25.5.2 Preparing a shared disk unit

There must be an external hard disk that can be shared between the primary system and the secondary system (or between the normal BES and the alternate BES in the case of the standby-less system switchover facility). This hard disk is called a shared disk unit.

Organization of this subsection
(1) Shared disk allocation
(2) Shared disk access control

(1) Shared disk allocation

Figure 25-29 shows shared disk allocation.

Figure 25-29 Shared disk allocation

[Figure]

Explanation
  1. When you use the standby system switchover facility or the standby-less system switchover (1:1) facility, allocate a shared disk to each unit because system switchover occurs on a unit-by-unit basis.
  2. When you use the standby-less system switchover (effects distributed) facility, allocate a shared disk to each server because system switchover occurs on a server-by-server basis. You cannot store information from multiple servers in a single shared disk.

The following HiRDB file system areas are created in a shared disk unit:

Notes
  • Set up these HiRDB file system areas so that the HiRDBs of both the primary system and the secondary system can reference the shared disk unit using the same path name. When you use the standby-less system switchover (1:1) facility, set up these HiRDB file system areas so that both the normal BES unit and the alternate BES unit can reference the shared disk unit using the same path name. When you use the standby-less system switchover (effects distributed) facility, set up these HiRDB file system areas so that all units within the HA group can reference the shared disk unit using the same path name. However, for the standby-less system switchover (effects distributed) facility, create the unit status file in an independent, non-shared disk that is different from those used for server status files, system log files, and synchronization point dump files.
  • The shared disk in which HiRDB file system areas for shared RDAREAs are created must be activated in the write mode from all units. For this reason, the disk must not be deactivated or activated in conjunction with system switchover.
  • Only HiRDB file system areas created in character special files can be shared. HiRDB file system areas in regular files cannot be shared.
  • Do not use regular files on a shared disk. If system switchover occurs when regular files are in a status that does not apply to shared disks (for example, data remains in the operating system cache even though HiRDB finished writing it to the target files), any updates made to the files may be lost.

(2) Shared disk access control

When the system switchover source and target both attempt to access the shared disk at the same time while the system switchover facility is being used, the database may become corrupted. For this reason, accesses from the system to the shared disk must be controlled. This shared disk access control is performed by either the cluster software or HiRDB.

Normally, the method described in (a) Shared disk access control by the cluster software is used to perform access control on the shared disk. To use the method described in (b) Shared disk access control by HiRDB, HA monitor 01-08 or later is required.

(a) Shared disk access control by the cluster software

The cluster software can perform access control on the shared disk. It exercises controls so that the running system is active and the standby and stopped systems are inactive, which means that only the running system can access the shared disk. Figure 25-30 shows how the cluster software exercises control over shared disk access.

Figure 25-30 Shared disk access control by the cluster software

[Figure]

Explanation
Because an inactive system cannot access the shared disk, only the running system is capable of accessing the shared disk.

For details about the shared disk switching method (between active and inactive), see the cluster software documentation.

If you are using HA monitor, you must specify the disk operand in the HA monitor's servers definition statement.

(b) Shared disk access control by HiRDB

To use HiRDB to perform access control on the shared disk, HA monitor 01-08 or later is required.

HiRDB can perform access control on the shared disk. In such a case, shared disk switching (between active and inactive) is not performed. System switchover takes place in the following sequence:

  1. A failure that results in system switchover occurs.
  2. HiRDB confirms that all processes (HiRDB processes) have terminated in the source system.
  3. System switchover takes place.
  4. The target system starts accessing the shared disk.

Figure 25-31 shows the shared disk access control that is provided by HiRDB.

Figure 25-31 Shared disk access control by HiRDB

[Figure]