+.. SPDX-License-Identifier: GPL-2.0
+
+====================
+The SCSI Tape Driver
+====================
+
This file contains brief information about the SCSI tape driver.
The driver is currently maintained by Kai Mäkisara (email
Kai.Makisara@kolumbus.fi)
Last modified: Tue Feb 9 21:54:16 2016 by kai.makisara
-BASICS
+Basics
+======
The driver is generic, i.e., it does not contain any code tailored
to any specific tape drive. The tape parameters can be specified with
attempted).
-MINOR NUMBERS
+Minor Numbers
+=============
The tape driver currently supports up to 2^17 drives if 4 modes for
each drive are used.
-The minor numbers consist of the following bit fields:
+The minor numbers consist of the following bit fields::
+
+ dev_upper non-rew mode dev-lower
+ 20 - 8 7 6 5 4 0
-dev_upper non-rew mode dev-lower
- 20 - 8 7 6 5 4 0
The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
byte). The bits defining the mode are below the non-rewind bit. The
remaining bits define the tape device number. This numbering is
only 8 bits wide.
-SYSFS SUPPORT
+Sysfs Support
+=============
The driver creates the directory /sys/class/scsi_tape and populates it with
directories corresponding to the existing tape devices. There are autorewind
options.
A link named 'tape' is made from the SCSI device directory to the class
-directory corresponding to the mode 0 auto-rewind device (e.g., st0).
+directory corresponding to the mode 0 auto-rewind device (e.g., st0).
-SYSFS AND STATISTICS FOR TAPE DEVICES
+Sysfs and Statistics for Tape Devices
+=====================================
The st driver maintains statistics for tape drives inside the sysfs filesystem.
The following method can be used to locate the statistics that are
1. Use opendir(3) on the directory /sys/class/scsi_tape
2. Use readdir(3) to read the directory contents
3. Use regcomp(3)/regexec(3) to match directory entries to the extended
- regular expression "^st[0-9]+$"
+ regular expression "^st[0-9]+$"
4. Access the statistics from the /sys/class/scsi_tape/<match>/stats
- directory (where <match> is a directory entry from /sys/class/scsi_tape
- that matched the extended regular expression)
+ directory (where <match> is a directory entry from /sys/class/scsi_tape
+ that matched the extended regular expression)
The reason for using this approach is that all the character devices
pointing to the same tape drive use the same statistics. That means
The directory contains the following statistics files:
-1. in_flight - The number of I/Os currently outstanding to this device.
-2. io_ns - The amount of time spent waiting (in nanoseconds) for all I/O
+1. in_flight
+ - The number of I/Os currently outstanding to this device.
+2. io_ns
+ - The amount of time spent waiting (in nanoseconds) for all I/O
to complete (including read and write). This includes tape movement
commands such as seeking between file or set marks and implicit tape
movement such as when rewind on close tape devices are used.
-3. other_cnt - The number of I/Os issued to the tape drive other than read or
+3. other_cnt
+ - The number of I/Os issued to the tape drive other than read or
write commands. The time taken to complete these commands uses the
following calculation io_ms-read_ms-write_ms.
-4. read_byte_cnt - The number of bytes read from the tape drive.
-5. read_cnt - The number of read requests issued to the tape drive.
-6. read_ns - The amount of time (in nanoseconds) spent waiting for read
+4. read_byte_cnt
+ - The number of bytes read from the tape drive.
+5. read_cnt
+ - The number of read requests issued to the tape drive.
+6. read_ns
+ - The amount of time (in nanoseconds) spent waiting for read
requests to complete.
-7. write_byte_cnt - The number of bytes written to the tape drive.
-8. write_cnt - The number of write requests issued to the tape drive.
-9. write_ns - The amount of time (in nanoseconds) spent waiting for write
+7. write_byte_cnt
+ - The number of bytes written to the tape drive.
+8. write_cnt
+ - The number of write requests issued to the tape drive.
+9. write_ns
+ - The amount of time (in nanoseconds) spent waiting for write
requests to complete.
