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SA(4)                  FreeBSD Kernel Interfaces Manual                  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 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:

     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).

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.)

BLOCK SIZES
     By default, the driver will NOT accept reads or writes to a tape device
     that are larger than may be written to or read from the mounted tape
     using a single write or read request.  Because of this, the application
     author may have confidence that his wishes are respected in terms of the
     block size written to tape.  For example, if the user tries to write a
     256KB block to the tape, but the controller can handle no more than
     128KB, the write will fail.  The previous FreeBSD behavior, prior to
     FreeBSD 10.0, was to break up large reads or writes into smaller blocks
     when going to the tape.  The problem with that behavior, though, is that
     it hides the actual on-tape block size from the application writer, at
     least in variable block mode.

     If the user would like his large reads and writes broken up into separate
     pieces, he may set the following loader tunables.  Note that these
     tunables WILL GO AWAY in FreeBSD 11.0.  They are provided for transition
     purposes only.

     kern.cam.sa.allow_io_split

         This variable, when set to 1, will configure all sa devices to split
         large buffers into smaller pieces when needed.

     kern.cam.sa.%d.allow_io_split

         This variable, when set to 1, will configure the given sa unit to
         split large buffers into multiple pieces.  This will override the
         global setting, if it exists.

     There are several sysctl(8) variables available to view block handling
     parameters:

     kern.cam.sa.%d.allow_io_split

         This variable allows the user to see, but not modify, the current I/O
         split setting.  The user is not permitted to modify this setting so
         that there is no chance of behavior changing for the application
         while a tape is mounted.

     kern.cam.sa.%d.maxio

         This variable shows the maximum I/O size in bytes that is allowed by
         the combination of kernel tuning parameters (MAXPHYS, DFLTPHYS) and
         the capabilities of the controller that is attached to the tape
         drive.  Applications may look at this value for a guide on how large
         an I/O may be permitted, but should keep in mind that the actual
         maximum may be restricted further by the tape drive via the SCSI READ
         BLOCK LIMITS command.

     kern.cam.sa.%d.cpi_maxio

         This variable shows the maximum I/O size supported by the controller,
         in bytes, that is reported via the CAM Path Inquiry CCB
         (XPT_PATH_INQ).  If this is 0, that means that the controller has not
         reported a maximum I/O size.

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 owner of record for many years was Matthew Jacob.  The current
     maintainer is Kenneth Merry

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.

     Partitions are only supported for status information and location.  It
     would be nice to add support for creating and editing tape partitions.

FreeBSD 11.0-PRERELEASE        February 12, 2015       FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | MOUNT SESSIONS | SUB-MODES | BLOCKING MODES | BLOCK SIZES | FILE MARK HANDLING | IOCTLS | FILES | DIAGNOSTICS | SEE ALSO | AUTHORS | BUGS

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