IBM General Parallel File System for AIX: Administration and Programming Reference

Specifying disk descriptors

You may use the rewritten DiskDesc file created by the mmcrvsd command in an SP environment or the mmcrlv command in an HACMP environment, or create your own file, or enter the disk descriptors on the command line. When using the DiskDesc file created by either the mmcrvsd command or the mmcrlv command command, the values supplied on input to the command for Disk Usage and FailureGroup are used. When creating your own file or entering the descriptors on the command line, you must specify these values or accept the system defaults.

Whether you create a DescFile or a DiskDesc, on your commands, the positional parameters of the disk descriptor are the same. The only difference is how the disk descriptors are delimited. When entering them on the command line using a DiskDesc, each descriptor is separated by a semicolon and the entire string must be enclosed in quotation marks. When entering them in a DescFile, each descriptor is on a separate line. The current maximum number of disk descriptors that can be defined for any single file system is 1024.

Each descriptor contains positional parameters that apply to the disk, separated by colons (:).

You must provide a descriptor for each disk to be passed to the GPFS file system. Each descriptor must be specified in the form:

DiskName:::DiskUsage:FailureGroup

DiskName

In an SP environment, you must specify the virtual shared disk name. For details on generating the DiskName for a virtual shared disk, see the mmcrvsd command. To use an existing virtual shared disk volume group in the file system, only the virtual shared disk name need be specified in the disk descriptor. GPFS performance and recovery processes function best with one disk per virtual shared disk. If you want to create virtual shared disks with more than one disk, refer to the IBM Parallel System Support Programs for AIX: Managing Shared Disks manual.

In an HACMP environment, you must specify the logical volume name. For details on generating the DiskName for a logical volume, see the mmcrlv command.

Disk Usage

Specify a disk usage or accept the default (see the IBM General Parallel File System for AIX: Concepts, Planning, and Installation Guide and search for recoverability considerations):

• dataAndMetadata (default)
• dataOnly
• metadataOnly

Failure Group

A number identifying the failure group to which this disk belongs. You can specify any value from -1 (where -1 indicates that the disk has no point of failure in common with any other disk) to 4000. If you do not specify a failure group, the value defaults to the server node number plus 4000. GPFS uses this information during data and metadata placement to assure that no two replicas of the same block are written in such a way as to become unavailable due to a single failure. All disks that have a common point of failure, such as all disks that are attached to the same virtual shared disk server node, should be placed in the same failure group. All disks that are attached to the same disk adapter, should be placed in the same failure group.

Any parameter that is allowed to default must be explicitly skipped. For example, if you want to use gpfslv01 for both data and metadata (the default) and have it placed in failure group 5, the disk descriptor would be:

gpfslv01::::5

If you also wanted to allow the last parameter, Failure Group, to default, you could omit the delimiting colons:

gpfslv01

or leave them in if you prefer:

gpfslv01::::