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Performance Considerations for Tape Drives and Libraries
5
Introduction
W
ith recent improvements in tape drive transfer rates, many host-side factors, such as RAID
(Redundant Array of Inexpensive [or Independent] Disks) configuration and hard-drive
specifications, must be considered when determining whether the host server and tape drive can
process data at the same rate. General configurations and attributes that may limit throughput
from the host server to the tape drive are discussed in "General Host Backup Considerations."
As multiple drives are placed into tape libraries, greater host bandwidths are needed to keep pace
with the potential throughput of multiple tape drives. Potential fibre limitations for multidrive
units, as well as recommended cabling configurations, are discussed in "General Performance
Considerations When Using Multiple Drives in Tape Libraries."
General Host Backup Considerations
The considerations in this section apply to both SCSI and storage area network (SAN) tape backup
configurations.
Tape Drive and Data Considerations
The following issues should be considered when evaluating performance:
Overhead from SCSI commands. Command overhead on the SCSI bus restrict
all
SCSI devices
in achieving theoretical maximum transfer speeds. Tape backup software does not account for this
overhead; instead, the software only measures the rate at which data is written to the tape. For
example, the drive may be processing 80 MB/sec of data, but only writing 77 MB/sec of data.
The latter rate is what the backup software will report.
Tape block sizes. 64 Kb block sizes are optimal for most tape drives. However, some backup
applications allow the user to change block size, even though a larger size will not enhance
performance. Using block sizes less than 64 Kb can actually hinder performance. See your backup
software User's Guide for information on adjusting the block size of your tape device.
Backup software buffer size. For optimal backup performance, backup software buffers should be
as large as possible. Some applications allow users to change the buffer size, which can help
maintain a steady stream of data to and from the drive and significantly increase transfer rates,
especially of small files. The larger the buffer, the more data it can hold and the less time the disk
spends seeking the data; however, this can affect memory and CPU performance. See your tape
backup application User’s Guide for specific details.
Drivers and firmware. Always ensure that the SCSI or fibre controller and tape drive have the latest
drivers and firmware installed. Visit support.dell.com to download the latest drivers and firmware
for your Dell PowerVault tape product.