NAME¶
sa —
SCSI Sequential Access device
driver
SYNOPSIS¶
device sa
DESCRIPTION¶
The
sa 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
sa 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.)
IOCTLS¶
The
sa driver supports all of the ioctls of
mtio(4).
FILES¶
- /dev/[n][e]sa[0-9]
- general form:
- /dev/sa0
- Rewind on close
- /dev/nsa0
- No rewind on close
- /dev/esa0
- Eject on close (if capable)
- /dev/sa0.ctl
- Control mode device (to examine state while another program
is accessing the device, e.g.).
DIAGNOSTICS¶
None.
SEE ALSO¶
cam(4),
mt(1)
AUTHORS¶
The
sa driver was written for the CAM SCSI subsystem by
Justin T. Gibbs and
Kenneth
Merry. Many ideas were gleaned from the
st device
driver written and ported from Mach 2.5 by
Julian
Elischer.
The current owner of record is
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
FreeBSD 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
FreeBSD 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.