Tandberg Data
About Tape Streamers
2-33
Tandberg SLR Product Line SCSI Functional Specifications
2.9.
Optimizing Streaming Operation
For a tape streamer device underruns and overruns are
typically time consuming. The stop, reposition and
restart sequence may take a second or two. When this
happens frequently, the average data transfer rate may
suffer greatly.
In many cases using a combination of Automatic Velocity
Control (AVC) and Forced Streaming can optimize
streaming operation. AVC will automatically reduce tape
speed if the transfer rate from host is weak or the
compression ratio is high. Forced streaming will keep
the drive streaming by writing dummy data while waiting
for more data from host.
2.9.1.
Forced Streaming
The Forced Streaming Count in the Mode Sense/Select -
Miscellaneous Parameters Page is used to enable or
disable the Forced Streaming mechanism in the drive. The
parameter is designed to give the maximum number of
times to rewrite the same block while waiting for more
data from host. When the maximum number is reached the
drive is entering underrun mode and tape will stop and
do a backspace while waiting for more data from host.
The Forced Streaming Count is by nature a write
parameter only. But, note that if forced streaming
blocks are written to the tape, the read speed will
reduce accordingly since the tape has to pass dummy data
while reading.
The Forced Streaming feature trades performance for
capacity. The number of underruns can be reduced, but
some capacity will be lost to the rewrite process.
Note however that if the host system is able to write
data at twice the speed of the native rate of the
current media and the data can be compressed 2:1 or
more, you will always have at least 2:1 compression no
matter how many forced streaming blocks written.
On SLR140, SLR100, SLR75, SLR60 and SLR7 drives the
Forced Streaming setting is ignored. Due to a large data
buffer and short underrun time, there is no reason to
use Forced Streaming.
2.9.2.
AVC (Automatic Velocity Control)
The AVC bit in the Mode Sense/Select - Device
Configuration page is used to enable or disable the AVC
mechanism in the drive. When an underrun (during write)
or an overrun (during read) occurs, the tape will stop
and do a backspace while waiting for more data to be
written or read from host. In this underrun/overrun
situation the AVC algorithm will try to predict if a
speed reduction is optimal or not. If a speed reduction
is chosen, the AVC algorithm will, while streaming, try
to predict if it is optimal to increase the speed again.
A speed increase will perform a forced underrun/overrun.
