mediator(7D)
NAME
mediator - support for HA configurations consisting of two
strings of drives
DESCRIPTION
Beginning with a prior version, Solaris Volume Manager pro-
vided support for high-availability (HA) configurations con-
sisting of two hosts that share at least three strings of
drives and that run software enabling exclusive access to
the data on those drives from one host. (Note: Volume
Manager, by itself, does not actually provide a high-
availability environment. The diskset feature is an enabler
for HA configurations.)
Volume Manager provides support for a low-end HA solution
consisting of two hosts that share only two strings of
drives. The hosts in this type of configuration, referred to
as mediators, run a special daemon, rpc.metamedd(1M). The
mediator hosts take on additional responsibilities to ensure
that data is available in the case of host or drive
failures.
In a mediator configuration, two hosts are physically con-
nected to two strings of drives. This configuration can sur-
vive the failure of a single host or a single string of
drives, without administrative intervention. If both a host
and a string of drives fail (multiple failures), the
integrity of the data cannot be guaranteed. At this point,
administrative intervention is required to make the data
accessible.
The following definitions pertain to a mediator configura-
tion:
diskset
A set of drives containing metadevices and hot spares
that can be shared exclusively (but not concurrently)
by two hosts.
Volume Manager state database
A replicated database that stores metadevice confi-
guration and state information.
mediator host
A host that runs the rpc.metamedd(1M) daemon and that
has been added to a diskset. The mediator host parti-
cipates in checking the state database and the media-
tor quorum.
mediator quorum
The condition achieved when the number of accessible
mediator hosts is equal to half+1 the total number of
configured mediator hosts. Because it is expected that
there will be two mediator hosts, this number will
normally be 2 ([(2/2) + 1] = 2.)
replica
A single copy of the Volume Manager metadevice state
database.
replica quorum
The condition achieved when the number of accessible
replicas is equal to half+1 the total number of con-
figured replicas. For example, if a system is config-
ured with ten replicas, the quorum is met when six are
accessible ([(10/2) + 1 = 6]).
A mediator host running the rpc.metamedd(1M) daemon keeps
track of replica updates. As long as the following condi-
tions are met, access to data occurs without any administra-
tive intervention:
o The replica quorum is not met.
o Half of the replicas is still accessible.
o The mediator quorum is met.
The following conditions describe the operation of mediator
hosts:
1. If the replica quorum is met, access to the diskset is
granted. At this point no mediator host is involved.
2. If the replica quorum is not met, half of the replicas is
accessible, the mediator quorum is met, and the replica
and mediator data match, access to the diskset is
granted. The mediator host contributes the deciding vote.
3. If the replica quorum is not met, half of the replicas is
accessible, the mediator quorum is not met, half of the
mediator hosts is accessible, and the replica and media-
tor data match, the system prompts you to grant or deny
access to the diskset.
4. If the replica quorum is not met, half of the replicas is
accessible, the mediator quorum is met, and the replica
and mediator data do not match, access to the diskset is
read-only. You can delete replicas, release the diskset,
and retake the diskset to gain read-write access to the
data in the diskset.
5. In all other cases, the diskset access is read-only. You
can delete replicas, release the diskset, and retake the
diskset to gain read-write access to the data in the
diskset.
The metaset(1M) command administers disksets and mediator
hosts. The following options to the metaset command pertain
only to administering mediator hosts.
-a -m mediator_host_list
Adds mediator hosts to the named set. A
mediator_host_list is the nodename of the mediator
host to be added and up to 2 other aliases for the
mediator host. The nodename and aliases for each medi-
ator host are separated by commas. Up to 2 mediator
hosts can be specified for the named diskset.
-d -m mediator_host_list
Deletes mediator hosts from the named diskset. Media-
tor hosts are deleted from the diskset by specifying
the nodename of mediator host to delete.
-q Displays an enumerated list of tags pertaining to
``tagged data'' that may be encountered during a take
of the ownership of a diskset.
-t [-f] -y
Takes ownership of a diskset safely, unless -f is
used, in which case the take is unconditional. If
metaset finds that another host owns the set, this
host will not be allowed to take ownership of the set.
If the set is not owned by any other host, all the
disks within the set will be owned by the host on
which metaset was executed. The metadevice state
database is read in and the shared metadevices con-
tained in the set become accessible. The -t option
will take a diskset that has stale databases. When the
databases are stale, metaset will exit with code 66,
and a message will be printed. At that point, the only
operations permitted are the addition and deletion of
replicas. Once the addition or deletion of the repli-
cas has been completed, the diskset should be released
and retaken to gain full access to the data. If media-
tor hosts have been configured, some additional exit
codes are possible. If half of the replicas and half
of the mediator hosts are operating properly, the take
will exit with code 3. At this point, you can add or
delete replicas, or use the -y option on a subsequent
take. If the take operation encounters ``tagged
data,'' the take operation will exit with code 2. You
can then run the metaset command with the -q option
to see an enumerated list of tags.
-t [-f] -u tagnumber
Once a tag has been selected, a subsequent take with
-u tagnumber can be executed to select the data asso-
ciated with the given tagnumber.
SEE ALSO
metaset(1M), md(7D), rpc.metamedd(1M), rpc.metad(1M)
Sun Cluster documentation, Solaris Volume Manager Adminis-
tration Guide
NOTES
Diskset administration, including the addition and deletion
of hosts and drives, requires all hosts in the set to be
accessible from the network.
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