I/O fencing for shared file systems

You can configure I/O fencing to protect shared resources in a high-availability cluster environment.

A software or hardware malfunction might cause unresolved operations to be written to shared storage devices. I/O fencing can be used to isolate a server and prevent it from accessing shared storage. I/O fencing must be used if maintenance or testing is being performed on a server in a high-availability cluster. If a problem is detected on a server or within an application, the cluster manager can detect the problem and prevent the server from connecting to the shared data.

You can configure a script to run when a primary server fails. Fencing commands are called by setting the FAILOVER_CALLBACK configuration parameter, which runs a script when a failover is initiated.

Although I/O fencing is not required in order to use Informix® database software, configuring I/O fencing must be used to protect shared-disk systems from inadvertent loss

Types of I/O fencing

Several types of I/O fencing are available, including:

  • Power fencing - Powers off nodes if a problem is detected.
  • Fibre Channel Switch Fencing (requires SCSI-3 persistent group reservation) - Blocks a port on the fibre channel device by removing the problem node's reservation.

Perhaps the most common method of implementing I/O fencing is to use fibre channel fencing. The fibre channel switch supports the industry-standard SCSI-3 persistent group reservation (PR) technology. PR technology allows a set of systems to have temporary registrations with the disk and to coordinate a write-exclusive reservation with the disk containing the data.

In most cases, cluster manager software must be installed. The cluster manager software provides the drivers and utilities required to issue commands to the fibre channel switch. For example, the Linux Cluster Suite provides a script named fence_scsi. Sun Cluster provides a command named scdidadm.

Other fencing methods are also available depending on the cluster manger software used and different hardware capabilities.

The IBM® General Parallel File System (GPFS™) is a high performance shared disk clustered file system developed by IBM. The file system is available for use with AIX®, Linux, and Windows platforms. The file system can be used with the IBM HACMP™ Cluster Management Software. GPFS uses the SCSI PR fencing mechanism to support I/O fencing. Fencing issues must be resolved by the IBM GPFS support team.

Implementing I/O fencing

I/O fencing can be configured on several platforms, including:

  • Linux
  • Solaris
  • AIX
  • Windows

see the documentation provided by the manufacturer of your equipment for specific information about configuring I/O fencing.

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