User Guide

Ampex 1308911-02 4-3
Tape Drive Operational Characteristics Tape Drive Block SizeTape Drive Block Size
4.2 Tape Drive Block Size
The DST/DIS tape drive is a fixed block device and enables the following block sizes to be
used during its operation:
• 1 byte (byte stream mode) - checksums are disabled.
• 80 to 1199840 bytes (even values only) - checksums can be enabled or disabled.
There are three block sizes that must be considered in tape drive operation:
• File block size – The block size used in a given file section. Every file section may have
a different block size, but only a single block size is used within a file section.
• Drive block size -- The block size number maintained within the tape drive. This is the
number returned by dd2_getparam_drive as %blksize, and dd2_status_drive as
%drvblksz.
• Write block size -- The block size number maintained within the device driver for new
writes. This is the number returned by dd2_status_drive as %wrblksz. This parameter,
and drive block size, are both set by dd2_setparam_drive. However, drive block size will
change as described below. Write block size will not change. Its default value is 8192.
Whenever you attempt a write at BOP or immediately after a filemark, drive block size will
first be set to the driver's write block size. This becomes the file block size for the new file
section. Whenever you attempt a read or a write within a file section, the drive block size will
first be set to the existing file's file block size. Then, the read() or write() will fail if the amount
of data to be transferred is not a multiple of the drive block size at it is now set.
4.3 Tape Partitions
All functions and utilities that read from, write to, or position the tape, will work only within
the boundaries of a partition, with the following exceptions:
• After an initial tape load, before establishing position, a rewind command rewinds a tape
to the beginning of the first partition.
• The dd2_setpos_tape function enables specification of a partition number. If this requires
a partition change, the behavior is to position to the beginning of the target partition and
then to the target location within the partition.