mirror of
https://github.com/ceph/ceph
synced 2024-12-21 19:02:10 +00:00
ba5e7042fc
Fixes: http://tracker.ceph.com/issues/42190 Signed-off-by: Venky Shankar <vshankar@redhat.com>
91 lines
3.7 KiB
ReStructuredText
91 lines
3.7 KiB
ReStructuredText
MDS Journaling
|
|
==============
|
|
|
|
CephFS Metadata Pool
|
|
--------------------
|
|
|
|
CephFS uses a separate (metadata) pool for managing file metadata (inodes and
|
|
dentries) in a Ceph File System. The metadata pool has all the information about
|
|
files in a Ceph File System including the File System hierarchy. Additionally,
|
|
CephFS maintains meta information related to other entities in a file system
|
|
such as file system journals, open file table, session map, etc.
|
|
|
|
This document describes how Ceph Metadata Servers use and rely on journaling.
|
|
|
|
CephFS MDS Journaling
|
|
---------------------
|
|
|
|
CephFS metadata servers stream a journal of metadata events into RADOS in the metadata
|
|
pool prior to executing a file system operation. Active MDS daemon(s) manage metadata
|
|
for files and directories in CephFS.
|
|
|
|
CephFS uses journaling for couple of reasons:
|
|
|
|
#. Consistency: On an MDS failover, the journal events can be replayed to reach a
|
|
consistent file system state. Also, metadata operations that require multiple
|
|
updates to the backing store need to be journaled for crash consistency (along
|
|
with other consistency mechanisms such as locking, etc..).
|
|
|
|
#. Performance: Journal updates are (mostly) sequential, hence updates to journals
|
|
are fast. Furthermore, updates can be batched into single write, thereby saving
|
|
disk seek time involved in updates to different parts of a file. Having a large
|
|
journal also helps a standby MDS to warm its cache which helps indirectly during
|
|
MDS failover.
|
|
|
|
Each active metadata server maintains its own journal in the metadata pool. Journals
|
|
are striped over multiple objects. Journal entries which are not required (deemed as
|
|
old) are trimmed by the metadata server.
|
|
|
|
Journal Events
|
|
--------------
|
|
|
|
Apart from journaling file system metadata updates, CephFS journals various other events
|
|
such as client session info and directory import/export state to name a few. These events
|
|
are used by the metadata sever to reestablish correct state as required, e.g., Ceph MDS
|
|
tries to reconnect clients on restart when journal events get replayed and a specific
|
|
event type in the journal specifies that a client entity type has a session with the MDS
|
|
before it was restarted.
|
|
|
|
To examine the list of such events recorded in the journal, CephFS provides a command
|
|
line utility `cephfs-journal-tool` which can be used as follows:
|
|
|
|
::
|
|
|
|
cephfs-journal-tool --rank=<fs>:<rank> event get list
|
|
|
|
`cephfs-journal-tool` is also used to discover and repair a damaged Ceph File System.
|
|
(See :doc:`/cephfs/cephfs-journal-tool` for more details)
|
|
|
|
Journal Event Types
|
|
-------------------
|
|
|
|
Following are various event types that are journaled by the MDS.
|
|
|
|
#. `EVENT_COMMITTED`: Mark a request (id) as committed.
|
|
|
|
#. `EVENT_EXPORT`: Maps directories to an MDS rank.
|
|
|
|
#. `EVENT_FRAGMENT`: Tracks various stages of directory fragmentation (split/merge).
|
|
|
|
#. `EVENT_IMPORTSTART`: Logged when an MDS rank starts importing directory fragments.
|
|
|
|
#. `EVENT_IMPORTFINISH`: Logged when an MDS rank finishes importing directory fragments.
|
|
|
|
#. `EVENT_NOOP`: No operation event type for skipping over a journal region.
|
|
|
|
#. `EVENT_OPEN`: Tracks which inodes have open file handles.
|
|
|
|
#. `EVENT_RESETJOURNAL`: Used to mark a journal as `reset` post truncation.
|
|
|
|
#. `EVENT_SESSION`: Tracks open client sessions.
|
|
|
|
#. `EVENT_SLAVEUPDATE`: Logs various stages of an operation that has been forwarded to a (slave) mds.
|
|
|
|
#. `EVENT_SUBTREEMAP`: Map of directory inodes to directory contents (subtree partition).
|
|
|
|
#. `EVENT_TABLECLIENT`: Log transition states of MDSs view of client tables (snap/anchor).
|
|
|
|
#. `EVENT_TABLESERVER`: Log transition states of MDSs view of server tables (snap/anchor).
|
|
|
|
#. `EVENT_UPDATE`: Log file operations on an inode.
|