Creating a DISK tiebreaker

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The DISK tiebreaker type is specific to AIX®. To create a DISK tiebreaker object, set the DeviceInfo persistent resource attribute to indicate the AIX device name or the PVID of the disk. The AIX device name must specify a SCSI or SCSI-like physical disk that is shared by all nodes of the peer domain. Disks with the DISK tie-breaker type only support these mpio pathing modules: FCPARRAY (for AIX 5.3), MPIO, and SDDPCM (with the single-path reserve policy). Any non-mpio disks can be supported if SCSI-2 reserve/release is supported.

Physical disks attached through Fiber Channel and iSCSI can serve as a DISK tiebreaker. However, IDE hard disks do not support the SCSI protocol and cannot serve as a DISK tiebreaker. Logical volumes also cannot serve as a DISK tiebreaker.

This type of tiebreaker can use a reserve/release mechanism or a persistent reserve. For reserve/release and persistent reserve, the disk needs to be re-reserved periodically to hold the reservation. For this reason, IBM® recommends that you also specify the HeartbeatPeriod persistent resource attribute when creating a tiebreaker of this type. The HeartbeatPeriod persistent resource attribute defines the interval at which the reservation is retried.

Attention: When defining tiebreaker resources, be aware that the disk on which IBM.Tiebreaker resources are stored should not also be used to store file systems.

To print every known physical volume in the system along with its physical disk name, enter the lspv command:

lspv

Output similar to the following is displayed:

hdisk0          000000371e5766b8                    rootvg          active
hdisk1          000069683404ed54                    None

To verify that a disk is a SCSI or SCSI-like disk and so a suitable candidate for a DISK tiebreaker, use the lsdev command. For example:

lsdev -C -l hdisk0

Output similar to the following is displayed

hdisk0 Available 10-60-00-0,0 16 Bit SCSI Disk Drive

In order to serve as a tie-breaker disk using reserve/release, the disk must be shared by all nodes of the peer domain. Check the physical volume ID returned by the lspv command to determine if the disk is shared between nodes (in the preceding output for the lspv command, the physical volume ID is listed in the second column; the volume ID for hdisk0 is 000000371e5766b8.) Be aware, however, that AIX remembers all disks that have been attached to the system, and the disks listed by the lspv command may no longer be attached. If such a disk was moved to another machine, it might appear that the disk is shared, when in fact it is no longer attached to the original machine.

The disk on which IBM.Tiebreaker resources are stored should not also be used to store file systems. If the nodes of the cluster share more than one disk, it may be difficult to determine which one is the tie-breaker disk, and which one is used for regular data. The output from the lsdev command shows the SCSI address associated with the disk. (In the preceding output for the lsdev command, the SCSI address is listed in the third column; the SCSI address for hdisk0 is 10-60-00-0,0.) This information will help you identify the correct disk if you are aware of the disk's address prior to its installation.

If the tie-breaker disk will be using persistent reserves, an additional step needs to be performed on each node.

Run the chdev command on the first node:


chdev -l hdisk -a PR_key_value=0x0001 -a reserve_policy=PR_exclusive

The PR_key_value must be a different value on the second node. Run the chdev command on the second node:
```
chdev -l hdisk -a PR_key_value=0x0002 -a reserve_policy=PR_exclusive
```

Once you know the device name, you can issue the mkrsrc command, as follows:

mkrsrc IBM.TieBreaker Name=disktb Type=DISK DeviceInfo="DEVICE=/dev/hdisk0" \
       HeartbeatPeriod=30

You can also use the PVID (instead of the device name) to specify the tiebreaker, as follows:

mkrsrc IBM.TieBreaker Name=disktb Type=DISK DeviceInfo="PVID=000000371e5766b8" \ 
       HeartbeatPeriod=30