Effect of fragment size on NetBackup restores
Fragment size can affect NetBackup restores for non-multiplexed and multiplexed images.
The fragment size affects where tape markers are placed and how many tape markers are used. (The default fragment size is 1 terabyte for tape storage units and 512 GB for disk.) As a rule, a larger fragment size results in faster backups, but may result in slower restores when recovering a small number of individual files.
The "Reduce fragment size to" setting on the Storage Unit dialog limits the largest fragment size of the image. By limiting the size of the fragment, the size of the largest read during restore is minimized, reducing restore time. The fragment size is especially important when restoring a small number of individual files rather than entire directories or file systems.
For many sites, a fragment size of approximately 10 GB results in good performance for backup and restore.
For a fragment size, consider the following:
Larger fragment sizes usually favor backup performance, especially when backing up large amounts of data. Smaller fragments can slow down large backups. Each time a new fragment is created, the backup stream is interrupted.
Larger fragment sizes do not hinder performance when restoring large amounts of data. But when restoring a few individual files, larger fragments may slow down the restore.
Larger fragment sizes do not hinder performance when restoring from non-multiplexed backups. For multiplexed backups, larger fragments may slow down the restore. In multiplexed backups, blocks from several images can be mixed together within a single fragment. During restore, NetBackup positions to the nearest fragment and starts reading the data from there, until it comes to the desired file. Splitting multiplexed backups into smaller fragments can improve restore performance.
During restores, newer, faster devices can handle large fragments well. Slower devices, especially if they do not use fast locate block positioning, restore individual files faster if fragment size is smaller. (In some cases, SCSI fast tape positioning can improve restore performance.)
Unless you have particular reasons for creating smaller fragments, larger fragment sizes are likely to yield better overall performance. For example, reasons for creating smaller fragments are the following: restoring a few individual files, restoring from multiplexed backups, or restoring from older equipment.