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Based on kernel version 3.4. Page generated on 2012-05-21 22:10 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)
4	
5	Last modified: Sun Aug 29 18:25:47 2010 by kai.makisara
6	
7	
8	BASICS
9	
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:
13	
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.
19	
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 :-).
34	
35	The user can override the parameters defined by the system
36	manager. The changes persist until the defaults again come into
37	effect.
38	
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.
46	
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).
52	
53	If more than one mode is used, all modes should contain definitions
54	for the same set of parameters.
55	
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.
63	
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.
67	
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.
72	
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.
77	
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.
82	
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.
87	
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.
98	
99	If rewind, offline, bsf, or seek is done and previous tape operation was
100	write, a filemark is written before moving tape.
101	
102	The compile options are defined in the file linux/drivers/scsi/st_options.h.
103	
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).
111	
112	
113	MINOR NUMBERS
114	
115	The tape driver currently supports 128 drives by default. This number
116	can be increased by editing st.h and recompiling the driver if
117	necessary. The upper limit is 2^17 drives if 4 modes for each drive
118	are used.
119	
120	The minor numbers consist of the following bit fields:
121	
122	dev_upper non-rew mode dev-lower
123	  20 -  8     7    6 5  4      0
124	The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
125	byte). The bits defining the mode are below the non-rewind bit. The
126	remaining bits define the tape device number. This numbering is
127	backward compatible with the numbering used when the minor number was
128	only 8 bits wide.
129	
130	
131	SYSFS SUPPORT
132	
133	The driver creates the directory /sys/class/scsi_tape and populates it with
134	directories corresponding to the existing tape devices. There are autorewind
135	and non-rewind entries for each mode. The names are stxy and nstxy, where x
136	is the tape number and y a character corresponding to the mode (none, l, m,
137	a). For example, the directories for the first tape device are (assuming four
138	modes): st0  nst0  st0l  nst0l  st0m  nst0m  st0a  nst0a.
139	
140	Each directory contains the entries: default_blksize  default_compression
141	default_density  defined  dev  device  driver. The file 'defined' contains 1
142	if the mode is defined and zero if not defined. The files 'default_*' contain
143	the defaults set by the user. The value -1 means the default is not set. The
144	file 'dev' contains the device numbers corresponding to this device. The links
145	'device' and 'driver' point to the SCSI device and driver entries.
146	
147	Each directory also contains the entry 'options' which shows the currently
148	enabled driver and mode options. The value in the file is a bit mask where the
149	bit definitions are the same as those used with MTSETDRVBUFFER in setting the
150	options.
151	
152	A link named 'tape' is made from the SCSI device directory to the class
153	directory corresponding to the mode 0 auto-rewind device (e.g., st0). 
154	
155	
156	BSD AND SYS V SEMANTICS
157	
158	The user can choose between these two behaviours of the tape driver by
159	defining the value of the symbol ST_SYSV. The semantics differ when a
160	file being read is closed. The BSD semantics leaves the tape where it
161	currently is whereas the SYS V semantics moves the tape past the next
162	filemark unless the filemark has just been crossed.
163	
164	The default is BSD semantics.
165	
166	
167	BUFFERING
168	
169	The driver tries to do transfers directly to/from user space. If this
170	is not possible, a driver buffer allocated at run-time is used. If
171	direct i/o is not possible for the whole transfer, the driver buffer
172	is used (i.e., bounce buffers for individual pages are not
173	used). Direct i/o can be impossible because of several reasons, e.g.:
174	- one or more pages are at addresses not reachable by the HBA
175	- the number of pages in the transfer exceeds the number of
176	  scatter/gather segments permitted by the HBA
177	- one or more pages can't be locked into memory (should not happen in
178	  any reasonable situation)
179	
180	The size of the driver buffers is always at least one tape block. In fixed
181	block mode, the minimum buffer size is defined (in 1024 byte units) by
182	ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
183	several blocks and using one SCSI read or write to transfer all of the
184	blocks. Buffering of data across write calls in fixed block mode is
185	allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
186	Buffer allocation uses chunks of memory having sizes 2^n * (page
187	size). Because of this the actual buffer size may be larger than the
188	minimum allowable buffer size.
