Root/Documentation/scsi/st.txt

1This file contains brief information about the SCSI tape driver.
2The driver is currently maintained by Kai Mäkisara (email
3Kai.Makisara@kolumbus.fi)
4
5Last modified: Sun Feb 24 21:59:07 2008 by kai.makisara
6
7
8BASICS
9
10The driver is generic, i.e., it does not contain any code tailored
11to any specific tape drive. The tape parameters can be specified with
12one of the following three methods:
13
141. Each user can specify the tape parameters he/she wants to use
15directly with ioctls. This is administratively a very simple and
16flexible method and applicable to single-user workstations. However,
17in a multiuser environment the next user finds the tape parameters in
18state the previous user left them.
19
202. The system manager (root) can define default values for some tape
21parameters, like block size and density using the MTSETDRVBUFFER ioctl.
22These parameters can be programmed to come into effect either when a
23new tape is loaded into the drive or if writing begins at the
24beginning of the tape. The second method is applicable if the tape
25drive performs auto-detection of the tape format well (like some
26QIC-drives). The result is that any tape can be read, writing can be
27continued using existing format, and the default format is used if
28the tape is rewritten from the beginning (or a new tape is written
29for the first time). The first method is applicable if the drive
30does 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
32used only in variable block mode (I don't know if this is sensible
33or not :-).
34
35The user can override the parameters defined by the system
36manager. The changes persist until the defaults again come into
37effect.
38
393. By default, up to four modes can be defined and selected using the minor
40number (bits 5 and 6). The number of modes can be changed by changing
41ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
42above. Additional modes are dormant until they are defined by the
43system manager (root). When specification of a new mode is started,
44the configuration of mode 0 is used to provide a starting point for
45definition of the new mode.
46
47Using the modes allows the system manager to give the users choices
48over some of the buffering parameters not directly accessible to the
49users (buffered and asynchronous writes). The modes also allow choices
50between formats in multi-tape operations (the explicitly overridden
51parameters are reset when a new tape is loaded).
52
53If more than one mode is used, all modes should contain definitions
54for the same set of parameters.
55
56Many Unices contain internal tables that associate different modes to
57supported devices. The Linux SCSI tape driver does not contain such
58tables (and will not do that in future). Instead of that, a utility
59program can be made that fetches the inquiry data sent by the device,
60scans its database, and sets up the modes using the ioctls. Another
61alternative is to make a small script that uses mt to set the defaults
62tailored to the system.
63
64The driver supports fixed and variable block size (within buffer
65limits). Both the auto-rewind (minor equals device number) and
66non-rewind devices (minor is 128 + device number) are implemented.
67
68In variable block mode, the byte count in write() determines the size
69of the physical block on tape. When reading, the drive reads the next
70tape block and returns to the user the data if the read() byte count
71is at least the block size. Otherwise, error ENOMEM is returned.
72
73In fixed block mode, the data transfer between the drive and the
74driver is in multiples of the block size. The write() byte count must
75be a multiple of the block size. This is not required when reading but
76may be advisable for portability.
77
78Support is provided for changing the tape partition and partitioning
79of the tape with one or two partitions. By default support for
80partitioned tape is disabled for each driver and it can be enabled
81with the ioctl MTSETDRVBUFFER.
82
83By default the driver writes one filemark when the device is closed after
84writing and the last operation has been a write. Two filemarks can be
85optionally written. In both cases end of data is signified by
86returning zero bytes for two consecutive reads.
87
88If rewind, offline, bsf, or seek is done and previous tape operation was
89write, a filemark is written before moving tape.
90
91The compile options are defined in the file linux/drivers/scsi/st_options.h.
92
934. If the open option O_NONBLOCK is used, open succeeds even if the
94drive is not ready. If O_NONBLOCK is not used, the driver waits for
95the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
96seconds, open fails with the errno value EIO. With O_NONBLOCK the
97device can be opened for writing even if there is a write protected
98tape in the drive (commands trying to write something return error if
99attempted).
100
101
102MINOR NUMBERS
103
104The tape driver currently supports 128 drives by default. This number
105can be increased by editing st.h and recompiling the driver if
106necessary. The upper limit is 2^17 drives if 4 modes for each drive
107are used.
108
109The minor numbers consist of the following bit fields:
110
111dev_upper non-rew mode dev-lower
112  20 - 8 7 6 5 4 0
113The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
114byte). The bits defining the mode are below the non-rewind bit. The
115remaining bits define the tape device number. This numbering is
116backward compatible with the numbering used when the minor number was
117only 8 bits wide.
