Computer Science
ST(4) Linux Programmer's Manual ST(4)
NAME
st - SCSI tape device
SYNOPSIS
#include <sys/mtio.h>
int ioctl(int fd, int request [, (void *)arg3])
int ioctl(int fd, MTIOCTOP, (struct mtop *)mt_cmd)
int ioctl(int fd, MTIOCGET, (struct mtget *)mt_status)
int ioctl(int fd, MTIOCPOS, (struct mtpos *)mt_pos)
DESCRIPTION
The st driver provides the interface to a variety of SCSI
tape devices. Currently, the driver takes control of all
detected devices of type "sequential-access." The st
driver uses major device number 9.
Each device uses eight minor device numbers. The lower-
most five bits in the minor numbers are assigned sequen-
tially in the order of detection. The minor numbers can be
grouped into two sets of four numbers: the principal
(auto-rewind) minor device numbers, n, and a "no-rewind"
device numbers, (n+ 128). Devices opened using the prin-
cipal device number will be sent a REWIND command when
they are closed. Devices opened using the "no-rewind"
device number will not. (Note that using an auto-rewind
device for positioning the tape with, for instance, mt
does not lead to the desired result: the tape is rewound
after the mt command and the next command starts from the
beginning of the tape).
Within each group, four minor numbers are available to
define devices with different characteristics (block size,
compression, density, etc.) When the system starts up,
only the first device is available. The other three are
activated when the default characteristics are defined
(see below). (By changing compile-time constants, it is
possible to change the balance between the maximum number
of tape drives and the number of minor numbers for each
drive. The default allocation allows control of 32 tape
drives. For instance, it is possible to control up to 64
tape drives with two minor numbers for different options.)
Devices are typically created by:
mknod -m 666 /dev/st0 c 9 0
mknod -m 666 /dev/st0l c 9 32
mknod -m 666 /dev/st0m c 9 64
mknod -m 666 /dev/st0a c 9 96
mknod -m 666 /dev/nst0 c 9 128
mknod -m 666 /dev/nst0l c 9 160
mknod -m 666 /dev/nst0l c 9 192
mknod -m 666 /dev/nst0a c 9 224
There is no corresponding block device.
The driver uses an internal buffer that has to be large
enough to hold at least one tape block. In kernels before
2.1.121, the buffer is allocated as one contiguous block.
This limits the block size to the largest contiguous block
of memory the kernel allocator can provide. The limit is
currently 128 kB for the 32-bit architectures and 256 kB
for the 64-bit architectures. In newer kernels the driver
allocates the buffer in several parts if necessary. By
default, the maximum number of parts is 16. This means
that the maximum block size is very large (2 MB if alloca-
tion of 16 blocks of 128 kB succeeds).
The driver's internal buffer size is determined by a com-
pile-time constant which can be overridden with a kernel
startup option. In addition to this, the driver tries to
allocate a larger temporary buffer at run-time if neces-
sary. However, run-time allocation of large contiguous
blocks of memory may fail and it is advisable not to rely
too much on dynamic buffer allocation with kernels older
than 2.1.121 (this applies also to demand-loading the
driver with kerneld or kmod).
The driver does not specifically support any tape drive
brand or model. After system start-up the tape device
options are defined by the drive firmware. For example, if
the drive firmware selects fixed block mode, the tape
device uses fixed block mode. The options can be changed
with explicit ioctl() calls and remain in effect when the
device is closed and reopened. Setting the options
affects both the auto-rewind and the non-rewind device.
Different options can be specified for the different
devices within the subgroup of four. The options take
effect when the device is opened. For example, the system
administrator can define one device that writes in fixed
block mode with a certain block size, and one which writes
in variable block mode (if the drive supports both modes).
The driver supports tape partitions if they are supported
by the drive. (Note that the tape partitions have nothing
to do with disk partitions. A partitioned tape can be seen
as several logical tapes within one medium.) Partition
support has to be enabled with an ioctl. The tape location
is preserved within each partition across partition
changes. The partition used for subsequent tape opera-
tions is selected with an ioctl. The partition switch is
executed together with the next tape operation in order to
avoid unnecessary tape movement. The maximum number of
partitions on a tape is defined by a compile-time constant
(originally four). The driver contains an ioctl that can
format a tape with either one or two partitions.
Device /dev/tape is usually created as a hard or soft link
to the default tape device on the system.
DATA TRANSFER
The driver supports operation in both fixed block mode and
variable block mode (if supported by the drive). In fixed
block mode the drive writes blocks of the specified size
and the block size is not dependent on the byte counts of
the write system calls. In variable block mode one tape
block is written for each write call and the byte count
determines the size of the corresponding tape block. Note
that the blocks on the tape do don't contain any informa-
tion about the writing mode: when reading, the only impor-
tant thing is to use commands that accept the block sizes
on the tape.
In variable block mode the read byte count does not have
to match the tape block size exactly. If the byte count is
larger than the next block on tape, the driver returns the
data and the function returns the actual block size. If
the block size is larger than the byte count, the
requested amount of data from the start of the block is
returned and the rest of the block is discarded.
In fixed block mode the read byte counts can be arbitrary
if buffering is enabled, or a multiple of the tape block
size if buffering is disabled. Kernels before 2.1.121
allow writes with arbitrary byte count if buffering is
enabled. In all other cases (kernel before 2.1.121 with
buffering disabled or newer kernel) the write byte count
must be a multiple of the tape block size.
A filemark is automatically written to tape if the last
tape operation before close was a write.
When a filemark is encountered while reading, the follow-
ing happens. If there are data remaining in the buffer
when the filemark is found, the buffered data is returned.
The next read returns zero bytes. The following read
returns data from the next file. The end of recorded data
is signaled by returning zero bytes for two consecutive
read calls. The third read returns an error.
IOCTLS
The driver supports three ioctl requests. Requests not
recognized by the st driver are passed to the SCSI driver.
The definitions below are from /usr/include/linux/mtio.h:
MTIOCTOP - Perform a tape operation
This request takes an argument of type (struct mtop *).
Not all drives support all operations. The driver returns
an EIO error if the drive rejects an operation.
/* Structure for MTIOCTOP - mag tape op command: */
struct mtop {
short mt_op; /* operations defined below */
int mt_count; /* how many of them */
};
Magnetic Tape operations for normal tape use:
MTBSF Backward space over mt_count filemarks.
MTBSFM Backward space over mt_count filemarks.
Reposition the tape to the EOT side of the
last filemark.
MTBSR Backward space over mt_count records (tape
blocks).
MTBSS Backward space over mt_count setmarks.
MTCOMPRESSION Enable compression of tape data within the
drive if mt_count is non-zero and disable
compression if mt_count is zero. This com-
mand uses the MODE page 15 supported by most
DATs.
MTEOM Go to the end of the recorded media (for
appending files).
MTERASE Erase tape.
MTFSF Forward space over mt_count filemarks.
MTFSFM Forward space over mt_count filemarks.
Reposition the tape to the BOT side of the
last filemark.
MTFSR Forward space over mt_count records (tape
blocks).
MTFSS Forward space over mt_count setmarks.
MTLOAD Execute the SCSI load command. A special
case is available for some HP autoloaders.
If mt_count is the constant
MT_ST_HPLOADER_OFFSET plus a number, the
number is sent to the drive to control the
autoloader.
MTLOCK Lock the tape drive door.
MTMKPART Format the tape into one or two partitions.
If mt_count is non-zero, it gives the size
of the first partition and the second parti-
tion contains the rest of the tape. If
mt_count is zero, the tape is formatted into
one partition. This command is not allowed
for a drive unless the partition support is
enabled for the drive (see MT_ST_CAN_PARTI-
TIONS below).
MTNOP No op - flushes the driver's buffer as a
side effect. Should be used before reading
status with MTIOCGET.
MTOFFL Rewind and put the drive off line.
MTRESET Reset drive.
MTRETEN Retension tape.
MTREW Rewind.
MTSEEK Seek to the tape block number specified in
mt_count. This operation requires either a
SCSI-2 drive that supports the LOCATE com-
mand (device-specific address) or a Tand-
berg-compatible SCSI-1 drive (Tandberg,
Archive Viper, Wangtek, ... ). The block
number should be one that was previously
returned by MTIOCPOS if device-specific
addresses are used.
MTSETBLK Set the drive's block length to the value
specified in mt_count. A block length of
zero sets the drive to variable block size
mode.
MTSETDENSITY Set the tape density to the code in
mt_count. The density codes supported by a
drive can be found from the drive documenta-
tion.
MTSETPART The active partition is switched to mt_count
. The partitions are numbered from zero.
This command is not allowed for a drive
unless the partition support is enabled for
the drive (see MT_ST_CAN_PARTITIONS below).
MTUNLOAD Execute the SCSI unload command (does not
eject the tape).
MTUNLOCK Unlock the tape drive door.
