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.)
PARAMETERS
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.
TIMEOUTS
The sa driver has a set of default timeouts for SCSI commands (READ,
WRITE, TEST UNIT READY, etc.) that will likely work in most cases for
many tape drives.
For newer tape drives that claim to support the SPC-4 standard (SCSI
Primary Commands 4) or later standards, the sa driver will attempt to use
the REPORT SUPPORTED OPERATION CODES command to fetch timeout descriptors
from the drive. If the drive does report timeout descriptors, the sa
driver will use the drive's recommended timeouts for commands.
The timeouts in use are reported in units of thousandths of a second via
the kern.cam.sa.%d.timeout.* sysctl(8) variables.
To override either the default timeouts, or the timeouts recommended by
the drive, you can set one of two sets of loader tunable values. If you
have a drive that supports the REPORT SUPPORTED OPERATION CODES timeout
descriptors (see the camcontrol(8) opcodes subcommand) it is generally
best to use those values. The global kern.cam.sa.timeout.* values will
override the timeouts for all sa driver instances. If there are 5 tape
drives in the system, they'll all get the same timeouts. The
kern.cam.sa.%d.timeout.* values (where %d is the numeric sa instance
number) will override the global timeouts as well as either the default
timeouts or the timeouts recommended by the drive.
To set timeouts after boot, the per-instance timeout values, for example:
kern.cam.sa.0.timeout.read, are available as sysctl variables.
If a tape drive arrives after boot, the global tunables or per-instance
tunables that apply to the newly arrived drive will be used.
Loader tunables:
kern.cam.sa.timeout.erase
kern.cam.sa.timeout.locate
kern.cam.sa.timeout.mode_select
kern.cam.sa.timeout.mode_sense
kern.cam.sa.timeout.prevent
kern.cam.sa.timeout.read
kern.cam.sa.timeout.read_position
kern.cam.sa.timeout.read_block_limits
kern.cam.sa.timeout.report_density
kern.cam.sa.timeout.reserve
kern.cam.sa.timeout.rewind
kern.cam.sa.timeout.space
kern.cam.sa.timeout.tur
kern.cam.sa.timeout.write
kern.cam.sa.timeout.write_filemarks
Loader tunable values and sysctl(8) values:
kern.cam.sa.%d.timeout.erase
kern.cam.sa.%d.timeout.locate
kern.cam.sa.%d.timeout.mode_select
kern.cam.sa.%d.timeout.mode_sense
kern.cam.sa.%d.timeout.prevent
kern.cam.sa.%d.timeout.read
kern.cam.sa.%d.timeout.read_position
kern.cam.sa.%d.timeout.read_block_limits
kern.cam.sa.%d.timeout.report_density
kern.cam.sa.%d.timeout.reserve
kern.cam.sa.%d.timeout.rewind
kern.cam.sa.%d.timeout.space
kern.cam.sa.%d.timeout.tur
kern.cam.sa.%d.timeout.write
kern.cam.sa.%d.timeout.write_filemarks
As mentioned above, the timeouts are set and reported in thousandths of a
second, so be sure to account for that when setting them.
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
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.
SEE ALSO
mt(1), cam(4)
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 13.1-RELEASE-p6 January 18, 2022 FreeBSD 13.1-RELEASE-p6
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