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 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  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 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 separated
				   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 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 exe-
				   cuted. The metadevice state database is read in and the shared metadevices contained in the set become accessi-
				   ble. 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 configured, some addi-
				   tional 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 subse-
				   quent 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 associ-
				   ated 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 5.11							    20 Jun 2008 						      mediator(7D)