Disk volumes with different characteristics
NetBackup allows multiple disk volumes to reside within a single disk pool. Load-balancing strategies are applied across all disk volumes within the disk pool during media and device selection (MDS). In other words, NetBackup assumes that all disk volumes within a disk pool are somewhat similar. This includes the areas of size and performance characteristics. While asymmetric configuration will not cause backups to fail, it is likely to result in unpredictable performance.
Example 1:
Assume a disk pool with one 800-GB disk volume and one 80-GB disk volume. When NetBackup selects a disk volume to be used for a backup job, this selection is essentially driven by the size (free space) of the disk volume. Because of the size difference in this case, the larger disk volume will initially receive a larger share of the backup traffic. While this is quite correct from the point of view of available space, it will limit the system-wide performance as the bulk of the I/O traffic will be directed to the one disk.
Example 2:
Assume a disk pool with two disk volumes of the same size. However, one volume has a transfer rate of 100 MB/sec and the other has a transfer rate of 25 MB/sec. Once again, load balancing will select a disk volume based on available free space. As a result both disk volumes will see the same amount of traffic but backups and restores will run much faster to one disk volume than the other.
Recommendation: All disk volumes within a NetBackup disk pool should be of similar size and should have similar performance characteristics. When dealing with disk volumes that have significantly different characteristics they should be grouped into multiple disk pools, each containing disk volumes with similar characteristics, rather than all being placed in one pool.