Devices 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 configura-
tion and state information.
mediator host
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Devices mediator(7D)
A host that runs the rpc.metamedd(1M) daemon and that
has 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 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 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 accessible
replicas is equal to half]1 the total number of config-
ured replicas. For example, if a system is configured
with 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 the
replicas 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 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 mediatorhostlist Adds mediator hosts to the named
set. A mediatorhostlist 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 mediatorhostlist Deletes mediator hosts from the
named diskset. Mediator hosts
are deleted from the diskset by
specifying 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.
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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 is
read in and the shared metadev-
ices 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
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 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 opera-
tion 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 tag-
number can be executed to select
the data associated with the
given tagnumber.
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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.
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