-10. resid_cnt - The number of times during a read or write we found
+10. resid_cnt
+ - The number of times during a read or write we found
the residual amount to be non-zero. This should mean that a program
is issuing a read larger thean the block size on tape. For write
not all data made it to tape.
-Note: The in_flight value is incremented when an I/O starts the I/O
-itself is not added to the statistics until it completes.
+.. Note::
+
+ The in_flight value is incremented when an I/O starts the I/O
+ itself is not added to the statistics until it completes.
The total of read_cnt, write_cnt, and other_cnt may not total to the same
value as iodone_cnt at the device level. The tape statistics only count
issued by the st driver. Tape statistics do not take into account any
I/O performed via the sg device.
-BSD AND SYS V SEMANTICS
+BSD and Sys V Semantics
+=======================
The user can choose between these two behaviours of the tape driver by
defining the value of the symbol ST_SYSV. The semantics differ when a
The default is BSD semantics.
-BUFFERING
+Buffering
+=========
The driver tries to do transfers directly to/from user space. If this
is not possible, a driver buffer allocated at run-time is used. If
direct i/o is not possible for the whole transfer, the driver buffer
is used (i.e., bounce buffers for individual pages are not
used). Direct i/o can be impossible because of several reasons, e.g.:
+
- one or more pages are at addresses not reachable by the HBA
- the number of pages in the transfer exceeds the number of
scatter/gather segments permitted by the HBA
allocated. To support all SCSI adapters (including those not
supporting scatter/gather), buffer allocation is using the following
three kinds of chunks:
+
1. The initial segment that is used for all SCSI adapters including
-those not supporting scatter/gather. The size of this buffer will be
-(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
-this size (and it is not larger than the buffer size specified by
-ST_BUFFER_BLOCKS). If this size is not available, the driver halves
-the size and tries again until the size of one page. The default
-settings in st_options.h make the driver to try to allocate all of the
-buffer as one chunk.
+ those not supporting scatter/gather. The size of this buffer will be
+ (PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
+ this size (and it is not larger than the buffer size specified by
+ ST_BUFFER_BLOCKS). If this size is not available, the driver halves
+ the size and tries again until the size of one page. The default
+ settings in st_options.h make the driver to try to allocate all of the
+ buffer as one chunk.
2. The scatter/gather segments to fill the specified buffer size are
-allocated so that as many segments as possible are used but the number
-of segments does not exceed ST_FIRST_SG.
+ allocated so that as many segments as possible are used but the number
+ of segments does not exceed ST_FIRST_SG.
3. The remaining segments between ST_MAX_SG (or the module parameter
-max_sg_segs) and the number of segments used in phases 1 and 2
-are used to extend the buffer at run-time if this is necessary. The
-number of scatter/gather segments allowed for the SCSI adapter is not
-exceeded if it is smaller than the maximum number of scatter/gather
-segments specified. If the maximum number allowed for the SCSI adapter
-is smaller than the number of segments used in phases 1 and 2,
-extending the buffer will always fail.
+ max_sg_segs) and the number of segments used in phases 1 and 2
+ are used to extend the buffer at run-time if this is necessary. The
+ number of scatter/gather segments allowed for the SCSI adapter is not
+ exceeded if it is smaller than the maximum number of scatter/gather
+ segments specified. If the maximum number allowed for the SCSI adapter
+ is smaller than the number of segments used in phases 1 and 2,
+ extending the buffer will always fail.
-EOM BEHAVIOUR WHEN WRITING
+EOM Behaviour When Writing
+==========================
When the end of medium early warning is encountered, the current write
is finished and the number of bytes is returned. The next write
alternately returned until the physical end of medium (or some other
error) is encountered.
-
-MODULE PARAMETERS
+Module Parameters
+=================
The buffer size, write threshold, and the maximum number of allocated buffers
are configurable when the driver is loaded as a module. The keywords are:
+========================== ===========================================
buffer_kbs=xxx the buffer size for fixed block mode is set
to xxx kilobytes
write_threshold_kbs=xxx the write threshold in kilobytes set to xxx
segments
try_direct_io=x try direct transfer between user buffer and
tape drive if this is non-zero
+========================== ===========================================
Note that if the buffer size is changed but the write threshold is not
set, the write threshold is set to the new buffer size - 2 kB.
