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     sa — SCSI Sequential Access device driver


     device sa


     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.


     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 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).


     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.


     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.)


     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.)


     The sa driver supports all of the ioctls of mtio(4).


     /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.).




     cam(4), mt(1)


     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

     The current owner of record is Matthew Jacob who has suffered too many years of breaking
     tape drivers.


     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.