freebsd-skq/share/man/man4/sa.4

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.\"	Julian Elischer <julian@freebsd.org>.  All rights reserved.
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.Dd June 6, 1999
.Dt SA 4
.Os FreeBSD
.Sh NAME
.Nm sa
.Nd SCSI Sequential Access Device Driver
.Sh SYNOPSIS
.Cd device sa
.Cd device sa1 at scbus0 target 4 unit 0
.Sh DESCRIPTION
The
.Nm sa
driver provides support for all
.Tn SCSI
devices of the sequential access class that are attached to the system
through a supported
.Tn SCSI
Host Adapter.
The sequential access class includes tape and other linear access devices.
.Pp
A
.Tn SCSI
Host
adapter must also be separately configured into the system
before a
.Tn SCSI
sequential access device can be configured.
.Sh MOUNT SESSIONS
The 
.Nm
driver is based around the concept of a 
.Dq Em 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:
.Bl -enum
.It
Closing a `rewind device',
referred to as sub-mode 00 below.
An example is 
.Pa /dev/rsa0 .
.It
Using the MTOFFL
.Xr ioctl 2
command, reachable through the
.Sq Cm offline
command of
.Xr mt 1 .
.El
.Pp
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).
.Sh SUB-MODES
Bits 0 and 1 of the minor number are interpreted as
.Sq sub-modes .
The sub-modes differ in the action taken when the device is closed:
.Bl -tag -width XXXX
.It 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.
.It 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.
.It 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.
.Sh BLOCKING MODES
.Tn SCSI
tapes may run in either 
.Sq Em variable
or
.Sq Em fixed
block-size modes.  Most 
.Tn QIC Ns -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:
.Bl -inset
.It 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 
.Em 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
.Tn SCSI
adapter and the system (usually between 1 byte and 64 Kbytes,
sometimes more).
.Pp
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.
.It 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.
.Pp
If one requests more blocks than remain in the file, the drive will
encounter the file mark.  Because 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.)
.El
.Sh 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 don't
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.)
.Sh IOCTLS
The
.Nm sa
driver supports all of the ioctls of
.Xr mtio 4 .
.Sh FILES
.Bl -tag -width /dev/[n][e]rsa[0-9] -compact
.It Pa /dev/[n][e]rsa[0-9]
general form:
.It Pa /dev/rsa0
Rewind on close
.It Pa /dev/nrsa0
No rewind on close
.It Pa /dev/ersa0
Eject on close (if capable)
.It Pa /dev/rsa0.ctl
Control mode device (to examine state while another program is
accessing the device, e.g.).
.Sh BUGS
This driver lacks many of the hacks required to deal with older devices.
Many older
.Tn SCSI-1
devices may not work properly with this driver yet.
.Pp
Additionally, certain 
tapes (QIC tapes mostly) that were written under FreeBSD 2.X
aren't 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.
.Pp
Fine grained density and compression mode support that is bound to specific
device names needs to be added.
.Pp
Support for fast indexing by use of partitions is missing.
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr mt 1 ,
.Xr scsi 4
.Sh HISTORY
The
.Nm sa
driver was written for the
.Tn CAM
.Tn SCSI
subsystem by Justin T. Gibbs and Kenneth Merry.
Many ideas were gleaned from the
.Nm st
device driver written and ported from
.Tn Mach
2.5
by Julian Elischer.
.sp
The current owner of record is Matthew Jacob who has suffered too many
years of breaking tape drivers.