-BOOT TIME CONFIGURATION
+Boot Time Configuration
+=======================
If the driver is compiled into the kernel, the same parameters can be
also set using, e.g., the LILO command line. The preferred syntax is
the driver as module. If several parameters are set, the keyword-value
pairs are separated with a comma (no spaces allowed). A colon can be
used instead of the equal mark. The definition is prepended by the
-string st=. Here is an example:
+string st=. Here is an example::
st=buffer_kbs:64,write_threshold_kbs:60
-The following syntax used by the old kernel versions is also supported:
+The following syntax used by the old kernel versions is also supported::
st=aa[,bb[,dd]]
-where
- aa is the buffer size for fixed block mode in 1024 byte units
- bb is the write threshold in 1024 byte units
- dd is the maximum number of scatter/gather segments
+where:
+ - aa is the buffer size for fixed block mode in 1024 byte units
+ - bb is the write threshold in 1024 byte units
+ - dd is the maximum number of scatter/gather segments
-IOCTLS
+
+IOCTLs
+======
The tape is positioned and the drive parameters are set with ioctls
defined in mtio.h The tape control program 'mt' uses these ioctls. Try
The following use the structure mtop:
-MTFSF Space forward over count filemarks. Tape positioned after filemark.
-MTFSFM As above but tape positioned before filemark.
-MTBSF Space backward over count filemarks. Tape positioned before
+MTFSF
+ Space forward over count filemarks. Tape positioned after filemark.
+MTFSFM
+ As above but tape positioned before filemark.
+MTBSF
+ Space backward over count filemarks. Tape positioned before
filemark.
-MTBSFM As above but ape positioned after filemark.
-MTFSR Space forward over count records.
-MTBSR Space backward over count records.
-MTFSS Space forward over count setmarks.
-MTBSS Space backward over count setmarks.
-MTWEOF Write count filemarks.
-MTWEOFI Write count filemarks with immediate bit set (i.e., does not
+MTBSFM
+ As above but ape positioned after filemark.
+MTFSR
+ Space forward over count records.
+MTBSR
+ Space backward over count records.
+MTFSS
+ Space forward over count setmarks.
+MTBSS
+ Space backward over count setmarks.
+MTWEOF
+ Write count filemarks.
+MTWEOFI
+ Write count filemarks with immediate bit set (i.e., does not
wait until data is on tape)
-MTWSM Write count setmarks.
-MTREW Rewind tape.
-MTOFFL Set device off line (often rewind plus eject).
-MTNOP Do nothing except flush the buffers.
-MTRETEN Re-tension tape.
-MTEOM Space to end of recorded data.
-MTERASE Erase tape. If the argument is zero, the short erase command
+MTWSM
+ Write count setmarks.
+MTREW
+ Rewind tape.
+MTOFFL
+ Set device off line (often rewind plus eject).
+MTNOP
+ Do nothing except flush the buffers.
+MTRETEN
+ Re-tension tape.
+MTEOM
+ Space to end of recorded data.
+MTERASE
+ Erase tape. If the argument is zero, the short erase command
is used. The long erase command is used with all other values
of the argument.
-MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA)
+MTSEEK
+ Seek to tape block count. Uses Tandberg-compatible seek (QFA)
for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
block numbers in the status are not valid after a seek.
-MTSETBLK Set the drive block size. Setting to zero sets the drive into
+MTSETBLK
+ Set the drive block size. Setting to zero sets the drive into
variable block mode (if applicable).
-MTSETDENSITY Sets the drive density code to arg. See drive
+MTSETDENSITY
+ Sets the drive density code to arg. See drive
documentation for available codes.
-MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door.
-MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the
+MTLOCK and MTUNLOCK
+ Explicitly lock/unlock the tape drive door.
