mirror of
https://github.com/ceph/ceph
synced 2024-12-26 05:25:09 +00:00
188 lines
6.9 KiB
ReStructuredText
188 lines
6.9 KiB
ReStructuredText
.. _mds-standby:
|
|
|
|
Terminology
|
|
-----------
|
|
|
|
A Ceph cluster may have zero or more CephFS *file systems*. Each CephFS has
|
|
a human readable name (set at creatiopn time with ``fs new``) and an integer
|
|
ID. The ID is called the file system cluster ID, or *FSCID*.
|
|
|
|
Each CephFS file system has a number of *ranks*, numbered beginning with zero.
|
|
By default there is one rank per file system. A rank may be thought of as a
|
|
metadata shard. Management of ranks is described in :doc:`/cephfs/multimds` .
|
|
|
|
Each CephFS ``ceph-mds`` daemon starts without a rank. It may be assigned one
|
|
by the cluster's monitors. A daemon may only hold one rank at a time, and only
|
|
give up a rank when the ``ceph-mds`` process stops.
|
|
|
|
If a rank is not associated with any daemon, that rank is considered ``failed``.
|
|
Once a rank is assigned to a daemon, the rank is considered ``up``.
|
|
|
|
Each ``ceph-mds`` daemon has a *name* that is assigned statically by the
|
|
administrator when the daemon is first configured. Each daemon's *name* is
|
|
typically that of the hostname where the process runs.
|
|
|
|
A ``ceph-mds`` daemon may be assigned to a specific file system by
|
|
setting its ``mds_join_fs`` configuration option to the file system's
|
|
``name``.
|
|
|
|
When a ``ceph-mds`` daemon starts, it is also assigned an integer ``GID``,
|
|
which is unique to this current daemon's process. In other words, when a
|
|
``ceph-mds`` daemon is restarted, it runs as a new process and is assigned a
|
|
*new* ``GID`` that is different from that of the previous process.
|
|
|
|
Referring to MDS daemons
|
|
------------------------
|
|
|
|
Most administrative commands that refer to a ``ceph-mds`` daemon (MDS)
|
|
accept a flexible argument format that may specify a ``rank``, a ``GID``
|
|
or a ``name``.
|
|
|
|
Where a ``rank`` is used, it may optionally be qualified by
|
|
a leading file system ``name`` or ``GID``. 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, say we have an MDS daemon with ``name`` 'myhost' and
|
|
``GID`` 5446, and which is assigned ``rank`` 0 for the file system 'myfs'
|
|
with ``FSCID`` 3. Any of the following are 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 # File System name and rank
|
|
|
|
Managing failover
|
|
-----------------
|
|
|
|
If an MDS daemon stops communicating with the cluster's monitors, the monitors
|
|
will wait ``mds_beacon_grace`` seconds (default 15) before marking the daemon as
|
|
*laggy*. If a standby MDS is available, the monitor will immediately replace the
|
|
laggy daemon.
|
|
|
|
Each file system may specify a minimum number of standby daemons in order to be
|
|
considered healthy. This number includes daemons in the ``standby-replay`` state
|
|
waiting for a ``rank`` to fail. Note that a ``standby-replay`` daemon will not
|
|
be assigned to take over a failure for another ``rank`` or a failure in a
|
|
different CephFS file system). The pool of standby daemons not in ``replay``
|
|
counts towards any file system count.
|
|
Each file system may set the desired number of standby daemons by:
|
|
|
|
::
|
|
|
|
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 in order to
|
|
reduce failover time in the event that the active MDS becomes unavailable. Each
|
|
active MDS may have only one ``standby-replay`` daemon following it.
|
|
|
|
Configuration of ``standby-replay`` on a file system is done using the below:
|
|
|
|
::
|
|
|
|
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.
|
|
|
|
.. _mds-join-fs:
|
|
|
|
Configuring MDS file system affinity
|
|
------------------------------------
|
|
|
|
You might elect to dedicate an MDS to a particular file system. Or, perhaps you
|
|
have MDSs that run on better hardware that should be preferred over a last-resort
|
|
standby on modest or over-provisioned systems. To configure this preference,
|
|
CephFS provides a configuration option for MDS called ``mds_join_fs`` which
|
|
enforces this affinity.
|
|
|
|
When failing over MDS daemons, a cluster's monitors will prefer standby daemons with
|
|
``mds_join_fs`` equal to the file system ``name`` with the failed ``rank``. If no
|
|
standby exists with ``mds_join_fs`` equal to the file system ``name``, it will
|
|
choose an unqualified standby (no setting for ``mds_join_fs``) for the replacement,
|
|
or any other available standby, as a last resort. Note, this does not change the
|
|
behavior that ``standby-replay`` daemons are always selected before
|
|
other standbys.
|
|
|
|
Even further, the monitors will regularly examine the CephFS file systems even when
|
|
stable to check if a standby with stronger affinity is available to replace an
|
|
MDS with lower affinity. This process is also done for ``standby-replay`` daemons:
|
|
if a regular standby has stronger affinity than the ``standby-replay`` MDS, it will
|
|
replace the standby-replay MDS.
|
|
|
|
For example, given this stable and healthy file system:
|
|
|
|
::
|
|
|
|
$ ceph fs dump
|
|
dumped fsmap epoch 399
|
|
...
|
|
Filesystem 'cephfs' (27)
|
|
...
|
|
e399
|
|
max_mds 1
|
|
in 0
|
|
up {0=20384}
|
|
failed
|
|
damaged
|
|
stopped
|
|
...
|
|
[mds.a{0:20384} state up:active seq 239 addr [v2:127.0.0.1:6854/966242805,v1:127.0.0.1:6855/966242805]]
|
|
|
|
Standby daemons:
|
|
|
|
[mds.b{-1:10420} state up:standby seq 2 addr [v2:127.0.0.1:6856/2745199145,v1:127.0.0.1:6857/2745199145]]
|
|
|
|
|
|
You may set ``mds_join_fs`` on the standby to enforce your preference: ::
|
|
|
|
$ ceph config set mds.b mds_join_fs cephfs
|
|
|
|
after automatic failover: ::
|
|
|
|
$ ceph fs dump
|
|
dumped fsmap epoch 405
|
|
e405
|
|
...
|
|
Filesystem 'cephfs' (27)
|
|
...
|
|
max_mds 1
|
|
in 0
|
|
up {0=10420}
|
|
failed
|
|
damaged
|
|
stopped
|
|
...
|
|
[mds.b{0:10420} state up:active seq 274 join_fscid=27 addr [v2:127.0.0.1:6856/2745199145,v1:127.0.0.1:6857/2745199145]]
|
|
|
|
Standby daemons:
|
|
|
|
[mds.a{-1:10720} state up:standby seq 2 addr [v2:127.0.0.1:6854/1340357658,v1:127.0.0.1:6855/1340357658]]
|
|
|
|
Note in the above example that ``mds.b`` now has ``join_fscid=27``. In this
|
|
output, the file system name from ``mds_join_fs`` is changed to the file system
|
|
identifier (27). If the file system is recreated with the same name, the
|
|
standby will follow the new file system as expected.
|
|
|
|
Finally, if the file system is degraded or undersized, no failover will occur
|
|
to enforce ``mds_join_fs``.
|