MTWEOF Write mt_count filemarks.
MTWSM Write mt_count setmarks.
Magnetic Tape operations for setting of device options (by
the superuser):
MTSETDRVBUFFER
Set various drive and driver options according to
bits encoded in mt_count. These consist of the
drive's buffering mode, 13 Boolean driver options,
the buffer write threshold, defaults for the block
size and density, and timeouts (only in kernels >=
2.1). A single operation can affect only one item
in the list above (the Booleans counted as one
item.)
A value having zeros in the high-order 4 bits will
be used to set the drive's buffering mode. The
buffering modes are:
0 The drive will not report GOOD status on
write commands until the data blocks are
actually written to the medium.
1 The drive may report GOOD status on write
commands as soon as all the data has been
transferred to the drive's internal
buffer.
2 The drive may report GOOD status on write
commands as soon as (a) all the data has
been transferred to the drive's internal
buffer, and (b) all buffered data from
different initiators has been successfully
written to the medium.
To control the write threshold the value in
mt_count must include the constant
MT_ST_WRITE_THRESHOLD logically ORed with a block
count in the low 28 bits. The block count refers
to 1024-byte blocks, not the physical block size
on the tape. The threshold cannot exceed the
driver's internal buffer size (see DESCRIPTION,
above).
To set and clear the Boolean options the value in
mt_count must include one the constants
MT_ST_BOOLEANS, MT_ST_SETBOOLEANS, MT_ST_CLEAR-
BOOLEANS, or MT_ST_DEFBOOLEANS logically ORed with
whatever combination of the following options is
desired. Using MT_ST_BOOLEANS the options can be
set to the values defined in the corresponding
bits. With MT_ST_SETBOOLEANS the options can be
selectively set and with MT_ST_DEFBOOLEANS selec-
tively cleared.
The default options for a tape device are set with
MT_ST_DEFBOOLEANS. A non-active tape device (e.g.,
device with minor 32 or 160) is activated when the
default options for it are defined the first time.
An activated device inherits from the device acti-
vated at start-up the options not set explicitly.
The Boolean options are:
MT_ST_BUFFER_WRITES (Default: true)
Buffer all write operations in fixed block
mode. If this option is false and the
drive uses a fixed block size, then all
write operations must be for a multiple of
the block size. This option must be set
false to write reliable multi-volume
archives.
MT_ST_ASYNC_WRITES (Default: true)
When this options is true write operations
return immediately without waiting for the
data to be transferred to the drive if the
data fits into the driver's buffer. The
write threshold determines how full the
buffer must be before a new SCSI write com-
mand is issued. Any errors reported by the
drive will be held until the next opera-
tion. This option must be set false to
write reliable multi-volume archives.
MT_ST_READ_AHEAD (Default: true)
This option causes the driver to provide
read buffering and read-ahead in fixed
block mode. If this option is false and
the drive uses a fixed block size, then all
read operations must be for a multiple of
the block size.
MT_ST_TWO_FM (Default: false)
This option modifies the driver behavior
when a file is closed. The normal action
is to write a single filemark. If the
option is true the driver will write two
filemarks and backspace over the second
one.
Note: This option should not be set true
for QIC tape drives since they are unable
to overwrite a filemark. These drives
detect the end of recorded data by testing
for blank tape rather than two consecutive
filemarks. Most other current drives also
detect the end of recorded data and using
two filemarks is usually necessary only
when interchanging tapes with some other
systems.
MT_ST_DEBUGGING (Default: false)
This option turns on various debugging mes-
sages from the driver (effective only if
the driver was compiled with DEBUG defined
non-zero).
MT_ST_FAST_EOM (Default: false)
This option causes the MTEOM operation to
be sent directly to the drive, potentially
speeding up the operation but causing the
driver to lose track of the current file
number normally returned by the MTIOCGET
request. If MT_ST_FAST_EOM is false the
driver will respond to an MTEOM request by
forward spacing over files.
MT_ST_AUTO_LOCK (Default: false)
When this option is true, the drive door is
locked when the device is opened and
unlocked when it is closed.
MT_ST_DEF_WRITES (Default: false)
The tape options (block size, mode, com-
pression, etc.) may change when changing
from one device linked to a drive to
another device linked to the same drive
depending on how the devices are defined.
This option defines when the changes are
enforced by the driver using SCSI-commands
and when the drives auto-detection capabil-
ities are relied upon. If this option is
false, the driver sends the SCSI-commands
immediately when the device is changed. If
the option is true, the SCSI-commands are
not sent until a write is requested. In
this case the drive firmware is allowed to
detect the tape structure when reading and
the SCSI-commands are used only to make
sure that a tape is written according to
the correct specification.
