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:
- Closing a `rewind device', referred to as sub-mode 00
below. An example is /dev/sa0.
- 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.)
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.
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 a number of parameters. The
user can query parameters using “mt param -l” (which uses the
MTIOCPARAMGET ioctl) and the user can set
parameters using “mt param -s” (which uses the
MTIOCPARAMSET ioctl). See
mt(1) and
mtio(4)
for more details on the interface.
Supported parameters:
- sili
- The default is 0. When set to 1, it sets the Suppress
Incorrect Length Indicator (SILI) bit on tape reads. Tape drives normally
return sense data (which contains the residual) when the application reads
a block that is not the same length as the amount of data requested. The
SILI bit suppresses that notification in most cases. See the SSC-5 spec
(available at t10.org), specifically the section on the READ(6) command,
for more information.
- eot_warn
- The default is 0. By default, the
sa driver reports entering Programmable Early
Warning, Early Warning and End of Media conditions by returning a write
with 0 bytes written, and
errno set to
0. If eot_warn is set to 1, the
sa driver will set
errno to
ENOSPC when it enters any of the out of
space conditions.
- protection.protection_supported
- This is a read-only parameter, and is set to 1 if the tape
drive supports protection information.
- protection.prot_method
- If protection is supported, set this to the desired
protection method supported by the tape drive. As of SSC-5r03 (available
at t10.org), the protection method values are:
- 0
- No protection.
- 1
- Reed-Solomon CRC, 4 bytes in length.
- 2
- CRC32C, 4 bytes in length.
- protection.pi_length
- Length of the protection information, see above for
lengths.
- protection.lbp_w
- If set to 1, enable logical block protection on writes. The
CRC must be appended to the end of the block written to the tape driver.
The tape drive will verify the CRC when it receives the block.
- protection.lbp_r
- If set to 1, enable logical block protection on reads. The
CRC will be appended to the end of the block read from the tape driver.
The application should verify the CRC when it receives the block.
- protection.rdbp
- If set to 1, enable logical block protection on the RECOVER
BUFFERED DATA command. The sa driver does not
currently use the RECOVER BUFFERED DATA command.
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.).
The
sa driver supports injecting End Of Media (EOM)
notification to aid application development and testing. EOM is indicated to
the application by returning the read or write with 0 bytes written. In
addition, when EOM is injected, the tape position status will be updated to
temporarily show Beyond of the Programmable Early Warning (BPEW) status. To
see BPEW status, use the
MTIOCEXTGET ioctl,
which is used by the “mt status” command. To inject an EOM
notification, set the
kern.cam.sa.%d.inject_eom
sysctl variable to 1. One EOM notification will be sent, BPEW status will be set
for one position query, and then the driver state will be reset to normal.
mt(1),
cam(4)
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
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.