ST(4) Linux Programmer's Manual ST(4)NAMEst - 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
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 com‐
mand 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 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 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 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,
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 dis‐
abled. 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.
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 avail‐
able for some HP autoloaders. If mt_count is the con‐
stant 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 parti‐
tion. 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 sup‐
ports 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-spe‐
cific 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 vari‐
able block size mode.
MTSETDENSITY Set the tape density to the code in mt_count. The den‐
sity codes supported by a drive can be found from the
drive documentation.
MTSETPART The active partition is switched to mt_count. The parti‐
tions 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 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_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 reliable multivolume archives.
MT_ST_ASYNC_WRITES (Default: true)
When this option is true, write opera‐
tions 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 multi‐
ple 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 test‐
ing for blank tape rather than two con‐
secutive 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, 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 is opened and
unlocked when it is closed.
MT_ST_DEF_WRITES (Default: false)
The tape options (block size, mode, com‐
pression, etc.) may change when changing
from one device linked to a drive to
another device linked to the same drive
depending on how the devices are defined.
This option defines when the changes are
enforced by the driver using SCSI-com‐
mands 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-com‐
mands 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 com‐
mand 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. 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 hap‐
pened 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 com‐
mands (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 val‐
ues for the parameters are or'ed with the opera‐
tion 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_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
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 guaran‐
tees 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 sup‐
ports the READ POSITION command (device-specific
address) or a Tandberg-compatible SCSI-1 drive (Tand‐
berg, 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 allocate a
buffer.
EFAULT The command parameters point to
memory not belonging to the calling
process.
EINVAL An ioctl(2) had an invalid argu‐
ment, 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 com‐
pleted 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 inter‐
nal 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
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 heli‐
cal 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.
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 con‐
sole/log. The SENSE codes written into some
messages are automatically translated to text
if verbose SCSI messages are enabled in ker‐
nel 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 ker‐
nel. 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 ALSOmt(1)
The file drivers/scsi/README.st or Documenta‐
tion/scsi/st.txt (kernel >= 2.6) in the Linux
kernel source tree contains the most recent
information about the driver and its configura‐
tion possibilities
COLOPHON
This page is part of release 3.44 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)
