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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	This file contains brief information about the SCSI tape driver.
2	The driver is currently maintained by Kai Mäkisara (email
3	Kai.Makisara@kolumbus.fi)
5	Last modified: Tue Feb  9 21:54:16 2016 by kai.makisara
10	The driver is generic, i.e., it does not contain any code tailored
11	to any specific tape drive. The tape parameters can be specified with
12	one of the following three methods:
14	1. Each user can specify the tape parameters he/she wants to use
15	directly with ioctls. This is administratively a very simple and
16	flexible method and applicable to single-user workstations. However,
17	in a multiuser environment the next user finds the tape parameters in
18	state the previous user left them.
20	2. The system manager (root) can define default values for some tape
21	parameters, like block size and density using the MTSETDRVBUFFER ioctl.
22	These parameters can be programmed to come into effect either when a
23	new tape is loaded into the drive or if writing begins at the
24	beginning of the tape. The second method is applicable if the tape
25	drive performs auto-detection of the tape format well (like some
26	QIC-drives). The result is that any tape can be read, writing can be
27	continued using existing format, and the default format is used if
28	the tape is rewritten from the beginning (or a new tape is written
29	for the first time). The first method is applicable if the drive
30	does not perform auto-detection well enough and there is a single
31	"sensible" mode for the device. An example is a DAT drive that is
32	used only in variable block mode (I don't know if this is sensible
33	or not :-).
35	The user can override the parameters defined by the system
36	manager. The changes persist until the defaults again come into
37	effect.
39	3. By default, up to four modes can be defined and selected using the minor
40	number (bits 5 and 6). The number of modes can be changed by changing
41	ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
42	above. Additional modes are dormant until they are defined by the
43	system manager (root). When specification of a new mode is started,
44	the configuration of mode 0 is used to provide a starting point for
45	definition of the new mode.
47	Using the modes allows the system manager to give the users choices
48	over some of the buffering parameters not directly accessible to the
49	users (buffered and asynchronous writes). The modes also allow choices
50	between formats in multi-tape operations (the explicitly overridden
51	parameters are reset when a new tape is loaded).
53	If more than one mode is used, all modes should contain definitions
54	for the same set of parameters.
56	Many Unices contain internal tables that associate different modes to
57	supported devices. The Linux SCSI tape driver does not contain such
58	tables (and will not do that in future). Instead of that, a utility
59	program can be made that fetches the inquiry data sent by the device,
60	scans its database, and sets up the modes using the ioctls. Another
61	alternative is to make a small script that uses mt to set the defaults
62	tailored to the system.
64	The driver supports fixed and variable block size (within buffer
65	limits). Both the auto-rewind (minor equals device number) and
66	non-rewind devices (minor is 128 + device number) are implemented.
68	In variable block mode, the byte count in write() determines the size
69	of the physical block on tape. When reading, the drive reads the next
70	tape block and returns to the user the data if the read() byte count
71	is at least the block size. Otherwise, error ENOMEM is returned.
73	In fixed block mode, the data transfer between the drive and the
74	driver is in multiples of the block size. The write() byte count must
75	be a multiple of the block size. This is not required when reading but
76	may be advisable for portability.
78	Support is provided for changing the tape partition and partitioning
79	of the tape with one or two partitions. By default support for
80	partitioned tape is disabled for each driver and it can be enabled
81	with the ioctl MTSETDRVBUFFER.
83	By default the driver writes one filemark when the device is closed after
84	writing and the last operation has been a write. Two filemarks can be
85	optionally written. In both cases end of data is signified by
86	returning zero bytes for two consecutive reads.
88	Writing filemarks without the immediate bit set in the SCSI command block acts
89	as a synchronization point, i.e., all remaining data form the drive buffers is
90	written to tape before the command returns. This makes sure that write errors
91	are caught at that point, but this takes time. In some applications, several
92	consecutive files must be written fast. The MTWEOFI operation can be used to
93	write the filemarks without flushing the drive buffer. Writing filemark at
94	close() is always flushing the drive buffers. However, if the previous
95	operation is MTWEOFI, close() does not write a filemark. This can be used if
96	the program wants to close/open the tape device between files and wants to
97	skip waiting.
99	If rewind, offline, bsf, or seek is done and previous tape operation was
100	write, a filemark is written before moving tape.
102	The compile options are defined in the file linux/drivers/scsi/st_options.h.
104	4. If the open option O_NONBLOCK is used, open succeeds even if the
105	drive is not ready. If O_NONBLOCK is not used, the driver waits for
106	the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
107	seconds, open fails with the errno value EIO. With O_NONBLOCK the
108	device can be opened for writing even if there is a write protected
109	tape in the drive (commands trying to write something return error if
110	attempted).
115	The tape driver currently supports up to 2^17 drives if 4 modes for
116	each drive are used.
118	The minor numbers consist of the following bit fields:
120	dev_upper non-rew mode dev-lower
121	  20 -  8     7    6 5  4      0
122	The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
123	byte). The bits defining the mode are below the non-rewind bit. The
124	remaining bits define the tape device number. This numbering is
125	backward compatible with the numbering used when the minor number was
126	only 8 bits wide.
131	The driver creates the directory /sys/class/scsi_tape and populates it with
132	directories corresponding to the existing tape devices. There are autorewind
133	and non-rewind entries for each mode. The names are stxy and nstxy, where x
134	is the tape number and y a character corresponding to the mode (none, l, m,
135	a). For example, the directories for the first tape device are (assuming four
136	modes): st0  nst0  st0l  nst0l  st0m  nst0m  st0a  nst0a.
138	Each directory contains the entries: default_blksize  default_compression
139	default_density  defined  dev  device  driver. The file 'defined' contains 1
140	if the mode is defined and zero if not defined. The files 'default_*' contain
141	the defaults set by the user. The value -1 means the default is not set. The
142	file 'dev' contains the device numbers corresponding to this device. The links
143	'device' and 'driver' point to the SCSI device and driver entries.
145	Each directory also contains the entry 'options' which shows the currently
146	enabled driver and mode options. The value in the file is a bit mask where the
147	bit definitions are the same as those used with MTSETDRVBUFFER in setting the
148	options.
150	A link named 'tape' is made from the SCSI device directory to the class
151	directory corresponding to the mode 0 auto-rewind device (e.g., st0). 
156	The st driver maintains statistics for tape drives inside the sysfs filesystem.
157	The following method can be used to locate the statistics that are
158	available (assuming that sysfs is mounted at /sys):
160	1. Use opendir(3) on the directory /sys/class/scsi_tape
161	2. Use readdir(3) to read the directory contents
162	3. Use regcomp(3)/regexec(3) to match directory entries to the extended
163	        regular expression "^st[0-9]+$"
164	4. Access the statistics from the /sys/class/scsi_tape/<match>/stats
165	        directory (where <match> is a directory entry from /sys/class/scsi_tape
166	        that matched the extended regular expression)
168	The reason for using this approach is that all the character devices
169	pointing to the same tape drive use the same statistics. That means
170	that st0 would have the same statistics as nst0.
172	The directory contains the following statistics files:
174	1.  in_flight - The number of I/Os currently outstanding to this device.
175	2.  io_ns - The amount of time spent waiting (in nanoseconds) for all I/O
176	        to complete (including read and write). This includes tape movement
177	        commands such as seeking between file or set marks and implicit tape
178	        movement such as when rewind on close tape devices are used.
179	3.  other_cnt - The number of I/Os issued to the tape drive other than read or
180	        write commands. The time taken to complete these commands uses the
181	        following calculation io_ms-read_ms-write_ms.
182	4.  read_byte_cnt - The number of bytes read from the tape drive.
183	5.  read_cnt - The number of read requests issued to the tape drive.
184	6.  read_ns - The amount of time (in nanoseconds) spent waiting for read
185	        requests to complete.
186	7.  write_byte_cnt - The number of bytes written to the tape drive.
187	8.  write_cnt - The number of write requests issued to the tape drive.
188	9.  write_ns - The amount of time (in nanoseconds) spent waiting for write
189	        requests to complete.
190	10. resid_cnt - The number of times during a read or write we found
191		the residual amount to be non-zero. This should mean that a program
192		is issuing a read larger thean the block size on tape. For write
193		not all data made it to tape.
195	Note: The in_flight value is incremented when an I/O starts the I/O
196	itself is not added to the statistics until it completes.
198	The total of read_cnt, write_cnt, and other_cnt may not total to the same
199	value as iodone_cnt at the device level. The tape statistics only count
200	I/O issued via the st module.
202	When read the statistics may not be temporally consistent while I/O is in
203	progress. The individual values are read and written to atomically however
204	when reading them back via sysfs they may be in the process of being
205	updated when starting an I/O or when it is completed.
207	The value shown in in_flight is incremented before any statstics are
208	updated and decremented when an I/O completes after updating statistics.
209	The value of in_flight is 0 when there are no I/Os outstanding that are
210	issued by the st driver. Tape statistics do not take into account any
211	I/O performed via the sg device.
215	The user can choose between these two behaviours of the tape driver by
216	defining the value of the symbol ST_SYSV. The semantics differ when a
217	file being read is closed. The BSD semantics leaves the tape where it
218	currently is whereas the SYS V semantics moves the tape past the next
219	filemark unless the filemark has just been crossed.
221	The default is BSD semantics.
226	The driver tries to do transfers directly to/from user space. If this
227	is not possible, a driver buffer allocated at run-time is used. If
228	direct i/o is not possible for the whole transfer, the driver buffer
229	is used (i.e., bounce buffers for individual pages are not
230	used). Direct i/o can be impossible because of several reasons, e.g.:
231	- one or more pages are at addresses not reachable by the HBA
232	- the number of pages in the transfer exceeds the number of
233	  scatter/gather segments permitted by the HBA
234	- one or more pages can't be locked into memory (should not happen in
235	  any reasonable situation)
237	The size of the driver buffers is always at least one tape block. In fixed
238	block mode, the minimum buffer size is defined (in 1024 byte units) by
239	ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
240	several blocks and using one SCSI read or write to transfer all of the
241	blocks. Buffering of data across write calls in fixed block mode is
242	allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
243	Buffer allocation uses chunks of memory having sizes 2^n * (page
244	size). Because of this the actual buffer size may be larger than the
245	minimum allowable buffer size.
247	NOTE that if direct i/o is used, the small writes are not buffered. This may
248	cause a surprise when moving from 2.4. There small writes (e.g., tar without
249	-b option) may have had good throughput but this is not true any more with
250	2.6. Direct i/o can be turned off to solve this problem but a better solution
251	is to use bigger write() byte counts (e.g., tar -b 64).
253	Asynchronous writing. Writing the buffer contents to the tape is
254	started and the write call returns immediately. The status is checked
255	at the next tape operation. Asynchronous writes are not done with
256	direct i/o and not in fixed block mode.
258	Buffered writes and asynchronous writes may in some rare cases cause
259	problems in multivolume operations if there is not enough space on the
260	tape after the early-warning mark to flush the driver buffer.
262	Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
263	attempted even if the user does not want to get all of the data at
264	this read command. Should be disabled for those drives that don't like
265	a filemark to truncate a read request or that don't like backspacing.
267	Scatter/gather buffers (buffers that consist of chunks non-contiguous
268	in the physical memory) are used if contiguous buffers can't be
269	allocated. To support all SCSI adapters (including those not
270	supporting scatter/gather), buffer allocation is using the following
271	three kinds of chunks:
272	1. The initial segment that is used for all SCSI adapters including
273	those not supporting scatter/gather. The size of this buffer will be
274	(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
275	this size (and it is not larger than the buffer size specified by
276	ST_BUFFER_BLOCKS). If this size is not available, the driver halves
277	the size and tries again until the size of one page. The default
278	settings in st_options.h make the driver to try to allocate all of the
279	buffer as one chunk.
280	2. The scatter/gather segments to fill the specified buffer size are
281	allocated so that as many segments as possible are used but the number
282	of segments does not exceed ST_FIRST_SG.
283	3. The remaining segments between ST_MAX_SG (or the module parameter
284	max_sg_segs) and the number of segments used in phases 1 and 2
285	are used to extend the buffer at run-time if this is necessary. The
286	number of scatter/gather segments allowed for the SCSI adapter is not
287	exceeded if it is smaller than the maximum number of scatter/gather
288	segments specified. If the maximum number allowed for the SCSI adapter
289	is smaller than the number of segments used in phases 1 and 2,
290	extending the buffer will always fail.
295	When the end of medium early warning is encountered, the current write
296	is finished and the number of bytes is returned. The next write
297	returns -1 and errno is set to ENOSPC. To enable writing a trailer,
298	the next write is allowed to proceed and, if successful, the number of
299	bytes is returned. After this, -1 and the number of bytes are
300	alternately returned until the physical end of medium (or some other
301	error) is encountered.
306	The buffer size, write threshold, and the maximum number of allocated buffers
307	are configurable when the driver is loaded as a module. The keywords are:
309	buffer_kbs=xxx             the buffer size for fixed block mode is set
310				   to xxx kilobytes
311	write_threshold_kbs=xxx    the write threshold in kilobytes set to xxx
312	max_sg_segs=xxx		   the maximum number of scatter/gather
313				   segments
314	try_direct_io=x		   try direct transfer between user buffer and
315				   tape drive if this is non-zero
317	Note that if the buffer size is changed but the write threshold is not
318	set, the write threshold is set to the new buffer size - 2 kB.
323	If the driver is compiled into the kernel, the same parameters can be
324	also set using, e.g., the LILO command line. The preferred syntax is
325	to use the same keyword used when loading as module but prepended
326	with 'st.'. For instance, to set the maximum number of scatter/gather
327	segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
328	number of scatter/gather segments).
330	For compatibility, the old syntax from early 2.5 and 2.4 kernel
331	versions is supported. The same keywords can be used as when loading
332	the driver as module. If several parameters are set, the keyword-value
333	pairs are separated with a comma (no spaces allowed). A colon can be
334	used instead of the equal mark. The definition is prepended by the
335	string st=. Here is an example:
337		st=buffer_kbs:64,write_threshold_kbs:60
339	The following syntax used by the old kernel versions is also supported:
341	           st=aa[,bb[,dd]]
343	where
344	  aa is the buffer size for fixed block mode in 1024 byte units
345	  bb is the write threshold in 1024 byte units
346	  dd is the maximum number of scatter/gather segments
351	The tape is positioned and the drive parameters are set with ioctls
352	defined in mtio.h The tape control program 'mt' uses these ioctls. Try
353	to find an mt that supports all of the Linux SCSI tape ioctls and
354	opens the device for writing if the tape contents will be modified
355	(look for a package mt-st* from the Linux ftp sites; the GNU mt does
356	not open for writing for, e.g., erase).
358	The supported ioctls are:
360	The following use the structure mtop:
362	MTFSF   Space forward over count filemarks. Tape positioned after filemark.
363	MTFSFM  As above but tape positioned before filemark.
364	MTBSF	Space backward over count filemarks. Tape positioned before
365	        filemark.
366	MTBSFM  As above but ape positioned after filemark.
367	MTFSR   Space forward over count records.
368	MTBSR   Space backward over count records.
369	MTFSS   Space forward over count setmarks.
370	MTBSS   Space backward over count setmarks.
371	MTWEOF  Write count filemarks.
372	MTWEOFI	Write count filemarks with immediate bit set (i.e., does not
373		wait until data is on tape)
374	MTWSM   Write count setmarks.
375	MTREW   Rewind tape.
376	MTOFFL  Set device off line (often rewind plus eject).
377	MTNOP   Do nothing except flush the buffers.
378	MTRETEN Re-tension tape.
379	MTEOM   Space to end of recorded data.
380	MTERASE Erase tape. If the argument is zero, the short erase command
381		is used. The long erase command is used with all other values
382		of the argument.
383	MTSEEK	Seek to tape block count. Uses Tandberg-compatible seek (QFA)
384	        for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
385		block numbers in the status are not valid after a seek.
386	MTSETBLK Set the drive block size. Setting to zero sets the drive into
387	        variable block mode (if applicable).
388	MTSETDENSITY Sets the drive density code to arg. See drive
389	        documentation for available codes.
390	MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door.
391	MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the
392		command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
393		MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
394		drive with the command and it selects the tape slot to use of
395		HP C1553A changer.
396	MTCOMPRESSION Sets compressing or uncompressing drive mode using the
397		SCSI mode page 15. Note that some drives other methods for
398		control of compression. Some drives (like the Exabytes) use
399		density codes for compression control. Some drives use another
400		mode page but this page has not been implemented in the
401		driver. Some drives without compression capability will accept
402		any compression mode without error.
403	MTSETPART Moves the tape to the partition given by the argument at the
404		next tape operation. The block at which the tape is positioned
405		is the block where the tape was previously positioned in the
406		new active partition unless the next tape operation is
407		MTSEEK. In this case the tape is moved directly to the block
408		specified by MTSEEK. MTSETPART is inactive unless
410	MTMKPART Formats the tape with one partition (argument zero) or two
411		partitions (argument non-zero). If the argument is positive,
412		it specifies the size of partition 1 in megabytes. For DDS
413		drives and several early drives this is the physically first
414		partition of the tape. If the argument is negative, its absolute
415		value specifies the size of partition 0 in megabytes. This is
416		the physically first partition of many later drives, like the
417		LTO drives from LTO-5 upwards. The drive has to support partitions
418		with size specified by the initiator. Inactive unless
421		Is used for several purposes. The command is obtained from count
422	        with mask MT_SET_OPTIONS, the low order bits are used as argument.
423		This command is only allowed for the superuser (root). The
424		subcommands are:
425		0
426	           The drive buffer option is set to the argument. Zero means
427	           no buffering.
428	        MT_ST_BOOLEANS
429	           Sets the buffering options. The bits are the new states
430	           (enabled/disabled) the following options (in the
431		   parenthesis is specified whether the option is global or
432		   can be specified differently for each mode):
433		     MT_ST_BUFFER_WRITES write buffering (mode)
434		     MT_ST_ASYNC_WRITES asynchronous writes (mode)
435	             MT_ST_READ_AHEAD  read ahead (mode)
436	             MT_ST_TWO_FM writing of two filemarks (global)
437		     MT_ST_FAST_EOM using the SCSI spacing to EOD (global)
438		     MT_ST_AUTO_LOCK automatic locking of the drive door (global)
439	             MT_ST_DEF_WRITES the defaults are meant only for writes (mode)
440		     MT_ST_CAN_BSR backspacing over more than one records can
441			be used for repositioning the tape (global)
442		     MT_ST_NO_BLKLIMS the driver does not ask the block limits
443			from the drive (block size can be changed only to
444			variable) (global)
445		     MT_ST_CAN_PARTITIONS enables support for partitioned
446			tapes (global)
447		     MT_ST_SCSI2LOGICAL the logical block number is used in
448			the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
449			the device dependent address. It is recommended to set
450			this flag unless there are tapes using the device
451			dependent (from the old times) (global)
452		     MT_ST_SYSV sets the SYSV semantics (mode)
453		     MT_ST_NOWAIT enables immediate mode (i.e., don't wait for
454		        the command to finish) for some commands (e.g., rewind)
455		     MT_ST_NOWAIT_EOF enables immediate filemark mode (i.e. when
456		        writing a filemark, don't wait for it to complete). Please
457			see the BASICS note about MTWEOFI with respect to the
458			possible dangers of writing immediate filemarks.
459		     MT_ST_SILI enables setting the SILI bit in SCSI commands when
460			reading in variable block mode to enhance performance when
461			reading blocks shorter than the byte count; set this only
462			if you are sure that the drive supports SILI and the HBA
463			correctly returns transfer residuals
464		     MT_ST_DEBUGGING debugging (global; debugging must be
465			compiled into the driver)
468		   Sets or clears the option bits.
470	           Sets the write threshold for this device to kilobytes
471	           specified by the lowest bits.
473		   Defines the default block size set automatically. Value
474		   0xffffff means that the default is not used any more.
477		   Used to set or clear the density (8 bits), and drive buffer
478		   state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
479		   (0xfffff) the default will not be used any more. Otherwise
480		   the lowermost bits of the value contain the new value of
481		   the parameter.
483		   The compression default will not be used if the value of
484		   the lowermost byte is 0xff. Otherwise the lowermost bit
485		   contains the new default. If the bits 8-15 are set to a
486		   non-zero number, and this number is not 0xff, the number is
487		   used as the compression algorithm. The value
488		   MT_ST_CLEAR_DEFAULT can be used to clear the compression
489		   default.
491		   Set the normal timeout in seconds for this device. The
492		   default is 900 seconds (15 minutes). The timeout should be
493		   long enough for the retries done by the device while
494		   reading/writing.
496		   Set the long timeout that is used for operations that are
497		   known to take a long time. The default is 14000 seconds
498		   (3.9 hours). For erase this value is further multiplied by
499		   eight.
501		   Set the cleaning request interpretation parameters using
502		   the lowest 24 bits of the argument. The driver can set the
503		   generic status bit GMT_CLN if a cleaning request bit pattern
504		   is found from the extended sense data. Many drives set one or
505		   more bits in the extended sense data when the drive needs
506		   cleaning. The bits are device-dependent. The driver is
507		   given the number of the sense data byte (the lowest eight
508		   bits of the argument; must be >= 18 (values 1 - 17
509		   reserved) and <= the maximum requested sense data sixe), 
510		   a mask to select the relevant bits (the bits 9-16), and the
511		   bit pattern (bits 17-23). If the bit pattern is zero, one
512		   or more bits under the mask indicate cleaning request. If
513		   the pattern is non-zero, the pattern must match the masked
514		   sense data byte.
516		   (The cleaning bit is set if the additional sense code and
517		   qualifier 00h 17h are seen regardless of the setting of
518		   MT_ST_SET_CLN.)
520	The following ioctl uses the structure mtpos:
521	MTIOCPOS Reads the current position from the drive. Uses
522	        Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
523	        command for the SCSI-2 drives.
525	The following ioctl uses the structure mtget to return the status:
526	MTIOCGET Returns some status information.
527	        The file number and block number within file are returned. The
528	        block is -1 when it can't be determined (e.g., after MTBSF).
529	        The drive type is either MTISSCSI1 or MTISSCSI2.
530	        The number of recovered errors since the previous status call
531	        is stored in the lower word of the field mt_erreg.
532	        The current block size and the density code are stored in the field
533	        mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
534	        MT_ST_DENSITY_SHIFT).
535		The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
536		is set if there is no tape in the drive. GMT_EOD means either
537		end of recorded data or end of tape. GMT_EOT means end of tape.
542	The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
543	is defined.
545	The maximum number of tape devices is determined by the define
546	ST_MAX_TAPES. If more tapes are detected at driver initialization, the
547	maximum is adjusted accordingly.
549	Immediate return from tape positioning SCSI commands can be enabled by
550	defining ST_NOWAIT. If this is defined, the user should take care that
551	the next tape operation is not started before the previous one has
552	finished. The drives and SCSI adapters should handle this condition
553	gracefully, but some drive/adapter combinations are known to hang the
554	SCSI bus in this case.
556	The MTEOM command is by default implemented as spacing over 32767
557	filemarks. With this method the file number in the status is
558	correct. The user can request using direct spacing to EOD by setting
559	ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
560	number will be invalid.
562	When using read ahead or buffered writes the position within the file
563	may not be correct after the file is closed (correct position may
564	require backspacing over more than one record). The correct position
565	within file can be obtained if ST_IN_FILE_POS is defined at compile
566	time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
567	(The driver always backs over a filemark crossed by read ahead if the
568	user does not request data that far.)
573	Debugging code is now compiled in by default but debugging is turned off
574	with the kernel module parameter debug_flag defaulting to 0.  Debugging
575	can still be switched on and off with an ioctl.  To enable debug at
576	module load time add debug_flag=1 to the module load options, the
577	debugging output is not voluminous. Debugging can also be enabled
578	and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
579	file /sys/bus/scsi/drivers/st/debug_flag.
581	If the tape seems to hang, I would be very interested to hear where
582	the driver is waiting. With the command 'ps -l' you can see the state
583	of the process using the tape. If the state is D, the process is
584	waiting for something. The field WCHAN tells where the driver is
585	waiting. If you have the current System.map in the correct place (in
586	/boot for the procps I use) or have updated /etc/psdatabase (for kmem
587	ps), ps writes the function name in the WCHAN field. If not, you have
588	to look up the function from System.map.
590	Note also that the timeouts are very long compared to most other
591	drivers. This means that the Linux driver may appear hung although the
592	real reason is that the tape firmware has got confused.
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