Dell PowerVault TL2000 - Manual (Page 6)

6
Performance Considerations for Tape Drives and Libraries
www
.dell.com | support.dell.com
Attach tape drives and hard drives on separate controllers (internal or external) on separate host
bus adapters (HBAs). This depends somewhat on the performance capabilities of your controller,
but best practice is to keep the tape drive HBA separate from the hard drive HBA to ensure
maximum throughput. Most onboard dual-mode SCSI/RAID controllers share one processor,
which must share bandwidth between the RAID array and tape drive. Thus, one controller is
handling reads and writes between the hard disks and the tape drive, as well as calculating and
writing any necessary parity information to the hard drives. See "Hardware RAID Configuration
Considerations" for specific information on RAID arrays and parity bytes.
Dirty drive heads or old media. A dirty tape drive head or old media can cause high error rates and
a corresponding reduction in read/write speeds. Each time a drive attempts to rewrite or reread a
block on a tape, performance is degraded. Once a certain threshold of read/write errors is reached,
the drive will usually request cleaning. It is important to clean the drive heads on regular intervals,
or when requested.
The chance of encountering a bad block increases as media ages or is excessively used. The typical
lifecycle of a piece of LTO media is approximately 75 full tape read/writes.
Speed matching. Newer LTO drives will match the speed of the data being provided to the drive,
down to approximately one-half of the maximum uncompressed data transfer speed. If data is
provided to the drive at less than the lower speed matching limit, the drive must stop, wait for the
buffer to fill, rewind, and then attempt to write the buffer (this is known as "back hitching").
For example, the Dell PowerVault 110T (LTO2 and LTO3) tape drive matches speed down to
30 MB/sec while writing to LTO-3 media. If the host server can only provide data at 20 MB/sec, the
drive will "back hitch" while waiting for its buffers to fill. In this situation, the effective throughput
will be something less than 20 MB/sec (probably closer to 15 MB/sec).
Confirming Performance of Your Tape Drive
Certain tape drive manufacturers have a performance diagnostic mode built into the drive that can
be used to confirm throughput. The PowerVault 110T LTO-2 and LTO-3 (firmware 53
XX or later)
offer a diagnostic mode "F," which performs a quick read/write performance test on the drive and
media. If the performance rate is not within 6 percent of the maximum specified drive speed, the
test fails with an error message. No error message is displayed if the test passes. Consult your tape
drive User’s Manual for specific details on diagnostic mode "F."
NOTICE:
Diagnostic mode "F" requires media that can be safely overwritten as part of the diagnostic test.
Do not use media containing critical data. Any data residing on the media used in the diagnostic test will
be lost.
Hard Drive and RAID Array Configuration
Several hard drive and disk array (both internal and external) attributes can affect backup or restore
performance. These attributes, as well as recommended configurations that help achieve
maximum backup and restore speeds, are discussed in the following subsections. If the tape drive’s
sustainable throughput exceeds that of the disk array, then the tape drive’s peak performance will
not be realized.