Multiple, concurrent data streams reduce the time that is required to back up or restore the database. You can specify the number of data streams that the Tivoli® Storage Manager server uses for backup and restore operations.
For example, if you assign four drives to database backup processing, Tivoli Storage Manager attempts to write to all four drives concurrently. For restore operations, the server uses the information that is in the volume history file to determine the number of data streams that were used during the backup operation. The server attempts to use the same number of data streams during the restore operation. For example, if the backup operation used four data streams, the server attempts the restore operation by using four data streams.
The following table summarizes server actions for different drive-availability scenarios. An available drive is a drive that is online and that is ready for use by the server.
| Operation | If the number of available drives exceeds the specified number of streams, the server uses | If the number of available drives equals the specified number of streams, the server uses | If the number of available drives is less than the specified number of streams, the server uses |
|---|---|---|---|
| Backup | The number of drives that is equal to the specified number of streams. | The number of drives that is equal to the specified number of streams. | All available drives. |
| Restore | The number of drives that is equal to the number of streams that were used in the backup operation. A restore process never uses more drives than the number of streams that were used to back up the database. | The number of drives that is equal to the number of streams that were used in the backup operation. | All available drives. At least one drive is required for restore processing |
Suppose that you specify four data streams for database backup operations. To indicate the maximum number of volumes that can be simultaneously mounted, you specify 4 as the value of the MOUNTLIMIT parameter in the device class definition. If only three drives are available at the time of the backup operation, the operation runs by using three drives. A message is issued that indicates that fewer drives are being used for the backup operation than the number requested. If all four drives for the device class are online, but one drive is in use by another operation, the backup operation has a higher priority and preempts use of the drive. If you specify four data streams, but the value of the MOUNTLIMIT parameter is 2, only two streams are used.
Another potential disadvantage is that more volumes are required for multistream processing than for single-stream processing. For example, if the backup of an 850 GB database requires a single linear tape open (LTO) volume, switching to four data streams requires four volumes. Furthermore, those volumes might be partially filled, especially if you use high-capacity volumes and device compression. For example, if the backup of an 850 GB database uses only 30% of a tape volume after compression, then four-stream processing results in larger amounts of wasted space.