| SA(4) | Device Drivers Manual | SA(4) |
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:
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:
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:
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.
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.
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 mounted.
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.
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:
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.sa driver does not currently use the
RECOVER BUFFERED DATA command.The sa driver supports all of the ioctls
of mtio(4).
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.
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.
| May 5, 2017 | Debian |