MT_ST_CAN_BSR (Default: false)
When read-ahead is used, the tape must
sometimes be spaced backward to the correct
position when the device is closed and the
SCSI command to space backwards over
records is used for this purpose. Some
older drives can't process this command
reliably and this option can be used to
instruct the driver not to use the command.
The end result is that, with read-ahead and
fixed block mode, the tape may not be cor-
rectly positioned within a file when the
device is closed.
MT_ST_NO_BLKLIMS (Default: false)
Some drives don't accept the READ BLOCK
LIMITS SCSI command. If this is used, the
driver does not use the command. The draw-
back is that the driver can't check before
sending commands if the selected block size
is acceptable to the drive.
MT_ST_CAN_PARTITIONS (Default: false)
This option enables support for several
partitions within a tape. The option
applies to all devices linked to a drive.
MT_ST_SCSI2LOGICAL (Default: false)
This option instructs the driver to use the
logical block addresses defined in the
SCSI-2 standard when performing the seek
and tell operations (both with MTSEEK and
MTIOCPOS commands and when changing tape
partition). Otherwise the device-specific
addresses are used. It is highly advisable
to set this option if the drive supports
the logical addresses because they count
also filemarks. There are some drives that
only support the logical block addresses.
MT_ST_SYSV (Default: false)
When this option is enabled, the tape
devices use the SystemV semantics. Other-
wise the BSD semantics are used. The most
important difference between the semantics
is what happens when a device used for
reading is closed: in SYSV semantics the
tape is spaced forward past the next file-
mark if this has not happened while using
the device. In BSD semantics the tape posi-
tion is not changed.
EXAMPLE
struct mtop mt_cmd;
mt_cmd.mt_op = MTSETDRVBUFFER;
mt_cmd.mt_count = MT_ST_BOOLEANS |
MT_ST_BUFFER_WRITES |
MT_ST_ASYNC_WRITES;
ioctl(fd, MTIOCTOP, &mt_cmd);
The default block size for a device can be set
with MT_ST_DEF_BLKSIZE and the default density
code can be set with MT_ST_DEFDENSITY. The values
for the parameters are ORed with the operation
code.
With kernels 2.1.x and later, the timeout values
can be set with the subcommand MT_ST_SET_TIMEOUT
or'ed with the timeout in seconds. The long time-
out (used for rewinds and other commands that may
take a long time) can be set with
MT_ST_SET_LONG_TIMEOUT. The kernel defaults are
very long to make sure that a successful command
is not timed out with any drive. Because of this
the driver may seem stuck even if it is only wait-
ing for the timeout. These commands can be used to
set more practical values for a specific drive.
The timeouts set for one device apply for all
devices linked to the same drive.
MTIOCGET - Get status
This request takes an argument of type (struct mtget *).
/* structure for MTIOCGET - mag tape get status command */
struct mtget {
long mt_type;
long mt_resid;
/* the following registers are device dependent */
long mt_dsreg;
long mt_gstat;
long mt_erreg;
/* The next two fields are not always used */
daddr_t mt_fileno;
daddr_t mt_blkno;
};
mt_type The header file defines many values for
mt_type, but the current driver reports only
the generic types MT_ISSCSI1 (Generic SCSI-1
tape) and MT_ISSCSI2 (Generic SCSI-2 tape).
mt_resid contains the current tape partition number.
mt_dsreg reports the drive's current settings for block
size (in the low 24 bits) and density (in the
high 8 bits). These fields are defined by
MT_ST_BLKSIZE_SHIFT, MT_ST_BLKSIZE_MASK,
MT_ST_DENSITY_SHIFT, and MT_ST_DENSITY_MASK.
mt_gstat reports generic (device independent) status
information. The header file defines macros
for testing these status bits:
GMT_EOF(x): The tape is positioned just after a
filemark (always false after an MTSEEK
operation).
GMT_BOT(x): The tape is positioned at the
beginning of the first file (always false
after an MTSEEK operation).
GMT_EOT(x): A tape operation has reached the
physical End Of Tape.
GMT_SM(x): The tape is currently positioned at
a setmark (always false after an MTSEEK
operation).
GMT_EOD(x): The tape is positioned at the end
of recorded data.
GMT_WR_PROT(x): The drive is write-protected.
For some drives this can also mean that the
drive does not support writing on the cur-
rent medium type.
