Planning for disk fencing

Disk fencing is one of the features for quarantine policies that is available in PowerHA® SystemMirror®.

Disk fencing prerequisites

To use disk fencing in PowerHA SystemMirror, your disks must meet the following requirements:
  • All disks that are managed by the storage systems must be enabled for SCSI-3 Persistent Reservation (PR). Some storage systems do not enable SCSI-3 PR capabilities by default.
  • All disks must not be in use when disk fencing is enabled (the volume groups must be offline).
  • The disks must not be reserved before you start PowerHA SystemMirror. You can use the storage systems software to release any disk reservations.

Critical resource group settings

To use the disk fencing option, you must identify the critical resource group and the storage subsystem must support SCSI-3 PR and ODM reserve_policy of PR_shared. This policy is applied to all disks in the volume groups that are configured for all resource groups.
Reservation type
The disk must have a persistent reservation type of Write Exclusive All Registrant (WEAR). You can run the following clmgr commands to check if your disk supports SCSI-3 capability (WEAR - type 7h) to support PowerHA SystemMirror disk fencing:
clmgr -a SCSIPR_CAPABLE query physical_volume <disk>
clmgr -a SCSIPR_CAPABLE query volume_group <vg>
where disk is the name of the disk and vg is the name of the volume group.
Note: If the command does not return Yes, you cannot use the disk for disk fencing. You must choose some other disk or quarantine policy.
reserve_policy
Defines whether a reservation method is running on the disk. The reserve_policy of PR_shared is the required policy that applies the shared host method for the disk. To view the attributes for the disk, run the lsattr -Rl <diskname> -a reserve_policy command.
To use the disk fencing feature, you must specify a critical resource group. The critical resource group that you specify must meet the following criteria:
  • The critical resource group cannot be added as a child in any parent_child, start_after, or stop_after, dependency relationship.
  • The critical resource group must have all nodes in the cluster as participating nodes.
  • The critical resource group must have the startup policy as Online On First Available Node.
The reserve_policy for a disk of a volume group that has cluster services that are running and disk fencing is enabled, must use the following settings:
Table 1. Disk settings
Option Value
Configured Reserve Policy PR_shared
Effective Reserve Policy PR_shared
Reservation Status SCSI PR reservation (Write Exclusive All Registrant)

EMC storage and SCSI-3 PR

EMC disks do not support SCSI-3 PR capabilities by default. If you attempt to configure disk fencing on EMC storage without enabling SCSI-3 PR, an error occurs.

The following is an example of steps to enable SCSI-3 PR on some EMC storage devices:
Note: Before you complete the following steps, each disk must not be in use on any nodes in the cluster and the volume groups must not be in a varyon state. The following commands are part of the EMC software packages that are installed on an AIX® LPAR.
  1. Identify the IDs for the storage devices and disks in the EMC storage subsystem by running the following command:
    
powermt display dev=hdiskpowerX
Pseudo name=hdiskpowerX
Symmetrix ID=000194900568                                
Logical device ID=0036                                   
Device WWN=6000097000019490056853303030xxxx
state=alive; policy=SymmOpt; queued-IOs=0
  2. Enable the SCSI-3 PR capabilities in the EMC storage subsystem by running the following command:
    symconfigure -sid 000194900568 -cmd "set device 0036 attribute=SCSI3_persist_reserv;" commit -v -noprompt
  3. Complete the following steps to rediscover the storage disks in the AIX operating system:
    1. Remove the storage devices or disks by running the rmdev -R -d -l hdiskpowerX command, where hdiskpowerX is the name of the disk.
    2. Run the cfgmgr command to discover the storage devices or disks.
  4. Verify that the SCSI-3 PR capabilities are enabled for the storage devices or disks by running the following command:
    /usr/symcli/bin/symdev -sid 000194900568 show 0036 | grep SCSI
    SCSI-3 Persistent Reserve: Enabled

For more information about configuring EMC for SCSI-3 PR, see the EMC Support website.

Hitachi storage and SCSI-3 PR

Hitachi disks do not support SCSI-3 PR capabilities by default. You must manually enable SCSI-3 PR capabilities for each disk that is part of a volume group that is managed by PowerHA SystemMirror. To enable the SCSI-3 PR capabilities, in the Hitachi Storage Navigator you must enable the Host Mode Options (HMHO) 2 and 72.