1.. SPDX-License-Identifier: GPL-2.0
2
3====================
4The SCSI Tape Driver
5====================
6
7This file contains brief information about the SCSI tape driver.
8The driver is currently maintained by Kai Mäkisara (email
9Kai.Makisara@kolumbus.fi)
10
11Last modified: Tue Feb  9 21:54:16 2016 by kai.makisara
12
13
14Basics
15======
16
17The driver is generic, i.e., it does not contain any code tailored
18to any specific tape drive. The tape parameters can be specified with
19one of the following three methods:
20
211. Each user can specify the tape parameters he/she wants to use
22directly with ioctls. This is administratively a very simple and
23flexible method and applicable to single-user workstations. However,
24in a multiuser environment the next user finds the tape parameters in
25state the previous user left them.
26
272. The system manager (root) can define default values for some tape
28parameters, like block size and density using the MTSETDRVBUFFER ioctl.
29These parameters can be programmed to come into effect either when a
30new tape is loaded into the drive or if writing begins at the
31beginning of the tape. The second method is applicable if the tape
32drive performs auto-detection of the tape format well (like some
33QIC-drives). The result is that any tape can be read, writing can be
34continued using existing format, and the default format is used if
35the tape is rewritten from the beginning (or a new tape is written
36for the first time). The first method is applicable if the drive
37does not perform auto-detection well enough and there is a single
38"sensible" mode for the device. An example is a DAT drive that is
39used only in variable block mode (I don't know if this is sensible
40or not :-).
41
42The user can override the parameters defined by the system
43manager. The changes persist until the defaults again come into
44effect.
45
463. By default, up to four modes can be defined and selected using the minor
47number (bits 5 and 6). The number of modes can be changed by changing
48ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
49above. Additional modes are dormant until they are defined by the
50system manager (root). When specification of a new mode is started,
51the configuration of mode 0 is used to provide a starting point for
52definition of the new mode.
53
54Using the modes allows the system manager to give the users choices
55over some of the buffering parameters not directly accessible to the
56users (buffered and asynchronous writes). The modes also allow choices
57between formats in multi-tape operations (the explicitly overridden
58parameters are reset when a new tape is loaded).
59
60If more than one mode is used, all modes should contain definitions
61for the same set of parameters.
62
63Many Unices contain internal tables that associate different modes to
64supported devices. The Linux SCSI tape driver does not contain such
65tables (and will not do that in future). Instead of that, a utility
66program can be made that fetches the inquiry data sent by the device,
67scans its database, and sets up the modes using the ioctls. Another
68alternative is to make a small script that uses mt to set the defaults
69tailored to the system.
70
71The driver supports fixed and variable block size (within buffer
72limits). Both the auto-rewind (minor equals device number) and
73non-rewind devices (minor is 128 + device number) are implemented.
74
75In variable block mode, the byte count in write() determines the size
76of the physical block on tape. When reading, the drive reads the next
77tape block and returns to the user the data if the read() byte count
78is at least the block size. Otherwise, error ENOMEM is returned.
79
80In fixed block mode, the data transfer between the drive and the
81driver is in multiples of the block size. The write() byte count must
82be a multiple of the block size. This is not required when reading but
83may be advisable for portability.
84
85Support is provided for changing the tape partition and partitioning
86of the tape with one or two partitions. By default support for
87partitioned tape is disabled for each driver and it can be enabled
88with the ioctl MTSETDRVBUFFER.
89
90By default the driver writes one filemark when the device is closed after
91writing and the last operation has been a write. Two filemarks can be
92optionally written. In both cases end of data is signified by
93returning zero bytes for two consecutive reads.
94
95Writing filemarks without the immediate bit set in the SCSI command block acts
96as a synchronization point, i.e., all remaining data form the drive buffers is
97written to tape before the command returns. This makes sure that write errors
98are caught at that point, but this takes time. In some applications, several
99consecutive files must be written fast. The MTWEOFI operation can be used to
100write the filemarks without flushing the drive buffer. Writing filemark at
101close() is always flushing the drive buffers. However, if the previous
102operation is MTWEOFI, close() does not write a filemark. This can be used if
103the program wants to close/open the tape device between files and wants to
104skip waiting.
105
106If rewind, offline, bsf, or seek is done and previous tape operation was
107write, a filemark is written before moving tape.
108
109The compile options are defined in the file linux/drivers/scsi/st_options.h.
110
1114. If the open option O_NONBLOCK is used, open succeeds even if the
112drive is not ready. If O_NONBLOCK is not used, the driver waits for
113the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
114seconds, open fails with the errno value EIO. With O_NONBLOCK the
115device can be opened for writing even if there is a write protected
116tape in the drive (commands trying to write something return error if
117attempted).
118
119
120Minor Numbers
121=============
122
123The tape driver currently supports up to 2^17 drives if 4 modes for
124each drive are used.
125
126The minor numbers consist of the following bit fields::
127
128    dev_upper non-rew mode dev-lower
129    20 -  8     7    6 5  4      0
130
131The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
132byte). The bits defining the mode are below the non-rewind bit. The
133remaining bits define the tape device number. This numbering is
134backward compatible with the numbering used when the minor number was
135only 8 bits wide.
136
137
138Sysfs Support
139=============
140
141The driver creates the directory /sys/class/scsi_tape and populates it with
142directories corresponding to the existing tape devices. There are autorewind
143and non-rewind entries for each mode. The names are stxy and nstxy, where x
144is the tape number and y a character corresponding to the mode (none, l, m,
145a). For example, the directories for the first tape device are (assuming four
146modes): st0  nst0  st0l  nst0l  st0m  nst0m  st0a  nst0a.
147
148Each directory contains the entries: default_blksize  default_compression
149default_density  defined  dev  device  driver. The file 'defined' contains 1
150if the mode is defined and zero if not defined. The files 'default_*' contain
151the defaults set by the user. The value -1 means the default is not set. The
152file 'dev' contains the device numbers corresponding to this device. The links
153'device' and 'driver' point to the SCSI device and driver entries.
154
155Each directory also contains the entry 'options' which shows the currently
156enabled driver and mode options. The value in the file is a bit mask where the
157bit definitions are the same as those used with MTSETDRVBUFFER in setting the
158options.
159
160A link named 'tape' is made from the SCSI device directory to the class
161directory corresponding to the mode 0 auto-rewind device (e.g., st0).
162
163
164Sysfs and Statistics for Tape Devices
165=====================================
166
167The st driver maintains statistics for tape drives inside the sysfs filesystem.
168The following method can be used to locate the statistics that are
169available (assuming that sysfs is mounted at /sys):
170
1711. Use opendir(3) on the directory /sys/class/scsi_tape
1722. Use readdir(3) to read the directory contents
1733. Use regcomp(3)/regexec(3) to match directory entries to the extended
174   regular expression "^st[0-9]+$"
1754. Access the statistics from the /sys/class/scsi_tape/<match>/stats
176   directory (where <match> is a directory entry from /sys/class/scsi_tape
177   that matched the extended regular expression)
178
179The reason for using this approach is that all the character devices
180pointing to the same tape drive use the same statistics. That means
181that st0 would have the same statistics as nst0.
182
183The directory contains the following statistics files:
184
1851.  in_flight
186      - The number of I/Os currently outstanding to this device.
1872.  io_ns
188      - The amount of time spent waiting (in nanoseconds) for all I/O
189        to complete (including read and write). This includes tape movement
190        commands such as seeking between file or set marks and implicit tape
191        movement such as when rewind on close tape devices are used.
1923.  other_cnt
193      - The number of I/Os issued to the tape drive other than read or
194        write commands. The time taken to complete these commands uses the
195        following calculation io_ms-read_ms-write_ms.
1964.  read_byte_cnt
197      - The number of bytes read from the tape drive.
1985.  read_cnt
199      - The number of read requests issued to the tape drive.
2006.  read_ns
201      - The amount of time (in nanoseconds) spent waiting for read
202        requests to complete.
2037.  write_byte_cnt
204      - The number of bytes written to the tape drive.
2058.  write_cnt
206      - The number of write requests issued to the tape drive.
2079.  write_ns
208      - The amount of time (in nanoseconds) spent waiting for write
209        requests to complete.
21010. resid_cnt
211      - The number of times during a read or write we found
212	the residual amount to be non-zero. This should mean that a program
213	is issuing a read larger thean the block size on tape. For write
214	not all data made it to tape.
215
216.. Note::
217
218   The in_flight value is incremented when an I/O starts the I/O
219   itself is not added to the statistics until it completes.
220
221The total of read_cnt, write_cnt, and other_cnt may not total to the same
222value as iodone_cnt at the device level. The tape statistics only count
223I/O issued via the st module.
224
225When read the statistics may not be temporally consistent while I/O is in
226progress. The individual values are read and written to atomically however
227when reading them back via sysfs they may be in the process of being
228updated when starting an I/O or when it is completed.
229
230The value shown in in_flight is incremented before any statstics are
231updated and decremented when an I/O completes after updating statistics.
232The value of in_flight is 0 when there are no I/Os outstanding that are
233issued by the st driver. Tape statistics do not take into account any
234I/O performed via the sg device.
235
236BSD and Sys V Semantics
237=======================
238
239The user can choose between these two behaviours of the tape driver by
240defining the value of the symbol ST_SYSV. The semantics differ when a
241file being read is closed. The BSD semantics leaves the tape where it
242currently is whereas the SYS V semantics moves the tape past the next
243filemark unless the filemark has just been crossed.
244
245The default is BSD semantics.
246
247
248Buffering
249=========
250
251The driver tries to do transfers directly to/from user space. If this
252is not possible, a driver buffer allocated at run-time is used. If
253direct i/o is not possible for the whole transfer, the driver buffer
254is used (i.e., bounce buffers for individual pages are not
255used). Direct i/o can be impossible because of several reasons, e.g.:
256
257- one or more pages are at addresses not reachable by the HBA
258- the number of pages in the transfer exceeds the number of
259  scatter/gather segments permitted by the HBA
260- one or more pages can't be locked into memory (should not happen in
261  any reasonable situation)
262
263The size of the driver buffers is always at least one tape block. In fixed
264block mode, the minimum buffer size is defined (in 1024 byte units) by
265ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
266several blocks and using one SCSI read or write to transfer all of the
267blocks. Buffering of data across write calls in fixed block mode is
268allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
269Buffer allocation uses chunks of memory having sizes 2^n * (page
270size). Because of this the actual buffer size may be larger than the
271minimum allowable buffer size.
272
273NOTE that if direct i/o is used, the small writes are not buffered. This may
274cause a surprise when moving from 2.4. There small writes (e.g., tar without
275-b option) may have had good throughput but this is not true any more with
2762.6. Direct i/o can be turned off to solve this problem but a better solution
277is to use bigger write() byte counts (e.g., tar -b 64).
278
279Asynchronous writing. Writing the buffer contents to the tape is
280started and the write call returns immediately. The status is checked
281at the next tape operation. Asynchronous writes are not done with
282direct i/o and not in fixed block mode.
283
284Buffered writes and asynchronous writes may in some rare cases cause
285problems in multivolume operations if there is not enough space on the
286tape after the early-warning mark to flush the driver buffer.
287
288Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
289attempted even if the user does not want to get all of the data at
290this read command. Should be disabled for those drives that don't like
291a filemark to truncate a read request or that don't like backspacing.
292
293Scatter/gather buffers (buffers that consist of chunks non-contiguous
294in the physical memory) are used if contiguous buffers can't be
295allocated. To support all SCSI adapters (including those not
296supporting scatter/gather), buffer allocation is using the following
297three kinds of chunks:
298
2991. The initial segment that is used for all SCSI adapters including
300   those not supporting scatter/gather. The size of this buffer will be
301   (PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
302   this size (and it is not larger than the buffer size specified by
303   ST_BUFFER_BLOCKS). If this size is not available, the driver halves
304   the size and tries again until the size of one page. The default
305   settings in st_options.h make the driver to try to allocate all of the
306   buffer as one chunk.
3072. The scatter/gather segments to fill the specified buffer size are
308   allocated so that as many segments as possible are used but the number
309   of segments does not exceed ST_FIRST_SG.
3103. The remaining segments between ST_MAX_SG (or the module parameter
311   max_sg_segs) and the number of segments used in phases 1 and 2
312   are used to extend the buffer at run-time if this is necessary. The
313   number of scatter/gather segments allowed for the SCSI adapter is not
314   exceeded if it is smaller than the maximum number of scatter/gather
315   segments specified. If the maximum number allowed for the SCSI adapter
316   is smaller than the number of segments used in phases 1 and 2,
317   extending the buffer will always fail.
318
319
320EOM Behaviour When Writing
321==========================
322
323When the end of medium early warning is encountered, the current write
324is finished and the number of bytes is returned. The next write
325returns -1 and errno is set to ENOSPC. To enable writing a trailer,
326the next write is allowed to proceed and, if successful, the number of
327bytes is returned. After this, -1 and the number of bytes are
328alternately returned until the physical end of medium (or some other
329error) is encountered.
