File system fragmentation for storage pools on disk

Fragmentation problems can occur when you are writing to scratch volumes in multiple storage pools simultaneously, in one file system. Because unfragmented volumes generally perform better than fragmented volumes, use the DEFINE VOLUME command to preallocate volumes for sequential disk storage pools (DEVTYPE is FILE).

In some operating system environments, preallocating several FILE-device class volumes or random-disk pool volumes in parallel in the same file system can also result in fragmentation. If you are running a Windows system or using JFS2 file systems on AIX® or ext4 file systems on Linux, fragmentation is not a problem with preallocated volumes. Tivoli® Storage Manager uses operating system tools to allocate files without fragmentation, even when created in parallel.

If you encounter slow performance, you can use operating system tools to help determine how fragmented a storage pool volume file might be. The tools report the number of fragments per file. To achieve high read performance from a volume, the number of fragments per megabyte should not exceed 0.25. For example, if a 50 GB volume has more than 12,800 fragments, then read performance for the volume might be poor because of fragmentation.

Use the following tools to determine fragmentation level:

On AIX, issue the fileplace command.
On Linux, issue the filefrag command.
On Windows, use the contig utility. You can find details and usage information about the contig utility on the Microsoft TechNet site: http://technet.microsoft.com/en-us/.

To avoid fragmentation, use preallocated volumes and use more file systems per storage pool. By using a greater number of file systems, you can usually avoid fragmentation because writing to different volumes is more likely to occur in different file systems.