Adding disks to a file system

Many file systems grow rapidly, so after creating a file system you might decide that more disk space is required.

Storage in a file system is divided in storage pools. The maximum size of any one disk that can be added to an existing storage pool is set approximately to the sum of the disk sizes when the storage pool is created. The actual value is shown in the mmdf command output.

Once a storage pool is created, the maximum size cannot be altered. However, you can create a new pool with larger disks, and then move data from the old pool to the new one.

When establishing a storage pool and when adding disks later to an existing storage pool, you should try to keep the sizes of the disks fairly uniform. GPFS™ allocates blocks round robin, and as the utilization level rises on all disks, the small ones will fill up first and all files created after that will be spread across fewer disks, which reduces the amount of prefetch that can be done for those files.

To add disks to a GPFS file system, first decide if you will:

Create new disks using the mmcrnsd command.
In this case, you must also decide whether to create a new set of NSD and pools stanzas or use the rewritten NSD and pool stanzas that the mmcrnsd command produces. In a rewritten file, the disk usage, failure group, and storage pool values are the same as the values that are specified in the mmcrnsd command.
Select disks no longer in use in any file system. Issue the mmlsnsd -F command to display the available disks.

The disk may then be added to the file system using the stanza file as input to the mmadddisk command.

Note: Rebalancing of files is an I/O intensive and time consuming operation, and is important only for file systems with large files that are mostly invariant. In many cases, normal file update and creation will rebalance your file system over time, without the cost of the rebalancing.

For more information, see mmadddisk command and mmcrnsd command.