189	
190	NOTE that if direct i/o is used, the small writes are not buffered. This may
191	cause a surprise when moving from 2.4. There small writes (e.g., tar without
192	-b option) may have had good throughput but this is not true any more with
193	2.6. Direct i/o can be turned off to solve this problem but a better solution
194	is to use bigger write() byte counts (e.g., tar -b 64).
195	
196	Asynchronous writing. Writing the buffer contents to the tape is
197	started and the write call returns immediately. The status is checked
198	at the next tape operation. Asynchronous writes are not done with
199	direct i/o and not in fixed block mode.
200	
201	Buffered writes and asynchronous writes may in some rare cases cause
202	problems in multivolume operations if there is not enough space on the
203	tape after the early-warning mark to flush the driver buffer.
204	
205	Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
206	attempted even if the user does not want to get all of the data at
207	this read command. Should be disabled for those drives that don't like
208	a filemark to truncate a read request or that don't like backspacing.
209	
210	Scatter/gather buffers (buffers that consist of chunks non-contiguous
211	in the physical memory) are used if contiguous buffers can't be
212	allocated. To support all SCSI adapters (including those not
213	supporting scatter/gather), buffer allocation is using the following
214	three kinds of chunks:
215	1. The initial segment that is used for all SCSI adapters including
216	those not supporting scatter/gather. The size of this buffer will be
217	(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
218	this size (and it is not larger than the buffer size specified by
219	ST_BUFFER_BLOCKS). If this size is not available, the driver halves
220	the size and tries again until the size of one page. The default
221	settings in st_options.h make the driver to try to allocate all of the
222	buffer as one chunk.
223	2. The scatter/gather segments to fill the specified buffer size are
224	allocated so that as many segments as possible are used but the number
225	of segments does not exceed ST_FIRST_SG.
226	3. The remaining segments between ST_MAX_SG (or the module parameter
227	max_sg_segs) and the number of segments used in phases 1 and 2
228	are used to extend the buffer at run-time if this is necessary. The
229	number of scatter/gather segments allowed for the SCSI adapter is not
230	exceeded if it is smaller than the maximum number of scatter/gather
231	segments specified. If the maximum number allowed for the SCSI adapter
232	is smaller than the number of segments used in phases 1 and 2,
233	extending the buffer will always fail.
234	
235	
236	EOM BEHAVIOUR WHEN WRITING
237	
238	When the end of medium early warning is encountered, the current write
239	is finished and the number of bytes is returned. The next write
240	returns -1 and errno is set to ENOSPC. To enable writing a trailer,
241	the next write is allowed to proceed and, if successful, the number of
242	bytes is returned. After this, -1 and the number of bytes are
243	alternately returned until the physical end of medium (or some other
244	error) is encountered.
245	
246	
247	MODULE PARAMETERS
248	
249	The buffer size, write threshold, and the maximum number of allocated buffers
250	are configurable when the driver is loaded as a module. The keywords are:
251	
252	buffer_kbs=xxx             the buffer size for fixed block mode is set
253				   to xxx kilobytes
254	write_threshold_kbs=xxx    the write threshold in kilobytes set to xxx
255	max_sg_segs=xxx		   the maximum number of scatter/gather
256				   segments
257	try_direct_io=x		   try direct transfer between user buffer and
258				   tape drive if this is non-zero
259	
260	Note that if the buffer size is changed but the write threshold is not
261	set, the write threshold is set to the new buffer size - 2 kB.
262	
263	
264	BOOT TIME CONFIGURATION
265	
266	If the driver is compiled into the kernel, the same parameters can be
267	also set using, e.g., the LILO command line. The preferred syntax is
268	to use the same keyword used when loading as module but prepended
269	with 'st.'. For instance, to set the maximum number of scatter/gather
270	segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
271	number of scatter/gather segments).
