Documentation / scsi / st.rst


Based on kernel version 5.15. Page generated on 2021-11-01 09:19 EST.

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.. 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
======

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
one of the following three methods:

1. Each user can specify the tape parameters he/she wants to use
directly with ioctls. This is administratively a very simple and
flexible method and applicable to single-user workstations. However,
in a multiuser environment the next user finds the tape parameters in
state the previous user left them.

2. The system manager (root) can define default values for some tape
parameters, like block size and density using the MTSETDRVBUFFER ioctl.
These parameters can be programmed to come into effect either when a
new tape is loaded into the drive or if writing begins at the
beginning of the tape. The second method is applicable if the tape
drive performs auto-detection of the tape format well (like some
QIC-drives). The result is that any tape can be read, writing can be
continued using existing format, and the default format is used if
the tape is rewritten from the beginning (or a new tape is written
for the first time). The first method is applicable if the drive
does not perform auto-detection well enough and there is a single
"sensible" mode for the device. An example is a DAT drive that is
used only in variable block mode (I don't know if this is sensible
or not :-).

The user can override the parameters defined by the system
manager. The changes persist until the defaults again come into
effect.

3. By default, up to four modes can be defined and selected using the minor
number (bits 5 and 6). The number of modes can be changed by changing
ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
above. Additional modes are dormant until they are defined by the
system manager (root). When specification of a new mode is started,
the configuration of mode 0 is used to provide a starting point for
definition of the new mode.

Using the modes allows the system manager to give the users choices
over some of the buffering parameters not directly accessible to the
users (buffered and asynchronous writes). The modes also allow choices
between formats in multi-tape operations (the explicitly overridden
parameters are reset when a new tape is loaded).

If more than one mode is used, all modes should contain definitions
for the same set of parameters.

Many Unices contain internal tables that associate different modes to
supported devices. The Linux SCSI tape driver does not contain such
tables (and will not do that in future). Instead of that, a utility
program can be made that fetches the inquiry data sent by the device,
scans its database, and sets up the modes using the ioctls. Another
alternative is to make a small script that uses mt to set the defaults
tailored to the system.

The driver supports fixed and variable block size (within buffer
limits). Both the auto-rewind (minor equals device number) and
non-rewind devices (minor is 128 + device number) are implemented.

In variable block mode, the byte count in write() determines the size
of the physical block on tape. When reading, the drive reads the next
tape block and returns to the user the data if the read() byte count
is at least the block size. Otherwise, error ENOMEM is returned.

In fixed block mode, the data transfer between the drive and the
driver is in multiples of the block size. The write() byte count must
be a multiple of the block size. This is not required when reading but
may be advisable for portability.

Support is provided for changing the tape partition and partitioning
of the tape with one or two partitions. By default support for
partitioned tape is disabled for each driver and it can be enabled
with the ioctl MTSETDRVBUFFER.

By default the driver writes one filemark when the device is closed after
writing and the last operation has been a write. Two filemarks can be
optionally written. In both cases end of data is signified by
returning zero bytes for two consecutive reads.

Writing filemarks without the immediate bit set in the SCSI command block acts
as a synchronization point, i.e., all remaining data form the drive buffers is
written to tape before the command returns. This makes sure that write errors
are caught at that point, but this takes time. In some applications, several
consecutive files must be written fast. The MTWEOFI operation can be used to
write the filemarks without flushing the drive buffer. Writing filemark at
close() is always flushing the drive buffers. However, if the previous
operation is MTWEOFI, close() does not write a filemark. This can be used if
the program wants to close/open the tape device between files and wants to
skip waiting.

If rewind, offline, bsf, or seek is done and previous tape operation was
write, a filemark is written before moving tape.

The compile options are defined in the file linux/drivers/scsi/st_options.h.

4. If the open option O_NONBLOCK is used, open succeeds even if the
drive is not ready. If O_NONBLOCK is not used, the driver waits for
the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
seconds, open fails with the errno value EIO. With O_NONBLOCK the
device can be opened for writing even if there is a write protected
tape in the drive (commands trying to write something return error if
attempted).


