechanisms such as locking, etc..).
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#. Performance: Journal updates are (mostly) sequential, hence updates to journals
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are fast. Furthermore, updates can be batched into single write, thereby saving
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disk seek time involved in updates to different parts of a file. Having a large
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journal also helps a standby MDS to warm its cache which helps indirectly during
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MDS failover.
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Each active metadata server maintains its own journal in the metadata pool. Journals
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are striped over multiple objects. Journal entries which are not required (deemed as
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old) are trimmed by the metadata server.
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Journal Events
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--------------
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Apart from journaling file system metadata updates, CephFS journals various other events
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such as client session info and directory import/export state to name a few. These events
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are used by the metadata sever to reestablish correct state as required, e.g., Ceph MDS
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tries to reconnect clients on restart when journal events get replayed and a specific
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event type in the journal specifies that a client entity type has a session with the MDS
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before it was restarted.
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To examine the list of such events recorded in the journal, CephFS provides a command
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line utility `cephfs-journal-tool` which can be used as follows:
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::
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cephfs-journal-tool --rank=<fs>:<rank> event get list
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`cephfs-journal-tool` is also used to discover and repair a damaged Ceph File System.
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(See :doc:`/cephfs/cephfs-journal-tool` for more details)
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