mediator(7D) Devices mediator(7D)NAMEmediator - support for HA configurations consisting of two strings of
drives
DESCRIPTION
Beginning with a prior version, Solaris Volume Manager provided support
for high-availability (HA) configurations consisting 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-availabil‐
ity environment. The diskset feature is an enabler for HA configura‐
tions.)
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 responsibili‐
ties to ensure that data is available in the case of host or drive
failures.
In a mediator configuration, two hosts are physically connected to two
strings of drives. This configuration can survive the failure of a sin‐
gle host or a single string of drives, without administrative interven‐
tion. If both a host and a string of drives fail (multiple failures),
the integrity of the data cannot be guaranteed. At this point, adminis‐
trative intervention is required to make the data accessible.
The following definitions pertain to a mediator configuration:
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 configuration 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 participates in checking the
state database and the mediator 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 configured replicas. For exam‐
ple, if a system is configured 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 conditions are met, access to
data occurs without any administrative intervention:
o The replica quorum is not met.
o Half of the replicas are still accessible.
o The mediator quorum is met.
The following conditions describe the operation of mediator hosts:
1. If the 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 are
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 are
accessible, the mediator quorum is not met, half of the
mediator hosts is accessible, and the replica and mediator
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 are
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 mediator host are sep‐
arated by commas. Up to 3 mediator hosts
can be specified for the named diskset.
-d -m mediator_host_list Deletes mediator hosts from the named
diskset. Mediator hosts are deleted from
the diskset by specifying the nodename of
mediator host to delete.
-q Displays an enumerated list of tags per‐
taining 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 mes‐
sage will be printed. At that point, the
only operations permitted are the addition
and deletion of replicas. Once the addition
or deletion of the replicas has been com‐
pleted, the diskset should be released and
retaken to gain full access to the data. If
mediator hosts have been configured, some
additional exit codes are possible. If
half of the replicas and half of the media‐
tor 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 associated with the given
tagnumber.
SEE ALSOmetaset(1M), md(7D), rpc.metamedd(1M), rpc.metad(1M)
Sun Cluster documentation, Solaris Volume Manager Administration 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.
SunOS 5.11 20 Jun 2008 mediator(7D)