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::

    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
backward compatible with the numbering used when the minor number was
only 8 bits wide.


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
and non-rewind entries for each mode. The names are stxy and nstxy, where x
is the tape number and y a character corresponding to the mode (none, l, m,
a). For example, the directories for the first tape device are (assuming four
modes): st0  nst0  st0l  nst0l  st0m  nst0m  st0a  nst0a.

Each directory contains the entries: default_blksize  default_compression
default_density  defined  dev  device  driver. The file 'defined' contains 1
if the mode is defined and zero if not defined. The files 'default_*' contain
the defaults set by the user. The value -1 means the default is not set. The
file 'dev' contains the device numbers corresponding to this device. The links
'device' and 'driver' point to the SCSI device and driver entries.

Each directory also contains the entry 'options' which shows the currently
enabled driver and mode options. The value in the file is a bit mask where the
bit definitions are the same as those used with MTSETDRVBUFFER in setting the
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).


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
available (assuming that sysfs is mounted at /sys):

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]+$"
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)

The reason for using this approach is that all the character devices
pointing to the same tape drive use the same statistics. That means
that st0 would have the same statistics as nst0.

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
        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
        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
        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
        requests to complete.
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.

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
I/O issued via the st module.

When read the statistics may not be temporally consistent while I/O is in
progress. The individual values are read and written to atomically however
when reading them back via sysfs they may be in the process of being
updated when starting an I/O or when it is completed.

The value shown in in_flight is incremented before any statstics are
updated and decremented when an I/O completes after updating statistics.
The value of in_flight is 0 when there are no I/Os outstanding that are
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
=======================

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
file being read is closed. The BSD semantics leaves the tape where it
currently is whereas the SYS V semantics moves the tape past the next
filemark unless the filemark has just been crossed.

The default is BSD semantics.


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
- one or more pages can't be locked into memory (should not happen in
  any reasonable situation)

The size of the driver buffers is always at least one tape block. In fixed
block mode, the minimum buffer size is defined (in 1024 byte units) by
ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
several blocks and using one SCSI read or write to transfer all of the
blocks. Buffering of data across write calls in fixed block mode is
allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
Buffer allocation uses chunks of memory having sizes 2^n * (page
size). Because of this the actual buffer size may be larger than the
minimum allowable buffer size.

NOTE that if direct i/o is used, the small writes are not buffered. This may
cause a surprise when moving from 2.4. There small writes (e.g., tar without
-b option) may have had good throughput but this is not true any more with
2.6. Direct i/o can be turned off to solve this problem but a better solution
is to use bigger write() byte counts (e.g., tar -b 64).

Asynchronous writing. Writing the buffer contents to the tape is
started and the write call returns immediately. The status is checked
at the next tape operation. Asynchronous writes are not done with
direct i/o and not in fixed block mode.

Buffered writes and asynchronous writes may in some rare cases cause
problems in multivolume operations if there is not enough space on the
tape after the early-warning mark to flush the driver buffer.

Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
attempted even if the user does not want to get all of the data at
this read command. Should be disabled for those drives that don't like
a filemark to truncate a read request or that don't like backspacing.

Scatter/gather buffers (buffers that consist of chunks non-contiguous
in the physical memory) are used if contiguous buffers can't be
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.
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.
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.


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
returns -1 and errno is set to ENOSPC. To enable writing a trailer,
the next write is allowed to proceed and, if successful, the number of
bytes is returned. After this, -1 and the number of bytes are
alternately returned until the physical end of medium (or some other
error) is encountered.

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
max_sg_segs=xxx		   the maximum number of scatter/gather
			   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
=======================

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
to use the same keyword used when loading as module but prepended
with 'st.'. For instance, to set the maximum number of scatter/gather
segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
number of scatter/gather segments).

