sa(4)

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 sup

ported 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:

1. Closing a `rewind device', referred to as sub-mode 00

below. An
example is /dev/sa0.

2. Using the MTOFFL ioctl(2) command, reachable through

the `offline'
command of mt(1).

It should be noted that tape devices are exclusive open de

vices, 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 re

quested. 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 re

quested. On
completion of the rewind an unload command will be is

sued. 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 sys

tem (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 sepa

rately, 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 suc

ceed, 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 behav

iors 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

mt(1), scsi(4)

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 de

vice driver
written and ported from Mach 2.5 by Julian Elischer.

The current owner of record is Matthew Jacob who has suf

fered 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 writ

ten 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.

BSD June 6, 1999

BSD