272	
273	For compatibility, the old syntax from early 2.5 and 2.4 kernel
274	versions is supported. The same keywords can be used as when loading
275	the driver as module. If several parameters are set, the keyword-value
276	pairs are separated with a comma (no spaces allowed). A colon can be
277	used instead of the equal mark. The definition is prepended by the
278	string st=. Here is an example:
279	
280		st=buffer_kbs:64,write_threshold_kbs:60
281	
282	The following syntax used by the old kernel versions is also supported:
283	
284	           st=aa[,bb[,dd]]
285	
286	where
287	  aa is the buffer size for fixed block mode in 1024 byte units
288	  bb is the write threshold in 1024 byte units
289	  dd is the maximum number of scatter/gather segments
290	
291	
292	IOCTLS
293	
294	The tape is positioned and the drive parameters are set with ioctls
295	defined in mtio.h The tape control program 'mt' uses these ioctls. Try
296	to find an mt that supports all of the Linux SCSI tape ioctls and
297	opens the device for writing if the tape contents will be modified
298	(look for a package mt-st* from the Linux ftp sites; the GNU mt does
299	not open for writing for, e.g., erase).
300	
301	The supported ioctls are:
302	
303	The following use the structure mtop:
304	
305	MTFSF   Space forward over count filemarks. Tape positioned after filemark.
306	MTFSFM  As above but tape positioned before filemark.
307	MTBSF	Space backward over count filemarks. Tape positioned before
308	        filemark.
309	MTBSFM  As above but ape positioned after filemark.
310	MTFSR   Space forward over count records.
311	MTBSR   Space backward over count records.
312	MTFSS   Space forward over count setmarks.
313	MTBSS   Space backward over count setmarks.
314	MTWEOF  Write count filemarks.
315	MTWEOFI	Write count filemarks with immediate bit set (i.e., does not
316		wait until data is on tape)
317	MTWSM   Write count setmarks.
318	MTREW   Rewind tape.
319	MTOFFL  Set device off line (often rewind plus eject).
320	MTNOP   Do nothing except flush the buffers.
321	MTRETEN Re-tension tape.
322	MTEOM   Space to end of recorded data.
323	MTERASE Erase tape. If the argument is zero, the short erase command
324		is used. The long erase command is used with all other values
325		of the argument.
326	MTSEEK	Seek to tape block count. Uses Tandberg-compatible seek (QFA)
327	        for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
328		block numbers in the status are not valid after a seek.
329	MTSETBLK Set the drive block size. Setting to zero sets the drive into
330	        variable block mode (if applicable).
331	MTSETDENSITY Sets the drive density code to arg. See drive
332	        documentation for available codes.
333	MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door.
334	MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the
335		command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
336		MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
337		drive with the command and it selects the tape slot to use of
338		HP C1553A changer.
339	MTCOMPRESSION Sets compressing or uncompressing drive mode using the
340		SCSI mode page 15. Note that some drives other methods for
341		control of compression. Some drives (like the Exabytes) use
342		density codes for compression control. Some drives use another
343		mode page but this page has not been implemented in the
344		driver. Some drives without compression capability will accept
345		any compression mode without error.
346	MTSETPART Moves the tape to the partition given by the argument at the
347		next tape operation. The block at which the tape is positioned
348		is the block where the tape was previously positioned in the
349		new active partition unless the next tape operation is
350		MTSEEK. In this case the tape is moved directly to the block
351		specified by MTSEEK. MTSETPART is inactive unless
352		MT_ST_CAN_PARTITIONS set.
353	MTMKPART Formats the tape with one partition (argument zero) or two
354		partitions (the argument gives in megabytes the size of
355		partition 1 that is physically the first partition of the
356		tape). The drive has to support partitions with size specified
357		by the initiator. Inactive unless MT_ST_CAN_PARTITIONS set.
