Choosing a Tape Drive
By Linus Chang
Tape drives remain the leading technology used by organizations for backup and
archiving. However, the plethora of tape drives on the market can make choosing the
appropriate tape drive a confusing task. How do you select a tape drive that satisfies
your needs without blowing the budget? The following are just some of the main factors to
consider.
1. Capacity
Select a tape drive that has sufficient capacity to store your backups. Tape drives are
able to compress data so that more data may fit on the tape, which is why manufacturers
specify both a native capacity and a compressed capacity, usually with a compression ratio
at 2:1. However, highly-compressed files such as those in video and sound formats are
hardly compressible at all. For this reason, do not heed the specified compressed capacity
when choosing a tape drive.
A good way to determine the size of the backup job after compression is to study logs
of past backups. If these are unavailable, it is safe to assume that the data can be
compressed at a ratio of 1.4:1, unless the hard drive contains an usually large number of
highly-compressed files.
2. Transfer rate
The transfer rate of the tape drive is becomes important when there is limited "window
of opportunity" in which backup jobs may run. It is often desirable for backups to take
place during the night when network use is at its lowest.
Select a tape drive that is capable of completing a backup job within your window of
opportunity. For instance, to back up 400GB per night, you will require a transfer rate of
about 30GB/hour.
A little known fact about tape drives is that data must be supplied to them at a sufficient
rate in order to keep them streaming, or else the tape suffers from start-stop motion. This
motion severely degrades the life of the drive and tapes and the reliability of backups.
There are two usual reasons why tape drive are not be supplied with data at a
sufficient rate. Firstly, the rate at which data is read from the hard disk of the server
is insufficient. This rate is dependent on the sizes and locations of the files on disk
and is generally unpredictable, but can be determined by the use of specialised software.
Secondly, if data is being transferred over a network of computers to a backup server,
the network may be incapable of supplying data at a sufficient rate. The maximum throughput
of a network is predictable and easy to measure, based on previous network performance.
Consider a network using 10BaseT Ethernet. This transfer rate through this type of
network cannot exceed 10MB/s, so it is immediately apparent that a tape drive requiring
20MB/s is inappropriate.
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