330
331Module Parameters
332=================
333
334The buffer size, write threshold, and the maximum number of allocated buffers
335are configurable when the driver is loaded as a module. The keywords are:
336
337========================== ===========================================
338buffer_kbs=xxx             the buffer size for fixed block mode is set
339			   to xxx kilobytes
340write_threshold_kbs=xxx    the write threshold in kilobytes set to xxx
341max_sg_segs=xxx		   the maximum number of scatter/gather
342			   segments
343try_direct_io=x		   try direct transfer between user buffer and
344			   tape drive if this is non-zero
345========================== ===========================================
346
347Note that if the buffer size is changed but the write threshold is not
348set, the write threshold is set to the new buffer size - 2 kB.
349
350
351Boot Time Configuration
352=======================
353
354If the driver is compiled into the kernel, the same parameters can be
355also set using, e.g., the LILO command line. The preferred syntax is
356to use the same keyword used when loading as module but prepended
357with 'st.'. For instance, to set the maximum number of scatter/gather
358segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
359number of scatter/gather segments).
360
361For compatibility, the old syntax from early 2.5 and 2.4 kernel
362versions is supported. The same keywords can be used as when loading
363the driver as module. If several parameters are set, the keyword-value
364pairs are separated with a comma (no spaces allowed). A colon can be
365used instead of the equal mark. The definition is prepended by the
366string st=. Here is an example::
367
368	st=buffer_kbs:64,write_threshold_kbs:60
369
370The following syntax used by the old kernel versions is also supported::
371
372           st=aa[,bb[,dd]]
373
374where:
375
376  - aa is the buffer size for fixed block mode in 1024 byte units
377  - bb is the write threshold in 1024 byte units
378  - dd is the maximum number of scatter/gather segments
379
380
381IOCTLs
382======
383
384The tape is positioned and the drive parameters are set with ioctls
385defined in mtio.h The tape control program 'mt' uses these ioctls. Try
386to find an mt that supports all of the Linux SCSI tape ioctls and
387opens the device for writing if the tape contents will be modified
388(look for a package mt-st* from the Linux ftp sites; the GNU mt does
389not open for writing for, e.g., erase).
390
391The supported ioctls are:
392
393The following use the structure mtop:
394
395MTFSF
396	Space forward over count filemarks. Tape positioned after filemark.
397MTFSFM
398	As above but tape positioned before filemark.
399MTBSF
400	Space backward over count filemarks. Tape positioned before
401        filemark.
402MTBSFM
403	As above but ape positioned after filemark.
404MTFSR
405	Space forward over count records.
406MTBSR
407	Space backward over count records.
408MTFSS
409	Space forward over count setmarks.
410MTBSS
411	Space backward over count setmarks.
412MTWEOF
413	Write count filemarks.
414MTWEOFI
415	Write count filemarks with immediate bit set (i.e., does not
416	wait until data is on tape)
417MTWSM
418	Write count setmarks.
419MTREW
420	Rewind tape.
421MTOFFL
422	Set device off line (often rewind plus eject).
423MTNOP
424	Do nothing except flush the buffers.
425MTRETEN
426	Re-tension tape.
427MTEOM
428	Space to end of recorded data.
429MTERASE
430	Erase tape. If the argument is zero, the short erase command
431	is used. The long erase command is used with all other values
432	of the argument.
433MTSEEK
434	Seek to tape block count. Uses Tandberg-compatible seek (QFA)
435        for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
436	block numbers in the status are not valid after a seek.
437MTSETBLK
438	Set the drive block size. Setting to zero sets the drive into
439        variable block mode (if applicable).
440MTSETDENSITY
441	Sets the drive density code to arg. See drive
442        documentation for available codes.
443MTLOCK and MTUNLOCK
444	Explicitly lock/unlock the tape drive door.
445MTLOAD and MTUNLOAD
446	Explicitly load and unload the tape. If the
447	command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
448	MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
449	drive with the command and it selects the tape slot to use of
450	HP C1553A changer.
451MTCOMPRESSION
452	Sets compressing or uncompressing drive mode using the
453	SCSI mode page 15. Note that some drives other methods for
454	control of compression. Some drives (like the Exabytes) use
455	density codes for compression control. Some drives use another
456	mode page but this page has not been implemented in the
457	driver. Some drives without compression capability will accept
458	any compression mode without error.
