mediator(7D)							      Devices							      mediator(7D)

NAME

       mediator - 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-availability environment. The diskset feature is an enabler for HA con-
       figurations.)

       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 responsibil-
       ities 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
       single  host  or a single string of drives, without administrative intervention. If both a host and a string of drives fail (multiple fail-
       ures), 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 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 datAbreplicated 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  config-
			       ured  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  example,  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 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 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 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_listAdds  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 separated by  com-
			       mas. Up to 2 mediator hosts can be specified for the named diskset.

       -d -m mediator_host_listDeletes	mediator hosts from the named diskset. Mediator hosts are deleted from the diskset by specifying the node-
			       name 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 own-
			       ership 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 contained 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 repli-
			       cas.  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 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	associated
			       with the given tagnumber.

SEE ALSO

       metaset(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 5.10							 20 September 2000						      mediator(7D)