MDS States¶
The Metadata Server (MDS) goes through several states during normal operation in CephFS. For example, some states indicate that the MDS is recovering from a failover by a previous instance of the MDS. Here we’ll document all of these states and include a state diagram to visualize the transitions.
State Descriptions¶
Common states¶
up:active
This is the normal operating state of the MDS. It indicates that the MDS and its rank in the file system is available.
up:standby
The MDS is available to takeover for a failed rank (see also Terminology). The monitor will automatically assign an MDS in this state to a failed rank once available.
up:standby_replay
The MDS is following the journal of another up:active MDS. Should the
active MDS fail, having a standby MDS in replay mode is desirable as the MDS is
replaying the live journal and will more quickly takeover. A downside to having
standby replay MDSs is that they are not available to takeover for any other
MDS that fails, only the MDS they follow.
Less common or transitory states¶
up:boot
This state is broadcast to the Ceph monitors during startup. This state is never visible as the Monitor immediately assign the MDS to an available rank or commands the MDS to operate as a standby. The state is documented here for completeness.
up:creating
The MDS is creating a new rank (perhaps rank 0) by constructing some per-rank metadata (like the journal) and entering the MDS cluster.
up:starting
The MDS is restarting a stopped rank. It opens associated per-rank metadata and enters the MDS cluster.
up:stopping
When a rank is stopped, the monitors command an active MDS to enter the
up:stopping state. In this state, the MDS accepts no new client
connections, migrates all subtrees to other ranks in the file system, flush its
metadata journal, and, if the last rank (0), evict all clients and shutdown
(see also CephFS Administrative commands).
up:replay
The MDS taking over a failed rank. This state represents that the MDS is recovering its journal and other metadata.
up:resolve
The MDS enters this state from up:replay if the Ceph file system has
multiple ranks (including this one), i.e. it’s not a single active MDS cluster.
The MDS is resolving any uncommitted inter-MDS operations. All ranks in the
file system must be in this state or later for progress to be made, i.e. no
rank can be failed/damaged or up:replay.
up:reconnect
An MDS enters this state from up:replay or up:resolve. This state is to
solicit reconnections from clients. Any client which had a session with this
rank must reconnect during this time, configurable via
mds_reconnect_timeout.
up:rejoin
The MDS enters this state from up:reconnect. In this state, the MDS is
rejoining the MDS cluster cache. In particular, all inter-MDS locks on metadata
are reestablished.
If there are no known client requests to be replayed, the MDS directly becomes
up:active from this state.
up:clientreplay
The MDS may enter this state from up:rejoin. The MDS is replaying any
client requests which were replied to but not yet durable (not journaled).
Clients resend these requests during up:reconnect and the requests are
replayed once again. The MDS enters up:active after completing replay.
Failed states¶
down:failed
No MDS actually holds this state. Instead, it is applied to the rank in the file system. For example:
$ ceph fs dump
...
max_mds 1
in 0
up {}
failed 0
...
Rank 0 is part of the failed set.
down:damaged
No MDS actually holds this state. Instead, it is applied to the rank in the file system. For example:
$ ceph fs dump
...
max_mds 1
in 0
up {}
failed
damaged 0
...
Rank 0 has become damaged (see also Disaster recovery) and placed in
the damaged set. An MDS which was running as rank 0 found metadata damage
that could not be automatically recovered. Operator intervention is required.
down:stopped
No MDS actually holds this state. Instead, it is applied to the rank in the file system. For example:
$ ceph fs dump
...
max_mds 1
in 0
up {}
failed
damaged
stopped 1
...
The rank has been stopped by reducing max_mds (see also Configuring multiple active MDS daemons).
State Diagram¶
This state diagram shows the possible state transitions for the MDS/rank. The legend is as follows:
Color¶
Green: MDS is active.
Orange: MDS is in transient state trying to become active.
Red: MDS is indicating a state that causes the rank to be marked failed.
Purple: MDS and rank is stopping.
Black: MDS is indicating a state that causes the rank to be marked damaged.
Shape¶
Circle: an MDS holds this state.
Hexagon: no MDS holds this state (it is applied to the rank).
Lines¶
A double-lined shape indicates the rank is “in”.
