Devices mediator(7D)
NAME
mediator - support for HA configurations consisting of two
strings of drivesDESCRIPTION
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: VolumeManager, 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 toas 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 databaseA replicated database that stores metadevice configura-
tion and state information.mediator host
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Devices mediator(7D)
A host that runs the rpc.metamedd(1M) daemon and thathas been added to a diskset. The mediator host partici-
pates in checking the state database and the mediator
quorum.mediator quorum
The condition achieved when the number of accessiblemediator 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 nor-
mally 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 accessiblereplicas is equal to half+1 the total number of config-
ured replicas. For example, if a system is configuredwith ten replicas, the quorum is met when six are acces-
sible ([(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 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 thereplicas are accessible, the mediator quorum is
met, and the replica and mediator data match,
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Devices mediator(7D)
access to the diskset is granted. The mediator host
contributes the deciding vote. 3. If the replica quorum is not met, half of thereplicas 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 thereplicas 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 retakethe 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 pertainonly 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 2other aliases for the mediator
host. The nodename and aliasesfor each mediator host are
separated by commas. Up to 3mediator 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 byspecifying the nodename of medi-
ator 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.SunOS 5.11 Last change: 20 Jun 2008 3
Devices mediator(7D)
-t [-f] -y Takes ownership of a diskset
safely, unless -f is used, in
which case the take is uncondi-
tional. 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 isread in and the shared metadev-
ices contained in the set becomeaccessible. 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 replicas has been completed, the diskset should be released and retaken to gain full access to the data.If mediator hosts have been con-
figured, some additional exit codes are possible. If half of the replicas and half of themediator hosts are operating
properly, the take will exit with code 3. At this point, you can add or delete replicas, oruse the -y option on a subse-
quent take. If the take opera-
tion encounters ``tagged data,'' the take operation will exit with code 2. You can then runthe 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 tag-
number can be executed to select the data associated with the given tagnumber.SunOS 5.11 Last change: 20 Jun 2008 4
Devices mediator(7D)
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.SunOS 5.11 Last change: 20 Jun 2008 5