Topics: Backup & restore, Spectrum Protect

Raw logical volumes vs JFS file systems for TSM

When using TSM on AIX, JFS file systems for storage of the database volumes are preferred, and not RAW logical volumes. This subject usually tends to cause discussion, so here are the reasons for using JFS and not use RAW logical volumes:

When TSM has JFS files open, they are locked by JFS and other applications cannot write to them. However, raw logical volumes are not locked and any application can write to them. TSM tries to prevent starting more than one instance of the same server from the same directory, but it can be done. If you are using raw logical volumes, multiple server instances can simultaneously update the same information. This could cause errors in the database, recovery log, or storage pool raw logical volumes. Auditing a corrupted TSM database and fixing corruptions can take up to a day downtime.

After a database, recovery log, or storage pool volume is defined to TSM, you cannot change its size. TSM uses size information to determine where data is placed and whether volumes have been modified by other applications or utilities. However, if you use raw logical volumes, smit lets you increase their sizes. If the volume is defined to TSM before its size is increased, TSM cannot use the volume or its data.

The use of JFS file systems for database, recovery log, and storage pool volumes requires slightly more CPU than is required for raw volumes. However, JFS read-ahead caching improves performance. Lab tests have proven that the use of raw logical volumes tend to give better performance. So as long as you have enough CPU, you may still use JFS.

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