st(4) Kernel Interfaces 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 lowermost five bits
in the minor numbers are assigned sequentially in the order of detec-
tion. In the 2.6 kernel, the bits above the eight lowermost bits are
concatenated to the five lowermost bits to form the tape number. The
minor numbers can be grouped into two sets of four numbers: the princi-
pal (auto-rewind) minor device numbers, n, and the “no-rewind” device
numbers, (n + 128). Devices opened using the principal device number
will be sent a REWIND command when they are closed. Devices opened us-
ing 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/nst0m 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. Before Linux 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 32-bit architectures and 256 kB for 64-bit ar-
chitectures. In newer kernels the driver allocates the buffer in sev-
eral 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 al-
location of 16 blocks of 128 kB succeeds).
The driver's internal buffer size is determined by a compile-time con-
stant which can be overridden with a kernel startup option. In addi-
tion to this, the driver tries to allocate a larger temporary buffer at
run time if necessary. However, run-time allocation of large contigu-
ous blocks of memory may fail and it is advisable not to rely too much
on dynamic buffer allocation before Linux 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(2) calls and remain in effect when the device is closed
and reopened. Setting the options affects both the auto-rewind and the
nonrewind 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(2). The
tape location is preserved within each partition across partition
changes. The partition used for subsequent tape operations is selected
with an ioctl(2). 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(2) 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 de-
fault tape device on the system.
Starting from Linux 2.6.2, the driver exports in the sysfs directory
/sys/class/scsi_tape the attached devices and some parameters assigned
to the devices.
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 deter-
mines the size of the corresponding tape block. Note that the blocks
on the tape don't contain any information about the writing mode: when
reading, the only important 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, an
error is returned.
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 dis-
abled. Before Linux 2.1.121 allow writes with arbitrary byte count if
buffering is enabled. In all other cases (before Linux 2.1.121 with
buffering disabled or newer kernel) the write byte count must be a mul-
tiple of the tape block size.
In Linux 2.6, the driver tries to use direct transfers between the user
buffer and the device. If this is not possible, the driver's internal
buffer is used. The reasons for not using direct transfers include im-
proper alignment of the user buffer (default is 512 bytes but this can
be changed by the HBA driver), one or more pages of the user buffer not
reachable by the SCSI adapter, and so on.
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 following 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(2) 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
nonzero 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. With Linux 2.6, short erase (mark tape empty) is
performed if the argument is zero. Otherwise, long erase (erase
all) is done.
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 pos-
itive, it gives the size of partition 1 and partition 0 contains
the rest of the tape. If mt_count is zero, the tape is format-
ted into one partition. From Linux 4.6, a negative mt_count
specifies the size of partition 0 and the rest of the tape con-
tains partition 1. The physical ordering of partitions depends
on the drive. This command is not allowed for a drive unless
the partition support is enabled for the drive (see
MT_ST_CAN_PARTITIONS 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
Re-tension tape.
MTREW Rewind.
MTSEEK Seek to the tape block number specified in mt_count. This oper-
ation requires either a SCSI-2 drive that supports the LOCATE
command (device-specific address) or a Tandberg-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 documentation.
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 supe-
ruser):
MTSETDRVBUFFER
Set various drive and driver options according to bits encoded
in mt_count. These consist of the drive's buffering mode, a set
of Boolean driver options, the buffer write threshold, defaults
for the block size and density, and timeouts (only since Linux
2.1). A single operation can affect only one item in the list
below (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 in-
clude the constant MT_ST_WRITE_THRESHOLD bitwise 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 of the constants MT_ST_BOOLEANS, MT_ST_SETBOOLEANS,
MT_ST_CLEARBOOLEANS, or MT_ST_DEFBOOLEANS bitwise 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 selectively
cleared.
The default options for a tape device are set with MT_ST_DEF-
BOOLEANS. A nonactive 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
activated 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 reli-
able multivolume archives.
MT_ST_ASYNC_WRITES (Default: true)
When this option is true, write operations return immedi-
ately 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 command 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
multivolume 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 file-
marks 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 messages from the
driver (effective only if the driver was compiled with
DEBUG defined nonzero).
MT_ST_FAST_EOM (Default: false)
This option causes the MTEOM operation to be sent di-
rectly to the drive, potentially speeding up the opera-
tion 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 file is opened and unlocked when it is closed.
MT_ST_DEF_WRITES (Default: false)
The tape options (block size, mode, compression, etc.)
may change when changing from one device linked to a
drive to another device linked to the same drive depend-
ing 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 capabilities
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 backward 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 correctly positioned within a
file when the device is closed. With Linux 2.6, the de-
fault is true for drives supporting SCSI-3.
