mirror of
https://github.com/ceph/ceph
synced 2024-12-14 07:25:50 +00:00
104 lines
3.7 KiB
ReStructuredText
104 lines
3.7 KiB
ReStructuredText
.. _mds-standby:
|
|
|
|
Terminology
|
|
-----------
|
|
|
|
A Ceph cluster may have zero or more CephFS *filesystems*. CephFS
|
|
filesystems have a human readable name (set in ``fs new``)
|
|
and an integer ID. The ID is called the filesystem cluster ID,
|
|
or *FSCID*.
|
|
|
|
Each CephFS filesystem has a number of *ranks*, one by default,
|
|
which start at zero. A rank may be thought of as a metadata shard.
|
|
Controlling the number of ranks in a filesystem is described
|
|
in :doc:`/cephfs/multimds`
|
|
|
|
Each CephFS ceph-mds process (a *daemon*) initially starts up
|
|
without a rank. It may be assigned one by the monitor cluster.
|
|
A daemon may only hold one rank at a time. Daemons only give up
|
|
a rank when the ceph-mds process stops.
|
|
|
|
If a rank is not associated with a daemon, the rank is
|
|
considered *failed*. Once a rank is assigned to a daemon,
|
|
the rank is considered *up*.
|
|
|
|
A daemon has a *name* that is set statically by the administrator
|
|
when the daemon is first configured. Typical configurations
|
|
use the hostname where the daemon runs as the daemon name.
|
|
|
|
Each time a daemon starts up, it is also assigned a *GID*, which
|
|
is unique to this particular process lifetime of the daemon. The
|
|
GID is an integer.
|
|
|
|
Referring to MDS daemons
|
|
------------------------
|
|
|
|
Most of the administrative commands that refer to an MDS daemon
|
|
accept a flexible argument format that may contain a rank, a GID
|
|
or a name.
|
|
|
|
Where a rank is used, this may optionally be qualified with
|
|
a leading filesystem name or ID. If a daemon is a standby (i.e.
|
|
it is not currently assigned a rank), then it may only be
|
|
referred to by GID or name.
|
|
|
|
For example, if we had an MDS daemon which was called 'myhost',
|
|
had GID 5446, and was assigned rank 0 in the filesystem 'myfs'
|
|
which had FSCID 3, then any of the following would be suitable
|
|
forms of the 'fail' command:
|
|
|
|
::
|
|
|
|
ceph mds fail 5446 # GID
|
|
ceph mds fail myhost # Daemon name
|
|
ceph mds fail 0 # Unqualified rank
|
|
ceph mds fail 3:0 # FSCID and rank
|
|
ceph mds fail myfs:0 # Filesystem name and rank
|
|
|
|
Managing failover
|
|
-----------------
|
|
|
|
If an MDS daemon stops communicating with the monitor, the monitor will wait
|
|
``mds_beacon_grace`` seconds (default 15 seconds) before marking the daemon as
|
|
*laggy*. If a standby is available, the monitor will immediately replace the
|
|
laggy daemon.
|
|
|
|
Each file system may specify a number of standby daemons to be considered
|
|
healthy. This number includes daemons in standby-replay waiting for a rank to
|
|
fail (remember that a standby-replay daemon will not be assigned to take over a
|
|
failure for another rank or a failure in a another CephFS file system). The
|
|
pool of standby daemons not in replay count towards any file system count.
|
|
Each file system may set the number of standby daemons wanted using:
|
|
|
|
::
|
|
|
|
ceph fs set <fs name> standby_count_wanted <count>
|
|
|
|
Setting ``count`` to 0 will disable the health check.
|
|
|
|
|
|
.. _mds-standby-replay:
|
|
|
|
Configuring standby-replay
|
|
--------------------------
|
|
|
|
Each CephFS file system may be configured to add standby-replay daemons. These
|
|
standby daemons follow the active MDS's metadata journal to reduce failover
|
|
time in the event the active MDS becomes unavailable. Each active MDS may have
|
|
only one standby-replay daemon following it.
|
|
|
|
Configuring standby-replay on a file system is done using:
|
|
|
|
::
|
|
|
|
ceph fs set <fs name> allow_standby_replay <bool>
|
|
|
|
Once set, the monitors will assign available standby daemons to follow the
|
|
active MDSs in that file system.
|
|
|
|
Once an MDS has entered the standby-replay state, it will only be used as a
|
|
standby for the rank that it is following. If another rank fails, this
|
|
standby-replay daemon will not be used as a replacement, even if no other
|
|
standbys are available. For this reason, it is advised that if standby-replay
|
|
is used then every active MDS should have a standby-replay daemon.
|