+MTLOAD and MTUNLOAD
+ Explicitly load and unload the tape. If the
command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
drive with the command and it selects the tape slot to use of
HP C1553A changer.
-MTCOMPRESSION Sets compressing or uncompressing drive mode using the
+MTCOMPRESSION
+ Sets compressing or uncompressing drive mode using the
SCSI mode page 15. Note that some drives other methods for
control of compression. Some drives (like the Exabytes) use
density codes for compression control. Some drives use another
mode page but this page has not been implemented in the
driver. Some drives without compression capability will accept
any compression mode without error.
-MTSETPART Moves the tape to the partition given by the argument at the
+MTSETPART
+ Moves the tape to the partition given by the argument at the
next tape operation. The block at which the tape is positioned
is the block where the tape was previously positioned in the
new active partition unless the next tape operation is
MTSEEK. In this case the tape is moved directly to the block
specified by MTSEEK. MTSETPART is inactive unless
MT_ST_CAN_PARTITIONS set.
-MTMKPART Formats the tape with one partition (argument zero) or two
+MTMKPART
+ Formats the tape with one partition (argument zero) or two
partitions (argument non-zero). If the argument is positive,
it specifies the size of partition 1 in megabytes. For DDS
drives and several early drives this is the physically first
with mask MT_SET_OPTIONS, the low order bits are used as argument.
This command is only allowed for the superuser (root). The
subcommands are:
- 0
+
+ * 0
The drive buffer option is set to the argument. Zero means
no buffering.
- MT_ST_BOOLEANS
+ * MT_ST_BOOLEANS
Sets the buffering options. The bits are the new states
(enabled/disabled) the following options (in the
parenthesis is specified whether the option is global or
can be specified differently for each mode):
- MT_ST_BUFFER_WRITES write buffering (mode)
- MT_ST_ASYNC_WRITES asynchronous writes (mode)
- MT_ST_READ_AHEAD read ahead (mode)
- MT_ST_TWO_FM writing of two filemarks (global)
- MT_ST_FAST_EOM using the SCSI spacing to EOD (global)
- MT_ST_AUTO_LOCK automatic locking of the drive door (global)
- MT_ST_DEF_WRITES the defaults are meant only for writes (mode)
- MT_ST_CAN_BSR backspacing over more than one records can
+
+ MT_ST_BUFFER_WRITES
+ write buffering (mode)
+ MT_ST_ASYNC_WRITES
+ asynchronous writes (mode)
+ MT_ST_READ_AHEAD
+ read ahead (mode)
+ MT_ST_TWO_FM
+ writing of two filemarks (global)
+ MT_ST_FAST_EOM
+ using the SCSI spacing to EOD (global)
+ MT_ST_AUTO_LOCK
+ automatic locking of the drive door (global)
+ MT_ST_DEF_WRITES
+ the defaults are meant only for writes (mode)
+ MT_ST_CAN_BSR
+ backspacing over more than one records can
be used for repositioning the tape (global)
- MT_ST_NO_BLKLIMS the driver does not ask the block limits
+ MT_ST_NO_BLKLIMS
+ the driver does not ask the block limits
from the drive (block size can be changed only to
variable) (global)
- MT_ST_CAN_PARTITIONS enables support for partitioned
+ MT_ST_CAN_PARTITIONS
+ enables support for partitioned
tapes (global)
- MT_ST_SCSI2LOGICAL the logical block number is used in
+ MT_ST_SCSI2LOGICAL
+ the logical block number is used in
the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
the device dependent address. It is recommended to set
this flag unless there are tapes using the device
dependent (from the old times) (global)
- MT_ST_SYSV sets the SYSV semantics (mode)
- MT_ST_NOWAIT enables immediate mode (i.e., don't wait for
+ MT_ST_SYSV
+ sets the SYSV semantics (mode)
+ MT_ST_NOWAIT
+ enables immediate mode (i.e., don't wait for
the command to finish) for some commands (e.g., rewind)
- MT_ST_NOWAIT_EOF enables immediate filemark mode (i.e. when
+ MT_ST_NOWAIT_EOF
+ enables immediate filemark mode (i.e. when
writing a filemark, don't wait for it to complete). Please
see the BASICS note about MTWEOFI with respect to the
possible dangers of writing immediate filemarks.