459MTSETPART
460	Moves the tape to the partition given by the argument at the
461	next tape operation. The block at which the tape is positioned
462	is the block where the tape was previously positioned in the
463	new active partition unless the next tape operation is
464	MTSEEK. In this case the tape is moved directly to the block
465	specified by MTSEEK. MTSETPART is inactive unless
466	MT_ST_CAN_PARTITIONS set.
467MTMKPART
468	Formats the tape with one partition (argument zero) or two
469	partitions (argument non-zero). If the argument is positive,
470	it specifies the size of partition 1 in megabytes. For DDS
471	drives and several early drives this is the physically first
472	partition of the tape. If the argument is negative, its absolute
473	value specifies the size of partition 0 in megabytes. This is
474	the physically first partition of many later drives, like the
475	LTO drives from LTO-5 upwards. The drive has to support partitions
476	with size specified by the initiator. Inactive unless
477	MT_ST_CAN_PARTITIONS set.
478MTSETDRVBUFFER
479	Is used for several purposes. The command is obtained from count
480        with mask MT_SET_OPTIONS, the low order bits are used as argument.
481	This command is only allowed for the superuser (root). The
482	subcommands are:
483
484	* 0
485           The drive buffer option is set to the argument. Zero means
486           no buffering.
487        * MT_ST_BOOLEANS
488           Sets the buffering options. The bits are the new states
489           (enabled/disabled) the following options (in the
490	   parenthesis is specified whether the option is global or
491	   can be specified differently for each mode):
492
493	     MT_ST_BUFFER_WRITES
494		write buffering (mode)
495	     MT_ST_ASYNC_WRITES
496		asynchronous writes (mode)
497             MT_ST_READ_AHEAD
498		read ahead (mode)
499             MT_ST_TWO_FM
500		writing of two filemarks (global)
501	     MT_ST_FAST_EOM
502		using the SCSI spacing to EOD (global)
503	     MT_ST_AUTO_LOCK
504		automatic locking of the drive door (global)
505             MT_ST_DEF_WRITES
506		the defaults are meant only for writes (mode)
507	     MT_ST_CAN_BSR
508		backspacing over more than one records can
509		be used for repositioning the tape (global)
510	     MT_ST_NO_BLKLIMS
511		the driver does not ask the block limits
512		from the drive (block size can be changed only to
513		variable) (global)
514	     MT_ST_CAN_PARTITIONS
515		enables support for partitioned
516		tapes (global)
517	     MT_ST_SCSI2LOGICAL
518		the logical block number is used in
519		the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
520		the device dependent address. It is recommended to set
521		this flag unless there are tapes using the device
522		dependent (from the old times) (global)
523	     MT_ST_SYSV
524		sets the SYSV semantics (mode)
525	     MT_ST_NOWAIT
526		enables immediate mode (i.e., don't wait for
527	        the command to finish) for some commands (e.g., rewind)
528	     MT_ST_NOWAIT_EOF
529		enables immediate filemark mode (i.e. when
530	        writing a filemark, don't wait for it to complete). Please
531		see the BASICS note about MTWEOFI with respect to the
532		possible dangers of writing immediate filemarks.
533	     MT_ST_SILI
534		enables setting the SILI bit in SCSI commands when
535		reading in variable block mode to enhance performance when
536		reading blocks shorter than the byte count; set this only
537		if you are sure that the drive supports SILI and the HBA
538		correctly returns transfer residuals
539	     MT_ST_DEBUGGING
540		debugging (global; debugging must be
541		compiled into the driver)
542
543	* MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS
544	   Sets or clears the option bits.
545        * MT_ST_WRITE_THRESHOLD
546           Sets the write threshold for this device to kilobytes
547           specified by the lowest bits.
548	* MT_ST_DEF_BLKSIZE
549	   Defines the default block size set automatically. Value
550	   0xffffff means that the default is not used any more.
551	* MT_ST_DEF_DENSITY, MT_ST_DEF_DRVBUFFER
552	   Used to set or clear the density (8 bits), and drive buffer
553	   state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
554	   (0xfffff) the default will not be used any more. Otherwise
555	   the lowermost bits of the value contain the new value of
556	   the parameter.
557	* MT_ST_DEF_COMPRESSION
558	   The compression default will not be used if the value of
559	   the lowermost byte is 0xff. Otherwise the lowermost bit
560	   contains the new default. If the bits 8-15 are set to a
561	   non-zero number, and this number is not 0xff, the number is
562	   used as the compression algorithm. The value
563	   MT_ST_CLEAR_DEFAULT can be used to clear the compression
564	   default.
565	* MT_ST_SET_TIMEOUT
566	   Set the normal timeout in seconds for this device. The
567	   default is 900 seconds (15 minutes). The timeout should be
568	   long enough for the retries done by the device while
569	   reading/writing.
570	* MT_ST_SET_LONG_TIMEOUT
571	   Set the long timeout that is used for operations that are
572	   known to take a long time. The default is 14000 seconds
573	   (3.9 hours). For erase this value is further multiplied by
574	   eight.
575	* MT_ST_SET_CLN
576	   Set the cleaning request interpretation parameters using
577	   the lowest 24 bits of the argument. The driver can set the
578	   generic status bit GMT_CLN if a cleaning request bit pattern
579	   is found from the extended sense data. Many drives set one or
580	   more bits in the extended sense data when the drive needs
581	   cleaning. The bits are device-dependent. The driver is
582	   given the number of the sense data byte (the lowest eight
583	   bits of the argument; must be >= 18 (values 1 - 17
584	   reserved) and <= the maximum requested sense data sixe),
585	   a mask to select the relevant bits (the bits 9-16), and the
586	   bit pattern (bits 17-23). If the bit pattern is zero, one
587	   or more bits under the mask indicate cleaning request. If
588	   the pattern is non-zero, the pattern must match the masked
589	   sense data byte.
590
591	   (The cleaning bit is set if the additional sense code and
592	   qualifier 00h 17h are seen regardless of the setting of
593	   MT_ST_SET_CLN.)
594
595The following ioctl uses the structure mtpos:
596
597MTIOCPOS
598	Reads the current position from the drive. Uses
599        Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
600        command for the SCSI-2 drives.
601
602The following ioctl uses the structure mtget to return the status:
603
604MTIOCGET
605	Returns some status information.
606        The file number and block number within file are returned. The
607        block is -1 when it can't be determined (e.g., after MTBSF).
608        The drive type is either MTISSCSI1 or MTISSCSI2.
609        The number of recovered errors since the previous status call
610        is stored in the lower word of the field mt_erreg.
611        The current block size and the density code are stored in the field
612        mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
613        MT_ST_DENSITY_SHIFT).
614	The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
615	is set if there is no tape in the drive. GMT_EOD means either
616	end of recorded data or end of tape. GMT_EOT means end of tape.
617
618
619Miscellaneous Compile Options
620=============================
621
622The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
623is defined.
624
625The maximum number of tape devices is determined by the define
626ST_MAX_TAPES. If more tapes are detected at driver initialization, the
627maximum is adjusted accordingly.
628
629Immediate return from tape positioning SCSI commands can be enabled by
630defining ST_NOWAIT. If this is defined, the user should take care that
631the next tape operation is not started before the previous one has
632finished. The drives and SCSI adapters should handle this condition
633gracefully, but some drive/adapter combinations are known to hang the
634SCSI bus in this case.
635
636The MTEOM command is by default implemented as spacing over 32767
637filemarks. With this method the file number in the status is
638correct. The user can request using direct spacing to EOD by setting
639ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
640number will be invalid.
641
642When using read ahead or buffered writes the position within the file
643may not be correct after the file is closed (correct position may
644require backspacing over more than one record). The correct position
645within file can be obtained if ST_IN_FILE_POS is defined at compile
646time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
647(The driver always backs over a filemark crossed by read ahead if the
648user does not request data that far.)
649
650
651Debugging Hints
652===============
653
654Debugging code is now compiled in by default but debugging is turned off
655with the kernel module parameter debug_flag defaulting to 0.  Debugging
656can still be switched on and off with an ioctl.  To enable debug at
657module load time add debug_flag=1 to the module load options, the
658debugging output is not voluminous. Debugging can also be enabled
659and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
660file /sys/bus/scsi/drivers/st/debug_flag.
661
662If the tape seems to hang, I would be very interested to hear where
663the driver is waiting. With the command 'ps -l' you can see the state
664of the process using the tape. If the state is D, the process is
665waiting for something. The field WCHAN tells where the driver is
666waiting. If you have the current System.map in the correct place (in
667/boot for the procps I use) or have updated /etc/psdatabase (for kmem
668ps), ps writes the function name in the WCHAN field. If not, you have
669to look up the function from System.map.
670
671Note also that the timeouts are very long compared to most other
672drivers. This means that the Linux driver may appear hung although the
673real reason is that the tape firmware has got confused.
674