SCSI Sequential Access device driver
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.
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
- Using the MTOFFL ioctl(2) command, reachable
through the ‘
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:
- 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
- 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
- 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
’ 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.)
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
behavior, prior to
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.
This variable, when set to 1, will configure all
sa devices to split large buffers into
smaller pieces when needed.
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:
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
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
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
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.)
driver supports all of the ioctls of
- general form:
- Rewind on close
- No rewind on close
- Eject on close (if capable)
- Control mode device (to examine state while another program is accessing
the device, e.g.).
driver was written for the CAM SCSI
subsystem by Justin T. Gibbs
. Many ideas were gleaned from
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.
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
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
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.