For compatibility, the old syntax from early 2.5 and 2.4 kernel
versions is supported. The same keywords can be used as when loading
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::

	st=buffer_kbs:64,write_threshold_kbs:60

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


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
to find an mt that supports all of the Linux SCSI tape ioctls and
opens the device for writing if the tape contents will be modified
(look for a package mt-st* from the Linux ftp sites; the GNU mt does
not open for writing for, e.g., erase).

The supported ioctls are:

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
        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
	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
	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)
        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
        variable block mode (if applicable).
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
	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
	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
	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
	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
	partition of the tape. If the argument is negative, its absolute
	value specifies the size of partition 0 in megabytes. This is
	the physically first partition of many later drives, like the
	LTO drives from LTO-5 upwards. The drive has to support partitions
	with size specified by the initiator. Inactive unless
	MT_ST_CAN_PARTITIONS set.
MTSETDRVBUFFER
	Is used for several purposes. The command is obtained from count
        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
           The drive buffer option is set to the argument. Zero means
           no buffering.
        * 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
		be used for repositioning the tape (global)
	     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
		tapes (global)
	     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
	        the command to finish) for some commands (e.g., rewind)
	     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
		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
		compiled into the driver)

	* MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS
	   Sets or clears the option bits.
        * MT_ST_WRITE_THRESHOLD
           Sets the write threshold for this device to kilobytes
           specified by the lowest bits.
	* 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
	   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
	   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
	   non-zero number, and this number is not 0xff, the number is
	   used as the compression algorithm. The value
	   MT_ST_CLEAR_DEFAULT can be used to clear the compression
	   default.
	* 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
	   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
	   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
	   is found from the extended sense data. Many drives set one or
	   more bits in the extended sense data when the drive needs
	   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),
	   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
	   the pattern is non-zero, the pattern must match the masked
	   sense data byte.

	   (The cleaning bit is set if the additional sense code and
	   qualifier 00h 17h are seen regardless of the setting of
	   MT_ST_SET_CLN.)

The following ioctl uses the structure mtpos:

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.
        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.
        The number of recovered errors since the previous status call
        is stored in the lower word of the field mt_erreg.
        The current block size and the density code are stored in the field
        mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
        MT_ST_DENSITY_SHIFT).
	The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
	is set if there is no tape in the drive. GMT_EOD means either
	end of recorded data or end of tape. GMT_EOT means end of tape.


Miscellaneous Compile Options
=============================

The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
is defined.

The maximum number of tape devices is determined by the define
ST_MAX_TAPES. If more tapes are detected at driver initialization, the
maximum is adjusted accordingly.

Immediate return from tape positioning SCSI commands can be enabled by
defining ST_NOWAIT. If this is defined, the user should take care that
the next tape operation is not started before the previous one has
finished. The drives and SCSI adapters should handle this condition
gracefully, but some drive/adapter combinations are known to hang the
SCSI bus in this case.

The MTEOM command is by default implemented as spacing over 32767
filemarks. With this method the file number in the status is
correct. The user can request using direct spacing to EOD by setting
ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
number will be invalid.

When using read ahead or buffered writes the position within the file
may not be correct after the file is closed (correct position may
require backspacing over more than one record). The correct position
within file can be obtained if ST_IN_FILE_POS is defined at compile
time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
(The driver always backs over a filemark crossed by read ahead if the
user does not request data that far.)


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
can still be switched on and off with an ioctl.  To enable debug at
module load time add debug_flag=1 to the module load options, the
debugging output is not voluminous. Debugging can also be enabled
and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
file /sys/bus/scsi/drivers/st/debug_flag.

If the tape seems to hang, I would be very interested to hear where
the driver is waiting. With the command 'ps -l' you can see the state
of the process using the tape. If the state is D, the process is
waiting for something. The field WCHAN tells where the driver is
waiting. If you have the current System.map in the correct place (in
/boot for the procps I use) or have updated /etc/psdatabase (for kmem
ps), ps writes the function name in the WCHAN field. If not, you have
to look up the function from System.map.

Note also that the timeouts are very long compared to most other
drivers. This means that the Linux driver may appear hung although the
real reason is that the tape firmware has got confused.