The nodes in a cluster must always agree on the status of
a disk. In particular, if one node cannot write to a given disk,
all nodes must stop accessing that disk before the results of the
write operation are returned to the caller. Therefore, if a node
cannot contact a disk, it should contact another node to check on
the disk's status. If the disk fails, no node can access
it and the nodes can agree to detach the disk. If the disk does
not fail, but rather the access paths from some of the nodes fail,
the nodes cannot agree on the status of the disk. A policy must
exist to resolve this type of discrepancy.
Disk Detach Policies |
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In order to address the above discrepancy, the following policies
(set for a diskgroup) are provided. These can be set by using the vxedit(1M) command.
Under the global connectivity policy for shared disk group(s),
the detach occurs cluster-wide (globally), if any node in the cluster
reports a disk(s) failure.
Under local connectivity policy, in the event of disk(s) failing,
the failures are confined to the particular node(s) which saw the
failure. Note that an attempt is made to communicate with all nodes
in the cluster to ascertain the disk(s) usability. If all nodes
report a problem with the disk(s), a cluster-wide detach occurs.
This is the default policy.
Disk Group Activation |
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Disk group activation controls volume I/O capability from
different nodes in the cluster. It is not possible to activate a
diskgroup on a given node if it is activated in a conflicting mode
on another node in the cluster. Table 7-2 “Allowed and Conflicting Activation Modes” summarizes the allowed and conflicting activation modes
for shared disk groups, as follows:
Table 7-2 Allowed and Conflicting Activation Modes
Disk
group activated in the cluster as.... | Attempt
to activate disk group on another node as.... |
| | Exclusive
write | Shared
write | Read
only | Shared read |
Exclusive
write | Fail | Fail | Fail | Succeed |
Shared
write | Fail | Succeed | Fail | Succeed |
Read
only | Fail | Fail | Succeed | Succeed |
Shared read | Succeed | Succeed | Succeed | Succeed |
Shared disk groups can be automatically activated in any mode
during disk group creation or during manual or auto-import. To control auto-activation
of shared disk groups, the defaults file /etc/default/vxdg must be created.
The defaults file /etc/default/vxdg must contain the following lines:
default_activation_mode=activation-mode
enable_activation=true |
where activation-mode is: off, shared-write, shared-read, read-only, or exclusive-write.
When a shared disk group is created or imported, it is activated
in the specified mode. When a node joins the cluster, all shared
disk groups accessible from the node are activated in the specified
mode.
If the default file is edited when the vxconfigd utility is running, the vxconfigd utility must be restarted for the changes in the
default file to take effect.
Notes: |
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When enabling activation using the defaults file,
it is recommended that the defaults file be identical on all nodes
in the cluster. Otherwise, the results of activation are unpredictable. If the default activation node is anything other
than off, an activation following a cluster join, or a disk group
creation or import may fail if another node in the cluster has activated
the disk group in a conflicting mode.
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