1mediator(7D) Devices mediator(7D)
2
6 mediator - support for HA configurations consisting of two strings of
7 drives
8
10 Beginning with a prior version, Solaris Volume Manager provided support
11 for high-availability (HA) configurations consisting of two hosts that
12 share at least three strings of drives and that run software enabling
13 exclusive access to the data on those drives from one host. (Note:
14 Volume Manager, by itself, does not actually provide a high-availabil‐
15 ity environment. The diskset feature is an enabler for HA configura‐
16 tions.)
17 Volume Manager provides support for a low-end HA solution consisting of
20 two hosts that share only two strings of drives. The hosts in this type
21 of configuration, referred to as mediators, run a special daemon,
22 rpc.metamedd(1M). The mediator hosts take on additional responsibili‐
23 ties to ensure that data is available in the case of host or drive
24 failures.
25 In a mediator configuration, two hosts are physically connected to two
28 strings of drives. This configuration can survive the failure of a sin‐
29 gle host or a single string of drives, without administrative interven‐
30 tion. If both a host and a string of drives fail (multiple failures),
31 the integrity of the data cannot be guaranteed. At this point, adminis‐
32 trative intervention is required to make the data accessible.
33 The following definitions pertain to a mediator configuration:
36 diskset
38 A set of drives containing metadevices and hot spares that can be
40 shared exclusively (but not concurrently) by two hosts.
41 Volume Manager state database
44 A replicated database that stores metadevice configuration and
46 state information.
47 mediator host
50 A host that runs the rpc.metamedd(1M) daemon and that has been
52 added to a diskset. The mediator host participates in checking the
53 state database and the mediator quorum.
54 mediator quorum
57 The condition achieved when the number of accessible mediator hosts
59 is equal to half+1 the total number of configured mediator hosts.
60 Because it is expected that there will be two mediator hosts, this
61 number will normally be 2 ([(2/2) + 1] = 2.)
62 replica
65 A single copy of the Volume Manager metadevice state database.
67 replica quorum
70 The condition achieved when the number of accessible replicas is
72 equal to half+1 the total number of configured replicas. For exam‐
73 ple, if a system is configured with ten replicas, the quorum is met
74 when six are accessible ([(10/2) + 1 = 6]).
75 A mediator host running the rpc.metamedd(1M) daemon keeps track of
79 replica updates. As long as the following conditions are met, access to
80 data occurs without any administrative intervention:
81 o The replica quorum is not met.
83 o Half of the replicas are still accessible.
85 o The mediator quorum is met.
87 The following conditions describe the operation of mediator hosts:
90 1. If the is met, access to the diskset is granted. At this
92 point no mediator host is involved.
93 2. If the replica quorum is not met, half of the replicas are
95 accessible, the mediator quorum is met, and the replica and
96 mediator data match, access to the diskset is granted. The
97 mediator host contributes the deciding vote.
98 3. If the replica quorum is not met, half of the replicas are
100 accessible, the mediator quorum is not met, half of the
101 mediator hosts is accessible, and the replica and mediator
102 data match, the system prompts you to grant or deny access
103 to the diskset.
104 4. If the replica quorum is not met, half of the replicas are
106 accessible, the mediator quorum is met, and the replica and
107 mediator data do not match, access to the diskset is read-
108 only. You can delete replicas, release the diskset, and
109 retake the diskset to gain read-write access to the data in
110 the diskset.
111 5. In all other cases, the diskset access is read-only. You can
113 delete replicas, release the diskset, and retake the diskset
114 to gain read-write access to the data in the diskset.
115 The metaset(1M) command administers disksets and mediator hosts. The
118 following options to the metaset command pertain only to administering
119 mediator hosts.
120 -a -m mediator_host_list Adds mediator hosts to the named set. A
122 mediator_host_list is the nodename of the
123 mediator host to be added and up to 2 other
124 aliases for the mediator host. The nodename
125 and aliases for each mediator host are sep‐
126 arated by commas. Up to 3 mediator hosts
127 can be specified for the named diskset.
128 -d -m mediator_host_list Deletes mediator hosts from the named
131 diskset. Mediator hosts are deleted from
132 the diskset by specifying the nodename of
133 mediator host to delete.
134 -q Displays an enumerated list of tags per‐
137 taining to ``tagged data'' that may be
138 encountered during a take of the ownership
139 of a diskset.
140 -t [-f] -y Takes ownership of a diskset safely, unless
143 -f is used, in which case the take is
144 unconditional. If metaset finds that
145 another host owns the set, this host will
146 not be allowed to take ownership of the
147 set. If the set is not owned by any other
148 host, all the disks within the set will be
149 owned by the host on which metaset was
150 executed. The metadevice state database is
151 read in and the shared metadevices con‐
152 tained in the set become accessible. The -t
153 option will take a diskset that has stale
154 databases. When the databases are stale,
155 metaset will exit with code 66, and a mes‐
156 sage will be printed. At that point, the
157 only operations permitted are the addition
158 and deletion of replicas. Once the addition
159 or deletion of the replicas has been com‐
160 pleted, the diskset should be released and
161 retaken to gain full access to the data. If
162 mediator hosts have been configured, some
163 additional exit codes are possible. If
164 half of the replicas and half of the media‐
165 tor hosts are operating properly, the take
166 will exit with code 3. At this point, you
167 can add or delete replicas, or use the -y
168 option on a subsequent take. If the take
169 operation encounters ``tagged data,'' the
170 take operation will exit with code 2. You
171 can then run the metaset command with the
172 -q option to see an enumerated list of
173 tags.
174 -t [-f] -u tagnumber Once a tag has been selected, a subsequent
177 take with -u tagnumber can be executed to
178 select the data associated with the given
179 tagnumber.
180
183 metaset(1M), md(7D), rpc.metamedd(1M), rpc.metad(1M)
184 Sun Cluster documentation, Solaris Volume Manager Administration Guide
187
189 Diskset administration, including the addition and deletion of hosts
190 and drives, requires all hosts in the set to be accessible from the
191 network.
192SunOS 5.11 20 Jun 2008 mediator(7D)