GMT_ONLINE(x): The last open() found the drive
with a tape in place and ready for opera-
tion.
GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x):
This "generic" status information reports
the current density setting for 9-track
1/2" tape drives only.
GMT_DR_OPEN(x): The drive does not have a tape
in place.
GMT_IM_REP_EN(x): Immediate report mode. This
bit is set if there are no guarantees that
the data has been physically written to the
tape when the write call returns. It is set
zero only when the driver does not buffer
data and the drive is set not to buffer
data.
mt_erreg The only field defined in mt_erreg is the
recovered error count in the low 16 bits (as
defined by MT_ST_SOFTERR_SHIFT and MT_ST_SOFT-
ERR_MASK). Due to inconsistencies in the way
drives report recovered errors, this count is
often not maintained (most drives do not by
default report soft errors but this can be
changed with a SCSI MODE SELECT command).
mt_fileno reports the current file number (zero-based).
This value is set to -1 when the file number is
unknown (e.g., after MTBSS or MTSEEK).
mt_blkno reports the block number (zero-based) within
the current file. This value is set to -1 when
the block number is unknown (e.g., after MTBSF,
MTBSS, or MTSEEK).
MTIOCPOS - Get tape position
This request takes an argument of type (struct mtpos *)
and reports the drive's notion of the current tape block
number, which is not the same as mt_blkno returned by MTI-
OCGET. This drive must be a SCSI-2 drive that supports
the READ POSITION command (device-specific address) or a
Tandberg-compatible SCSI-1 drive (Tandberg, Archive Viper,
Wangtek, ... ).
/* structure for MTIOCPOS - mag tape get position command */
struct mtpos {
long mt_blkno; /* current block number */
};
RETURN VALUE
EIO The requested operation could not be com-
pleted.
ENOSPC A write operation could not be completed
because the tape reached end-of-medium.
EACCES An attempt was made to write or erase a
write-protected tape. (This error is not
detected during open().)
EFAULT The command parameters point to memory not
belonging to the calling process.
ENXIO During opening, the tape device does not
exist.
EBUSY The device is already in use or the driver
was unable to allocate a buffer.
EOVERFLOW An attempt was made to read or write a vari-
able-length block that is larger than the
driver's internal buffer.
EINVAL An ioctl() had an illegal argument, or a
requested block size was illegal.
ENOSYS Unknown ioctl().
EROFS Open is attempted with O_WRONLY or O_RDWR
when the tape in the drive is write-pro-
tected.
FILES
/dev/st* : the auto-rewind SCSI tape devices
/dev/nst* : the non-rewind SCSI tape devices
AUTHOR
The driver has been written by Kai Mkisara (Kai.Mak-
isara@metla.fi) starting from a driver written by Dwayne
Forsyth. Several other people have also contributed to the
driver.
SEE ALSO
mt(1)
The file README.st in the kernel sources contains the most
recent information about the driver and it's configuration
possibilities.
NOTES
1. When exchanging data between systems, both systems have
to agree on the physical tape block size. The parameters
of a drive after startup are often not the ones most oper-
ating systems use with these devices. Most systems use
drives in variable block mode if the drive supports that
mode. This applies to most modern drives, including DATs,
8mm helical scan drives, DLTs, etc. It may be advisable
use these drives in variable block mode also in Linux
(i.e., use MTSETBLK or MTSETDEFBLK at system startup to
set the mode), at least when exchanging data with foreign
system. The drawback of this is that a fairly large tape
block size has to be used to get acceptable data transfer
rates on the SCSI bus.
2. Many programs (e.g., tar) allow the user to specify the
blocking factor on command line. Note that this determines
the physical block size on tape only in variable block
mode.
3. In order to use SCSI tape drives, the basic SCSI
driver, a SCSI-adapter driver and the SCSI tape driver
must be either configured into the kernel or loaded as
modules. If the SCSI-tape driver is not present, the drive
is recognized but the tape support described in this page
is not available.
4. The driver writes error messages to the console/log.
The SENSE codes written into some messages are automati-
cally translated to text if verbose SCSI messages are
enabled in kernel configuration.
COPYRIGHT
Copyright (C) 1995 Robert K. Nichols.
Copyright (C) 1999 Kai Mkisara.
Permission is granted to make and distribute verbatim
copies of this manual provided the copyright notice and
this permission notice are preserved on all copies. Addi-
tional permissions are contained in the header of the
source file.
Linux 2.0 - 2.2 January 18, 1999 1
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