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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.
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, 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.
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 of more pages of the user buffer not reachable by the
SCSI adapter, etc.
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.
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:
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
nonzero, it gives the size of the first partition and the second
partition 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_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 only support 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.
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 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.
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).
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 kernel
sources contains the most recent information about the
driver and its configuration possibilities.
This page is part of release 3.32 of the Linux man-pages
project. A description of the project, and information
about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2010-09-04 ST(4)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface