Users Guide

12 Performance Considerations for Tape Drives and Libraries
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In conclusion, RAID 0 tends to be the best overall configuration for read and write performance,
but does not allow for redundancy. RAID 1 is the worst performer overall, as all data written to
the array is mirrored and reads come from a single disk. RAID 5 tends to be a good read performer
but average write performer; however, RAID 5 improves if more disks are added to the array. If
the RAID is within an enclosure that offers significant levels of cache memory, then performance
limitations during restore operations may be abated. Backup operations will still be subject to
limitations of the RAID configuration.
In addition, the characteristics of the array still depend
heavily on the specific hard drive characteristics listed in "Hard Drive Performance."
General Performance Considerations When Using Multiple
Drives in Tape Libraries
When multiple tape drives are utilized simultaneously to perform data backups (such as in tape
libraries), additional aspects of the hardware configuration must be considered. By employing
simple performance-minded methods in setting up hardware and cabling configurations, additional
throughput bottlenecks can be limited.
SCSI Configurations
The latest high-performance tape drives offered in tape libraries support the Ultra160 specification
of the SCSI interface standard. Therefore, to achieve maximum performance, backup servers
utilizing SCSI must have an HBA installed that supports data speeds of Ultra160 or higher. A SCSI
HBA that meets this requirement will allow each tape drive to communicate with the host at a rate
of 160 MB/sec on the SCSI bus. The higher data rate of the SCSI bus compared to tape drive
speeds allows multiple devices to be connected to the same bus without sacrificing device
performance. But only to a point.
The 160 MB/sec data rate of an Ultra160 bus is the maximum possible data throughput rate to all
devices connected to the bus. Therefore, a single tape drive will not consume the full bandwidth of
the bus because it can read or write data to tape at up to 80 MB/sec (native). Multiple tape drives,
however, can combine to consume the full 160 MB/sec offered by the bus if each is operating at its
maximum native performance. Each additional drive connected to the same bus after this point
will reduce the average performance of each drive.
Therefore, to achieve maximum performance from a tape library, it is recommended to connect
no more than two tape drives to each SCSI bus. See "Recommended Cabling Configurations" for
specific details and illustrations. A SCSI HBA supporting at least Ultra160 should be used, but
upgrading to an Ultra320 HBA will not lead to an additional improvement in performance if the
tape drive's specification is Ultra160.