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
below (the Booleans counted as one item.)
A value having zeros in the high-order 4 bits will be used
to set the drive's buffering mode. The buffering modes
are:
0 The drive will not report GOOD status on write
commands until the data blocks are actually
written to the medium.
1 The drive may report GOOD status on write
commands as soon as all the data has been
transferred to the drive's internal buffer.
2 The drive may report GOOD status on write
commands as soon as (a) all the data has been
transferred to the drive's internal buffer, and
(b) all buffered data from different initiators
has been successfully written to the medium.
To control the write threshold the value in mt_count must
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 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-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 System V 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 */
};