- MT_ST_SILI enables setting the SILI bit in SCSI commands when
+ MT_ST_SILI
+ enables setting the SILI bit in SCSI commands when
reading in variable block mode to enhance performance when
reading blocks shorter than the byte count; set this only
if you are sure that the drive supports SILI and the HBA
correctly returns transfer residuals
- MT_ST_DEBUGGING debugging (global; debugging must be
+ MT_ST_DEBUGGING
+ debugging (global; debugging must be
compiled into the driver)
- MT_ST_SETBOOLEANS
- MT_ST_CLEARBOOLEANS
+
+ * MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS
Sets or clears the option bits.
- MT_ST_WRITE_THRESHOLD
+ * MT_ST_WRITE_THRESHOLD
Sets the write threshold for this device to kilobytes
specified by the lowest bits.
- MT_ST_DEF_BLKSIZE
+ * MT_ST_DEF_BLKSIZE
Defines the default block size set automatically. Value
0xffffff means that the default is not used any more.
- MT_ST_DEF_DENSITY
- MT_ST_DEF_DRVBUFFER
+ * MT_ST_DEF_DENSITY, MT_ST_DEF_DRVBUFFER
Used to set or clear the density (8 bits), and drive buffer
state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
(0xfffff) the default will not be used any more. Otherwise
the lowermost bits of the value contain the new value of
the parameter.
- MT_ST_DEF_COMPRESSION
+ * MT_ST_DEF_COMPRESSION
The compression default will not be used if the value of
the lowermost byte is 0xff. Otherwise the lowermost bit
contains the new default. If the bits 8-15 are set to a
used as the compression algorithm. The value
MT_ST_CLEAR_DEFAULT can be used to clear the compression
default.
- MT_ST_SET_TIMEOUT
+ * MT_ST_SET_TIMEOUT
Set the normal timeout in seconds for this device. The
default is 900 seconds (15 minutes). The timeout should be
long enough for the retries done by the device while
reading/writing.
- MT_ST_SET_LONG_TIMEOUT
+ * MT_ST_SET_LONG_TIMEOUT
Set the long timeout that is used for operations that are
known to take a long time. The default is 14000 seconds
(3.9 hours). For erase this value is further multiplied by
eight.
- MT_ST_SET_CLN
+ * MT_ST_SET_CLN
Set the cleaning request interpretation parameters using
the lowest 24 bits of the argument. The driver can set the
generic status bit GMT_CLN if a cleaning request bit pattern
cleaning. The bits are device-dependent. The driver is
given the number of the sense data byte (the lowest eight
bits of the argument; must be >= 18 (values 1 - 17
- reserved) and <= the maximum requested sense data sixe),
+ reserved) and <= the maximum requested sense data sixe),
a mask to select the relevant bits (the bits 9-16), and the
bit pattern (bits 17-23). If the bit pattern is zero, one
or more bits under the mask indicate cleaning request. If
MT_ST_SET_CLN.)
The following ioctl uses the structure mtpos:
-MTIOCPOS Reads the current position from the drive. Uses
+
+MTIOCPOS
+ Reads the current position from the drive. Uses
Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
command for the SCSI-2 drives.
The following ioctl uses the structure mtget to return the status:
-MTIOCGET Returns some status information.
+
+MTIOCGET
+ Returns some status information.
The file number and block number within file are returned. The
block is -1 when it can't be determined (e.g., after MTBSF).
The drive type is either MTISSCSI1 or MTISSCSI2.
end of recorded data or end of tape. GMT_EOT means end of tape.
-MISCELLANEOUS COMPILE OPTIONS
+Miscellaneous Compile Options
+=============================
The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
is defined.
user does not request data that far.)
-DEBUGGING HINTS
+Debugging Hints
+===============
Debugging code is now compiled in by default but debugging is turned off
with the kernel module parameter debug_flag defaulting to 0. Debugging
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