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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 lowermost five bits in the minor numbers
are assigned sequentially in the order of detection. 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 principal (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 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 charac‐
teristics (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 de‐
fined (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. 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 pro‐
vide. The limit is currently 128 kB for 32-bit architectures and 256 kB for 64-bit architec‐
tures. 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 allocation of 16 blocks of 128 kB succeeds).

The driver's internal buffer size is determined by a compile-time constant which can be over‐
ridden with a kernel startup option. In addition to this, the driver tries to allocate a
larger temporary buffer at run time if necessary. However, run-time allocation of large con‐
tiguous 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(2) calls and remain in effect when the device is closed and re‐
opened. 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 sev‐
eral 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 parti‐
tions.

Device /dev/tape is usually created as a hard or soft link to the default tape device on the
system.

Starting from kernel 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 determines 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 ex‐
actly. 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 mul‐
tiple 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.

In the 2.6 kernel, 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 improper 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 reach‐
able 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 re‐
maining 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 opera‐
tions. 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 command 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 2.6 kernel, short erase (mark tape empty) is performed if the argu‐
ment 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 positive, it gives the
size of partition 1 and partition 0 contains the rest of the tape. If mt_count is
zero, the tape is formatted into one partition. From kernel version 4.6, a negative
mt_count specifies the size of partition 0 and the rest of the tape contains partition
1. The physical ordering of partitions depends on the drive. This command is not al‐
lowed 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 operation 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 superuser):

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 in ker‐
nels 2.1 and later). 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 buffer‐
ing 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 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_DEFBOOLEANS. 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 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 mul‐
tiple of the block size. This option must be set false to write reliable mul‐
tivolume archives.

MT_ST_ASYNC_WRITES (Default: true)
When this option 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 command is issued. Any errors reported by the drive will be
held until the next operation. 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 ac‐
tion 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 un‐
able to overwrite a filemark. These drives detect the end of recorded data by
testing for blank tape rather than two consecutive filemarks. Most other cur‐
rent 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 directly to the drive, poten‐
tially 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 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
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-detec‐
tion 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 cor‐
rect 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 2.6 kernel, the default is true for drives supporting SCSI-3.

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 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 MT‐‐
SEEK 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 support only the logical block addresses.

MT_ST_SYSV (Default: false)
When this option is enabled, the tape devices use the System V semantics. Oth‐
erwise, the BSD semantics are used. The most important difference between the
semantics 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 using 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 kernels 2.1.x and later, the timeout values can be set with the subcommand
MT_ST_SET_TIMEOUT ORed with the timeout in seconds. The long timeout (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 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 kernels 2.4.19 and 2.5.43, the driver supports a status bit which indi‐
cates whether the drive requests cleaning. The method used by the drive to return
cleaning information 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 pattern is zero, one or more bits under the mask in‐
dicate 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 defined by MT_ST_BLKSIZE_SHIFT, MT_ST_BLK‐‐
SIZE_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 current medium type.

GMT_ONLINE(x): The last open(2) found the drive with a tape in place and ready for op‐
eration.

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 in kernels since 2.4.19 and
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 com‐
mand).

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 MTIOCGET. This
drive must be a SCSI-2 drive that supports the READ POSITION command (device-specific ad‐
dress) 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 de‐
tected during open(2).)

EBUSY The device is already in use or the driver was unable to allocate 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. (Be‐
fore 2.2.18 and 2.4.0 the extra 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-pro‐
tected.

FILES
/dev/st*
the auto-rewind SCSI tape devices

/dev/nst*
the nonrewind SCSI tape devices

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 operat‐
ing 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 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.

2. 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.

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 automatically translated to text if verbose SCSI messages are enabled in
kernel configuration.

5. 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 possi‐
ble, direct transfers can be disabled.