Интерактивная система просмотра системных руководств (man-ов)
sa (4)
>> sa (4) ( FreeBSD man: Специальные файлы /dev/* )
sa (8) ( FreeBSD man: Команды системного администрирования )
sa (8) ( Linux man: Команды системного администрирования )
BSD mandoc
NAME
sa
- SCSI Sequential Access device driver
SYNOPSIS
device sa
DESCRIPTION
The
driver provides support for all
SCSI
devices of the sequential access class that are attached to the system
through a supported
SCSI
Host Adapter.
The sequential access class includes tape and other linear access devices.
A
SCSI
Host
adapter must also be separately configured into the system
before a
SCSI
sequential access device can be configured.
MOUNT SESSIONS
The
driver is based around the concept of a
``mount session
''
which is defined as the period between the time that a tape is
mounted, and the time when it is unmounted.
Any parameters set during
a mount session remain in effect for the remainder of the session or
until replaced.
The tape can be unmounted, bringing the session to a
close in several ways.
These include:
Closing a `rewind device',
referred to as sub-mode 00 below.
An example is
/dev/sa0
Using the MTOFFL
ioctl(2)
command, reachable through the
`offline
'
command of
mt(1).
It should be noted that tape devices are exclusive open devices, except in
the case where a control mode device is opened.
In the latter case, exclusive
access is only sought when needed (e.g., to set parameters).
SUB-MODES
Bits 0 and 1 of the minor number are interpreted as
`sub-modes'
The sub-modes differ in the action taken when the device is closed:
00
A close will rewind the device; if the tape has been
written, then a file mark will be written before the rewind is requested.
The device is unmounted.
01
A close will leave the tape mounted.
If the tape was written to, a file mark will be written.
No other head positioning takes place.
Any further reads or writes will occur directly after the
last read, or the written file mark.
10
A close will rewind the device.
If the tape has been
written, then a file mark will be written before the rewind is requested.
On completion of the rewind an unload command will be issued.
The device is unmounted.
BLOCKING MODES
SCSI
tapes may run in either
`variable
'
or
`fixed
'
block-size modes.
Most
QIC -type
devices run in fixed block-size mode, where most nine-track tapes and
many new cartridge formats allow variable block-size.
The difference between the two is as follows:
Variable block-size:
Each write made to the device results in a single logical record
written to the tape.
One can never read or write
part
of a record from tape (though you may request a larger block and read
a smaller record); nor can one read multiple blocks.
Data from a single write is therefore read by a single read.
The block size used
may be any value supported by the device, the
SCSI
adapter and the system (usually between 1 byte and 64 Kbytes,
sometimes more).
When reading a variable record/block from the tape, the head is
logically considered to be immediately after the last item read,
and before the next item after that.
If the next item is a file mark,
but it was never read, then the next
process to read will immediately hit the file mark and receive an end-of-file notification.
Fixed block-size:
Data written by the user is passed to the tape as a succession of
fixed size blocks.
It may be contiguous in memory, but it is
considered to be a series of independent blocks.
One may never write
an amount of data that is not an exact multiple of the blocksize.
One may read and write the same data as a different set of records.
In other words, blocks that were written together may be read separately,
and vice-versa.
If one requests more blocks than remain in the file, the drive will
encounter the file mark.
As there is some data to return (unless
there were no records before the file mark), the read will succeed,
returning that data.
The next read will return immediately with a value
of 0.
(As above, if the file mark is never read, it remains for the next
process to read if in no-rewind mode.)
FILE MARK HANDLING
The handling of file marks on write is automatic.
If the user has
written to the tape, and has not done a read since the last write,
then a file mark will be written to the tape when the device is
closed.
If a rewind is requested after a write, then the driver
assumes that the last file on the tape has been written, and ensures
that there are two file marks written to the tape.
The exception to
this is that there seems to be a standard (which we follow, but do not
understand why) that certain types of tape do not actually write two
file marks to tape, but when read, report a `phantom' file mark when the
last file is read.
These devices include the QIC family of devices.
(It might be that this set of devices is the same set as that of fixed
block devices.
This has not been determined yet, and they are treated
as separate behaviors by the driver at this time.)
An -nosplit
The
driver was written for the
CAMSCSI
subsystem by
An Justin T. Gibbs
and
An Kenneth Merry .
Many ideas were gleaned from the
st
device driver written and ported from
Mach
2.5
by
An Julian Elischer .
The current owner of record is
An Matthew Jacob
who has suffered too many
years of breaking tape drivers.
BUGS
This driver lacks many of the hacks required to deal with older devices.
Many older
SCSI-1
devices may not work properly with this driver yet.
Additionally, certain
tapes (QIC tapes mostly) that were written under
Fx 2.X
are not automatically read correctly with this driver: you may need to
explicitly set variable block mode or set to the blocksize that works best
for your device in order to read tapes written under
Fx 2.X.
Fine grained density and compression mode support that is bound to specific
device names needs to be added.
Support for fast indexing by use of partitions is missing.