358	MTSETDRVBUFFER
359		Is used for several purposes. The command is obtained from count
360	        with mask MT_SET_OPTIONS, the low order bits are used as argument.
361		This command is only allowed for the superuser (root). The
362		subcommands are:
363		0
364	           The drive buffer option is set to the argument. Zero means
365	           no buffering.
366	        MT_ST_BOOLEANS
367	           Sets the buffering options. The bits are the new states
368	           (enabled/disabled) the following options (in the
369		   parenthesis is specified whether the option is global or
370		   can be specified differently for each mode):
371		     MT_ST_BUFFER_WRITES write buffering (mode)
372		     MT_ST_ASYNC_WRITES asynchronous writes (mode)
373	             MT_ST_READ_AHEAD  read ahead (mode)
374	             MT_ST_TWO_FM writing of two filemarks (global)
375		     MT_ST_FAST_EOM using the SCSI spacing to EOD (global)
376		     MT_ST_AUTO_LOCK automatic locking of the drive door (global)
377	             MT_ST_DEF_WRITES the defaults are meant only for writes (mode)
378		     MT_ST_CAN_BSR backspacing over more than one records can
379			be used for repositioning the tape (global)
380		     MT_ST_NO_BLKLIMS the driver does not ask the block limits
381			from the drive (block size can be changed only to
382			variable) (global)
383		     MT_ST_CAN_PARTITIONS enables support for partitioned
384			tapes (global)
385		     MT_ST_SCSI2LOGICAL the logical block number is used in
386			the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
387			the device dependent address. It is recommended to set
388			this flag unless there are tapes using the device
389			dependent (from the old times) (global)
390		     MT_ST_SYSV sets the SYSV semantics (mode)
391		     MT_ST_NOWAIT enables immediate mode (i.e., don't wait for
392		        the command to finish) for some commands (e.g., rewind)
393		     MT_ST_NOWAIT_EOF enables immediate filemark mode (i.e. when
394		        writing a filemark, don't wait for it to complete). Please
395			see the BASICS note about MTWEOFI with respect to the
396			possible dangers of writing immediate filemarks.
397		     MT_ST_SILI enables setting the SILI bit in SCSI commands when
398			reading in variable block mode to enhance performance when
399			reading blocks shorter than the byte count; set this only
400			if you are sure that the drive supports SILI and the HBA
401			correctly returns transfer residuals
402		     MT_ST_DEBUGGING debugging (global; debugging must be
403			compiled into the driver)
404		MT_ST_SETBOOLEANS
405		MT_ST_CLEARBOOLEANS
406		   Sets or clears the option bits.
407	        MT_ST_WRITE_THRESHOLD
408	           Sets the write threshold for this device to kilobytes
409	           specified by the lowest bits.
410		MT_ST_DEF_BLKSIZE
411		   Defines the default block size set automatically. Value
412		   0xffffff means that the default is not used any more.
413		MT_ST_DEF_DENSITY
414		MT_ST_DEF_DRVBUFFER
415		   Used to set or clear the density (8 bits), and drive buffer
416		   state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
417		   (0xfffff) the default will not be used any more. Otherwise
418		   the lowermost bits of the value contain the new value of
419		   the parameter.
420		MT_ST_DEF_COMPRESSION
421		   The compression default will not be used if the value of
422		   the lowermost byte is 0xff. Otherwise the lowermost bit
423		   contains the new default. If the bits 8-15 are set to a
424		   non-zero number, and this number is not 0xff, the number is
425		   used as the compression algorithm. The value
426		   MT_ST_CLEAR_DEFAULT can be used to clear the compression
427		   default.
428		MT_ST_SET_TIMEOUT
429		   Set the normal timeout in seconds for this device. The
430		   default is 900 seconds (15 minutes). The timeout should be
431		   long enough for the retries done by the device while
432		   reading/writing.
433		MT_ST_SET_LONG_TIMEOUT
434		   Set the long timeout that is used for operations that are
435		   known to take a long time. The default is 14000 seconds
436		   (3.9 hours). For erase this value is further multiplied by
437		   eight.
