ST(4) | Device Drivers Manual | ST(4) |
st
— SCSI/ATAPI
tape driver
st* at scsibus? target ? lun ?
st1 at scsibus0 target 4 lun 0
st* at atapibus? drive ? flags 0x0000
The st
driver provides support for SCSI
and Advanced Technology Attachment Packet Interface (ATAPI) tape drives. It
allows a tape drive to be run in several different modes depending on minor
numbers and supports several different ‘sub-modes’. The device
can have both a
raw
interface and a
block
interface; however, only the raw interface is usually used (or
recommended).
SCSI and ATAPI devices have a relatively high level interface and talk to the system via a SCSI or ATAPI adapter and a SCSI or ATAPI adapter driver (e.g. ahc(4), pciide(4)). A SCSI or ATAPI adapter must also be separately configured into the system before a SCSI or ATAPI tape can be configured.
As the SCSI or ATAPI adapter is probed during
boot, the SCSI or ATAPI bus is scanned for devices. Any devices found which
answer as
‘Sequential’
type devices will be attached to the st
driver.
The st
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:
MTOFFL
ioctl(2) command, reachable
through the ‘offline
’ command of
mt(1).There are several different ‘operation’ modes. These are controlled by bits 2 and 3 of the minor number and are designed to allow users to easily read and write different formats of tape on devices that allow multiple formats. The parameters for each mode can be set individually by hand with the mt(1) command. When a device corresponding to a particular mode is first mounted, The operating parameters for that mount session are copied from that mode. Further changes to the parameters during the session will change those in effect for the session but not those set in the operation mode. To change the parameters for an operation mode, one must compile them into the “quirk” table in the driver's source code.
In addition to the operating modes mentioned above, 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. Because 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 an EOF (as above, if the file mark is never read, it remains for the next process to read if in no-rewind mode).
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 don't 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).
Attempts to write past EOM and how EOM is reported are handled slightly differently based upon whether EARLY WARNING recognition is enabled in the driver.
If EARLY WARNING recognitions is not enabled, then detection of EOM (as reported in SCSI Sense Data with an EOM indicator) causes the write operation to be flagged with I/O error (EIO). This has the effect for the user application of not knowing actually how many bytes were read (since the return of the read(2) system call is set to −1).
If EARLY WARNING recognition is enabled, then detection of EOM (as reported in SCSI Sense Data with an EOM indicator) has no immediate effect except that the driver notes that EOM has been detected. If the write completing didn't transfer all data that was requested, then the residual count (counting bytes not written) is returned to the user application. In any event, the next attempt to write (if that is the next action the user application takes) is immediately completed with no data transferred, and a residual returned to the user application indicating that no data was transferred. This is the traditional UNIX EOF indication. The state that EOM had been seen is then cleared.
In either mode of operation, the driver does not prohibit the user application from writing more data, if it chooses to do so. This will continue up until the physical end of media, which is usually signalled internally to the driver as a CHECK CONDITION with the Sense Key set to VOLUME OVERFLOW. When this or any otherwise unhandled error occurs, an error return of EIO will be transmitted to the user application. This does indeed mean that if EARLY WARNING is enables and the device continues to set EOM indicators prior to hitting physical end of media, that an indeterminate number of 'short write returns' as described in the previous paragraph will occur. However, the expected user application behaviour (in common with other systems) is to close the tape and rewind and request another tape upon the receipt of the first EOM indicator, possibly after writing one trailer record.
Because different tape drives behave differently, there is a
mechanism within the source to st
to quickly and
conveniently recognize and deal with brands and models of drive that have
special requirements.
There is a table (called the “quirk table”) in which the identification strings of known errant drives can be stored. Alongside each is a set of flags that allows the setting of densities and blocksizes for each of the modes, along with a set of `QUIRK' flags that can be used to enable or disable sections of code within the driver if a particular drive is recognized.
The following
ioctl(2) calls apply to SCSI
tapes. Some also apply to other tapes. They are defined in the header file
<sys/mtio.h>
.
MTIOCGET
struct mtget
) Retrieve the status and parameters
of the tape. Error status and residual is unlatched and cleared by the
driver when it receives this ioctl.MTIOCTOP
struct mtop
) Perform a multiplexed operation. The
argument structure is as follows:
struct mtop { short mt_op; daddr_t mt_count; };
The following operation values are defined for mt_op:
MTWEOF
MTFSF
MTBSF
MTFSR
MTBSR
MTREW
MTOFFL
MTNOP
MTERASE
MTCACHE
MTNOCACHE
MTSETBSIZ
MTSETDNSTY
MTCMPRESS
MTEWARN
MTIOCRDSPOS
uint32_t
) Read device logical block position. Not
all drives support this option.MTIOCRDHPOS
uint32_t
) Read device hardware block position.
Not all drives support this option.MTIOCSLOCATE
uint32_t
) Position the tape to the specified
device logical block position.MTIOCHLOCATE
uint32_t
) Position the tape to the specified
hardware block position. Not all drives support this option.This st
driver was originally written for
Mach 2.5 by Julian Elischer, and was ported to
NetBSD by Charles Hannum. This man page was edited
for NetBSD by Jon Buller.
The selection of compression could possibly also be usefully done as with a minor device bit.
August 23, 1996 | NetBSD 10.99 |