What Slows Down Tape Drives; Maintaining The Flow Of Data - HP BB118BV - StorageWorks Data Protector Express Package User's Manual & Technical Reference

What Slows Down Tape Drives?

Tape drives work most efficiently when they are "streaming," that is, when the tape itself is constantly
moving forward and a steady flow of data is being written to the tape as it passes the write head. In
general, tape drives will "stream" if there is a constant flow of data available to write to the tape.
Whenever there is an interruption in the data and the drive must wait for the data, the "stream" breaks off
and the tape stops. Additionally, after the tape stops, the drive must reposition the write head and regain
the velocity of the tape. To do so, the drive will rewind the tape prior to where it stopped writing and then
restart the tape forward again. This can be a time-consuming process, especially if it occurs repeatedly.
For this reason, it is important to keep data constantly flowing to the tape drive. The performance of your
job will be maximized when the tape drive constantly has data available to it.
Maintaining the flow of data
There are several steps you can take to make sure that data is constantly available to the backup device.
Local buses compared to network connections: In general, data is transmitted more efficiently over a
local bus than over a network connection. Thus, for example, performance increases if the backup
device is connected to the file or application server rather than to a different machine on the network,
although either arrangement is possible.
NOTE:
The catalog need not be placed on the same machine as the backup device. The location of the catalog
does not affect the rate of data transmission. (Other job tasks, such as building backup selection lists
and opening and closing files, create network traffic and require CPU calculations. While the location
of the catalog will affect these parts of the job, the advantages of a remote location of the catalog often
outweigh any speed loss.)
Add a second device controller
local connection between them. If the backup device and the volume share the same device controller,
this limits the speed of data transmission and may cause an error in Data Protector Express. Therefore,
you should add a second device controller specifically for the backup device. This can effectively double
the rate of data transmission and enhance operation. At the least, you should connect the backup
device to a different SCSI chain or IDE channel.
Use a wider data stream
the rate at which data can be transferred. Many tape drives are capable of writing data much faster
than can be transferred over older device controllers and network connections. By upgrading to more
recent technology with wider data streams, there will be fewer interruptions in the stream of data to the
backup device. For example, consider using a Wide or Ultra Wide SCSI controller for backup devices
that support such an interface. Additionally, for network connections, consider upgrading from 10Base-T
Ethernet to 100Base-TX.
TIP:
Consult your manufacturer's documentation to see if your backup device would benefit from a wider
data stream.
Use the proper number of data streams
data streams per backup device simultaneously. This feature can greatly increase your job speed since
multiple devices can simultaneously send data. For example, four backup devices can support up to 32
simultaneous data streams.
You control the data streams of a volume, directory or file from its Storage page. Normally, the Backup
stream field on the Storage page of a volume is set to Create new stream, while the Backup stream of a
directory or file is set to Use existing stream. By changing these options, you can change the number of
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Tips, Techniques and Strategies
A backup device and volume may be on the same machine with a
For both network and local connections, the width of the data stream affects
Data Protector Express is capable of controlling up to eight