438		MT_ST_SET_CLN
439		   Set the cleaning request interpretation parameters using
440		   the lowest 24 bits of the argument. The driver can set the
441		   generic status bit GMT_CLN if a cleaning request bit pattern
442		   is found from the extended sense data. Many drives set one or
443		   more bits in the extended sense data when the drive needs
444		   cleaning. The bits are device-dependent. The driver is
445		   given the number of the sense data byte (the lowest eight
446		   bits of the argument; must be >= 18 (values 1 - 17
447		   reserved) and <= the maximum requested sense data sixe), 
448		   a mask to select the relevant bits (the bits 9-16), and the
449		   bit pattern (bits 17-23). If the bit pattern is zero, one
450		   or more bits under the mask indicate cleaning request. If
451		   the pattern is non-zero, the pattern must match the masked
452		   sense data byte.
453	
454		   (The cleaning bit is set if the additional sense code and
455		   qualifier 00h 17h are seen regardless of the setting of
456		   MT_ST_SET_CLN.)
457	
458	The following ioctl uses the structure mtpos:
459	MTIOCPOS Reads the current position from the drive. Uses
460	        Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
461	        command for the SCSI-2 drives.
462	
463	The following ioctl uses the structure mtget to return the status:
464	MTIOCGET Returns some status information.
465	        The file number and block number within file are returned. The
466	        block is -1 when it can't be determined (e.g., after MTBSF).
467	        The drive type is either MTISSCSI1 or MTISSCSI2.
468	        The number of recovered errors since the previous status call
469	        is stored in the lower word of the field mt_erreg.
470	        The current block size and the density code are stored in the field
471	        mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
472	        MT_ST_DENSITY_SHIFT).
473		The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
474		is set if there is no tape in the drive. GMT_EOD means either
475		end of recorded data or end of tape. GMT_EOT means end of tape.
476	
477	
478	MISCELLANEOUS COMPILE OPTIONS
479	
480	The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
481	is defined.
482	
483	The maximum number of tape devices is determined by the define
484	ST_MAX_TAPES. If more tapes are detected at driver initialization, the
485	maximum is adjusted accordingly.
486	
487	Immediate return from tape positioning SCSI commands can be enabled by
488	defining ST_NOWAIT. If this is defined, the user should take care that
489	the next tape operation is not started before the previous one has
490	finished. The drives and SCSI adapters should handle this condition
491	gracefully, but some drive/adapter combinations are known to hang the
492	SCSI bus in this case.
493	
494	The MTEOM command is by default implemented as spacing over 32767
495	filemarks. With this method the file number in the status is
496	correct. The user can request using direct spacing to EOD by setting
497	ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
498	number will be invalid.
499	
500	When using read ahead or buffered writes the position within the file
501	may not be correct after the file is closed (correct position may
502	require backspacing over more than one record). The correct position
503	within file can be obtained if ST_IN_FILE_POS is defined at compile
504	time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
505	(The driver always backs over a filemark crossed by read ahead if the
506	user does not request data that far.)
507	
508	
509	DEBUGGING HINTS
510	
511	To enable debugging messages, edit st.c and #define DEBUG 1. As seen
512	above, debugging can be switched off with an ioctl if debugging is
513	compiled into the driver. The debugging output is not voluminous.
514	
515	If the tape seems to hang, I would be very interested to hear where
516	the driver is waiting. With the command 'ps -l' you can see the state
517	of the process using the tape. If the state is D, the process is
518	waiting for something. The field WCHAN tells where the driver is
519	waiting. If you have the current System.map in the correct place (in
520	/boot for the procps I use) or have updated /etc/psdatabase (for kmem
521	ps), ps writes the function name in the WCHAN field. If not, you have
522	to look up the function from System.map.
523	
524	Note also that the timeouts are very long compared to most other
525	drivers. This means that the Linux driver may appear hung although the
526	real reason is that the tape firmware has got confused.
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