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ST(4) Linux Programmer's Manual ST(4)
st - SCSI tape device
#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);
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 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. 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 32-bit architectures and
256 kB for 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 allocation of 16 blocks of 128 kB
succeeds).
The driver's internal buffer size is determined by a compile-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 necessary. 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(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
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 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 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 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 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 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 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 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 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 kernels 2.1 and later). 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 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 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 reliable multivolume 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 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 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, 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, 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-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 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
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
support only the logical block addresses.
MT_ST_SYSV (Default: false)
When this option is enabled, the tape devices use the
SystemV semantics. Otherwise, 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 successful 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 indicates 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 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 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 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
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 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 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 */
};
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 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. (Before 2.2.18 and
2.4.0-test6 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-protected.
/dev/st* the auto-rewind SCSI tape devices
/dev/nst* the nonrewind SCSI tape devices
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 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, 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 possible, direct transfers can be
disabled.
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
information about the driver and its configuration possibilities
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-06-05 ST(4)
Pages that refer to this page: cciss(4), hpsa(4)