MT_ST_NO_BLKLIMS (Default: false)
Some drives don't accept the READ BLOCK LIMITS SCSI com-
mand. If this is used, the driver does not use the com-
mand. The drawback 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 MTIOC-
POS commands and when changing tape partition). Other-
wise, 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 support only the logical block
addresses.
MT_ST_SYSV (Default: false)
When this option is enabled, the tape devices use the
System V semantics. Otherwise, the BSD semantics are
used. The most important difference between the seman-
tics is what happens when a device used for reading is
closed: in System V semantics the tape is spaced forward
past the next filemark if this has not happened while us-
ing the device. In BSD semantics the tape position is
not changed.
MT_NO_WAIT (Default: false)
Enables immediate mode (i.e., don't wait for the command
to finish) for some commands (e.g., rewind).
An 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 or'ed with
the operation code.
With Linux 2.1.x and later, the timeout values can be set with
the subcommand MT_ST_SET_TIMEOUT ORed with the timeout in sec-
onds. The long timeout (used for rewinds and other commands
that may take a long time) can be set with MT_ST_SET_LONG_TIME-
OUT. The kernel defaults are very long to make sure that a suc-
cessful command is not timed out with any drive. Because of
this, the driver may seem stuck even if it is only waiting 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.
Starting from Linux 2.4.19 and Linux 2.5.43, the driver supports
a status bit which indicates whether the drive requests clean-
ing. The method used by the drive to return cleaning informa-
tion is set using the MT_ST_SEL_CLN subcommand. If the value is
zero, the cleaning bit is always zero. If the value is one, the
TapeAlert data defined in the SCSI-3 standard is used (not yet
implemented). Values 2–17 are reserved. If the lowest eight
bits are >= 18, bits from the extended sense data are used. The
bits 9–16 specify a mask to select the bits to look at and the
bits 17–23 specify the bit pattern to look for. If the bit pat-
tern is zero, one or more bits under the mask indicate the
cleaning request. If the pattern is nonzero, the pattern must
match the masked sense data byte.
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 de-
fined by MT_ST_BLKSIZE_SHIFT, MT_ST_BLKSIZE_MASK, MT_ST_DEN-
SITY_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
current medium type.
GMT_ONLINE(x)
The last open(2) found the drive with a tape in place and
ready for operation.
GMT_D_6250(x)
GMT_D_1600(x)
GMT_D_800(x)
This “generic” status information reports the current
density setting for 9-track ½" 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.
GMT_CLN(x)
The drive has requested cleaning. Implemented since
Linux 2.4.19 and Linux 2.5.43.
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_SOFTERR_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 MT-
SEEK).
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 MTIOCGET. 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
EACCES An attempt was made to write or erase a write-protected tape.
(This error is not detected during open(2).)
EBUSY The device is already in use or the driver was unable to allo-
cate a buffer.
EFAULT The command parameters point to memory not belonging to the
calling process.
EINVAL An ioctl(2) had an invalid argument, or a requested block size
was invalid.
EIO The requested operation could not be completed.
ENOMEM The byte count in read(2) is smaller than the next physical
block on the tape. (Before Linux 2.2.18 and Linux 2.4.0 the ex-
tra bytes have been silently ignored.)
ENOSPC A write operation could not be completed because the tape
reached end-of-medium.
ENOSYS Unknown ioctl(2).
ENXIO During opening, the tape device does not exist.
EOVERFLOW
An attempt was made to read or write a variable-length block
that is larger than the driver's internal buffer.
EROFS Open is attempted with O_WRONLY or O_RDWR when the tape in the
drive is write-protected.
FILES
/dev/st*
the auto-rewind SCSI tape devices
/dev/nst*
the nonrewind SCSI tape devices
NOTES
• 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 operating 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, in-
cluding DATs, 8mm helical scan drives, DLTs, etc. It may be advis-
able to 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 a 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.
• Many programs (e.g., tar(1)) allow the user to specify the blocking
factor on the command line. Note that this determines the physical
block size on tape only in variable-block mode.
• 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.
• The driver writes error messages to the console/log. The SENSE
codes written into some messages are automatically translated to
text if verbose SCSI messages are enabled in kernel configuration.
• The driver's internal buffering allows good throughput in fixed-
block mode also with small read(2) and write(2) byte counts. With
direct transfers this is not possible and may cause a surprise when
moving to the 2.6 kernel. The solution is to tell the software to
use larger transfers (often telling it to use larger blocks). If
this is not possible, direct transfers can be disabled.
SEE ALSO
mt(1)
The file drivers/scsi/README.st or Documentation/scsi/st.txt (kernel >=
2.6) in the Linux kernel source tree contains the most recent informa-
tion about the driver and its configuration possibilities
Linux man-pages 6.03 2023-02-05 st(4)
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