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.

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*.

Each CephFS filesystem has a *max_mds* setting, which controls
how many ranks will be created.  The actual number of ranks
in the filesystem will only be increased if a spare daemon is
available to take on the new rank.

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*.

Configuring standby daemons
---------------------------

There are four configuration settings that control how a daemon
will behave while in standby:

::

    mds_standby_for_name
    mds_standby_for_rank
    mds_standby_for_fscid
    mds_standby_replay

These may be set in the ceph.conf on the host where the MDS daemon
runs (as opposed to on the monitor).  The daemon loads these settings
when it starts, and sends them to the monitor.

By default, if none of these settings are used, all MDS daemons
which do not hold a rank will be used as standbys for any rank.

The settings which associate a standby daemon with a particular
name or rank do not guarantee that the daemon will *only* be used
for that rank.  They mean that when several standbys are available,
the associated standby daemon will be used.  If a rank is failed,
and a standby is available, it will be used even if it is associated
with a different rank or named daemon.

mds_standby_replay
~~~~~~~~~~~~~~~~~~

If this is set to true, then the standby daemon will continuously read
the metadata journal of an up rank.  This will give it
a warm metadata cache, and speed up the process of failing over
if the daemon serving the rank fails.

An up rank may only have one standby replay daemon assigned to it,
if two daemons are both set to be standby replay then one of them
will arbitrarily win, and the other will become a normal non-replay
standby.

Once a daemon 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.

*Historical note:* In Ceph prior to v10.2.1, this setting (when ``false``) is
always true when ``mds_standby_for_*`` is also set.

mds_standby_for_name
~~~~~~~~~~~~~~~~~~~~

Set this to make the standby daemon only take over a failed rank
if the last daemon to hold it matches this name.

mds_standby_for_rank
~~~~~~~~~~~~~~~~~~~~

Set this to make the standby daemon only take over the specified
rank.  If another rank fails, this daemon will not be used to
replace it.

Use in conjunction with ``mds_standby_for_fscid`` to be specific
about which filesystem's rank you are targeting, if you have
multiple filesystems.

mds_standby_for_fscid
~~~~~~~~~~~~~~~~~~~~~

If ``mds_standby_for_rank`` is set, this is simply a qualifier to
say which filesystem's rank is referred to.

If ``mds_standby_for_rank`` is not set, then setting FSCID will
cause this daemon to target any rank in the specified FSCID.  Use
this if you have a daemon that you want to use for any rank, but
only within a particular filesystem.

mon_force_standby_active
~~~~~~~~~~~~~~~~~~~~~~~~

This setting is used on monitor hosts.  It defaults to true.

If it is false, then daemons configured with standby_replay=true
will **only** become active if the rank/name that they have
been configured to follow fails.  On the other hand, if this
setting is true, then a daemon configured with standby_replay=true
may be assigned some other rank.

Examples
--------

These are example ceph.conf snippets.  In practice you can either
copy a ceph.conf with all daemons' configuration to all your servers,
or you can have a different file on each server that contains just
that server's daemons' configuration.

Simple pair
~~~~~~~~~~~

Two MDS daemons 'a' and 'b' acting as a pair, where whichever one is not
currently assigned a rank will be the standby replay follower
of the other.

::

    [mds.a]
    mds standby replay = true
    mds standby for rank = 0

    [mds.b]
    mds standby replay = true
    mds standby for rank = 0

Floating standby
~~~~~~~~~~~~~~~~

Three MDS daemons 'a', 'b' and 'c', in a filesystem that has
``max_mds`` set to 2.

::
    
    # No explicit configuration required: whichever daemon is
    # not assigned a rank will go into 'standby' and take over
    # for whichever other daemon fails.

Two MDS clusters
~~~~~~~~~~~~~~~~

With two filesystems, I have four MDS daemons, and I want two
to act as a pair for one filesystem and two to act as a pair
for the other filesystem.

::

    [mds.a]
    mds standby for fscid = 1

    [mds.b]
    mds standby for fscid = 1

    [mds.c]
    mds standby for fscid = 2

    [mds.d]
    mds standby for fscid = 2