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NAME
ct - Command-Set 80 (CS/80) cartridge tape access
DESCRIPTION
This entry describes the actions of the general HP-UX Command-Set 1980
(CS/80) cartridge tape drivers when referring to a CS/80 cartridge
tape as either a block- or character-special (raw) device.
Cartridge tapes are designed to work optimally as "streaming" devices,
and are not designed to start and stop frequently. Technically, they
are "random access" devices such as disks, but such access is both
less efficient and more stressful than streaming mode. Thus it is
possible to use a cartridge tape as a file system, or in general
access it randomly, but such use will more rapidly wear either or both
the tape drive and the media.
Cartridge tape units in either CS/80 disk drives or in stand-alone
devices can be accessed as blocked or raw devices.
Block special files access cartridge tapes via the system's normal
buffering mechanism. Buffering is done in such a way that concurrent
access through multiple opens or a mount of the same physical device
do not get out of phase. Block special files may be read and written
without regard to physical cartridge tape records. Each I/O operation
results in one or more logical block transactions. In general, this
mode is not recommended as it stresses the hardware.
There is also a raw interface via a character special file which
provides for direct transmission between the cartridge tape and the
user's read or write buffer. A single read or write operation results
in exactly one transaction. Therefore raw I/O is considerably more
efficient when many bytes are transmitted in a single operation
because blocked cartridge tape access requires potentially several
transactions and does not transmit directly to user space.
In raw I/O, there may be implementation dependent restrictions on the
alignment of the user buffer in memory and its maximum size. Also,
each transfer must occur on a record boundary, and must read a whole
number of records. The record size is a hardware-dependent value.
Selecting the proper buffer size when accessing a cartridge tape
device through the raw interface is critical to the performance of the
cartridge tape device and other devices connected on the same HPIB. A
large buffer in certain situations can increase performance but has
the potential to block other devices on the HPIB until all the data
for a request has been transferred. On the other hand when a small
buffer is used and the application is unable to keep the cartridge
tape device streaming, performance and the wear and tear of the device
suffer because of tape repositioning. The optimal solution is to keep
the tape streaming while using a small buffer. To select the proper
buffer size, consider two factors: the cartridge tape device being
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accessed and the application which is accessing the cartridge tape
device.
Some cartridge tape units (see DEPENDENCIES) support a feature called
immediate report mode. During writing, this mode enables the drive to
complete a write transaction with the host before the data has
actually been written to the tape from the drive's buffer. This
allows the host to start gathering data for the next write request
while the data for the previous request is still in the process of
being written. During reading, this mode enables the drive to read
ahead after completing a host read request. This allows the drive to
gather data for future read requests while the host is still
processing data from the previous read request. When data is
requested or supplied at a sufficient rate, immediate report mode
allows the drive to stream the tape continuously across multiple
read/write requests, as opposed to having to reposition the tape
between each read/write request. Repositioning adds to the wear and
tear of the cartridge tape device and decreases the performance. Some
cartridge tape devices (see DEPENDENCIES) do not support immediate
report mode and as such cannot stream across multiple requests.
If the cartridge tape device being accessed supports immediate report
mode and the application can maintain a data rate that allows the
cartridge tape device to stream multiple requests, a small buffer (1
Kbyte to 12 Kbytes) is suggested so that the HP-IB is not blocked for
a significant amount of time. For cartridge tape devices that do not
support immediate report mode or applications that cannot maintain a
data rate that allows the cartridge tape device to stream multiple
requests, a large buffer (64 Kbytes) is suggested so that the number
of tape repositions is reduced.
Each raw access is independent of other raw accesses and of block
accesses to the same physical device. Thus, transfers are not
guaranteed to occur in any particular order. Having multiple programs
access the cartridge tape is, in effect, random access, and is subject
to the warnings above.
In raw I/O, each operation is completed to the device before the call
returns. For block-mode writes, the data may be cached until it is
convenient for the system to write it. In addition, block-mode reads
potentially do a one (or more) block read-ahead. The interaction of
block-mode and raw access to the same cartridge tape is not specified,
and in general is unpredictable. Because block-mode writes can be
delayed, it is possible for a program to generate requests much more
rapidly than the drive can actually process them. Flushing a large
number of requests could take several minutes, and during that time
the system will not have use of the buffers taken by these requests,
and thus will suffer a possibly severe performance degradation. If
the tape is integral with the system disk, very little disk activity
may be possible until the buffers are flushed.
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Cartridge tape device file names are in the following format:
/dev/[r]ct/[r]c#[d#][s#)
where the first r indicates a raw interface to the cartridge tape, the
second r is reserved to indicate that this cartridge tape is on a
remote system, the c# indicates the controller number, the d#
optionally indicates the drive, and the s# optionally indicates a
section number. The assignment of controller, drive, and section
numbers is described in the system administrator's manual for your
system.
WARNINGS
Like disks, the cartridge tape units in CS/80 disk drives can be
accessed as blocked or raw devices. However, using a cartridge tape
as a file system severely limits the life expectancy of the tape
drive. Tapes should be used only for system back-up and other needs
where data must be stored on tape for transport or other purposes.
ct does not support access of DDS and QIC cartridge tape devices.
DEPENDENCIES
HP7941CT/HP9144A/HP35401
These cartridge tape devices support the immediate report
mode.
HP7942/HP7946
These cartridge tape devices support the immediate report
mode. The use of a small buffer size is not recommended with
these shared controller devices when there is simultaneous
access to the disk, because the disk accesses will prevent
proper tape streaming.
HP7908/HP7911/HP7912/HP7914
These cartridge tape devices do not support the immediate
report mode.
AUTHOR
ct was developed by HP and AT&T.
SEE ALSO
mkdev(1M), mknod(1M), tcio(1), disk(7), intro(7), mt(7).
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