118
119
120SYSFS SUPPORT
121
122The driver creates the directory /sys/class/scsi_tape and populates it with
123directories corresponding to the existing tape devices. There are autorewind
124and non-rewind entries for each mode. The names are stxy and nstxy, where x
125is the tape number and y a character corresponding to the mode (none, l, m,
126a). For example, the directories for the first tape device are (assuming four
127modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a.
128
129Each directory contains the entries: default_blksize default_compression
130default_density defined dev device driver. The file 'defined' contains 1
131if the mode is defined and zero if not defined. The files 'default_*' contain
132the defaults set by the user. The value -1 means the default is not set. The
133file 'dev' contains the device numbers corresponding to this device. The links
134'device' and 'driver' point to the SCSI device and driver entries.
135
136Each directory also contains the entry 'options' which shows the currently
137enabled driver and mode options. The value in the file is a bit mask where the
138bit definitions are the same as those used with MTSETDRVBUFFER in setting the
139options.
140
141A link named 'tape' is made from the SCSI device directory to the class
142directory corresponding to the mode 0 auto-rewind device (e.g., st0).
143
144
145BSD AND SYS V SEMANTICS
146
147The user can choose between these two behaviours of the tape driver by
148defining the value of the symbol ST_SYSV. The semantics differ when a
149file being read is closed. The BSD semantics leaves the tape where it
150currently is whereas the SYS V semantics moves the tape past the next
151filemark unless the filemark has just been crossed.
152
153The default is BSD semantics.
154
155
156BUFFERING
157
158The driver tries to do transfers directly to/from user space. If this
159is not possible, a driver buffer allocated at run-time is used. If
160direct i/o is not possible for the whole transfer, the driver buffer
161is used (i.e., bounce buffers for individual pages are not
162used). Direct i/o can be impossible because of several reasons, e.g.:
163- one or more pages are at addresses not reachable by the HBA
164- the number of pages in the transfer exceeds the number of
165  scatter/gather segments permitted by the HBA
166- one or more pages can't be locked into memory (should not happen in
167  any reasonable situation)
168
169The size of the driver buffers is always at least one tape block. In fixed
170block mode, the minimum buffer size is defined (in 1024 byte units) by
171ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
172several blocks and using one SCSI read or write to transfer all of the
173blocks. Buffering of data across write calls in fixed block mode is
174allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
175Buffer allocation uses chunks of memory having sizes 2^n * (page
176size). Because of this the actual buffer size may be larger than the
177minimum allowable buffer size.
178
179NOTE that if direct i/o is used, the small writes are not buffered. This may
180cause a surprise when moving from 2.4. There small writes (e.g., tar without
181-b option) may have had good throughput but this is not true any more with
1822.6. Direct i/o can be turned off to solve this problem but a better solution
183is to use bigger write() byte counts (e.g., tar -b 64).
184
185Asynchronous writing. Writing the buffer contents to the tape is
186started and the write call returns immediately. The status is checked
187at the next tape operation. Asynchronous writes are not done with
188direct i/o and not in fixed block mode.
189
190Buffered writes and asynchronous writes may in some rare cases cause
191problems in multivolume operations if there is not enough space on the
192tape after the early-warning mark to flush the driver buffer.
193
194Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
195attempted even if the user does not want to get all of the data at
196this read command. Should be disabled for those drives that don't like
197a filemark to truncate a read request or that don't like backspacing.
198
199Scatter/gather buffers (buffers that consist of chunks non-contiguous
200in the physical memory) are used if contiguous buffers can't be
201allocated. To support all SCSI adapters (including those not
202supporting scatter/gather), buffer allocation is using the following
203three kinds of chunks:
2041. The initial segment that is used for all SCSI adapters including
205those not supporting scatter/gather. The size of this buffer will be
206(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
207this size (and it is not larger than the buffer size specified by
208ST_BUFFER_BLOCKS). If this size is not available, the driver halves
209the size and tries again until the size of one page. The default
210settings in st_options.h make the driver to try to allocate all of the
211buffer as one chunk.
2122. The scatter/gather segments to fill the specified buffer size are
213allocated so that as many segments as possible are used but the number
214of segments does not exceed ST_FIRST_SG.
2153. The remaining segments between ST_MAX_SG (or the module parameter
216max_sg_segs) and the number of segments used in phases 1 and 2
217are used to extend the buffer at run-time if this is necessary. The
218number of scatter/gather segments allowed for the SCSI adapter is not
219exceeded if it is smaller than the maximum number of scatter/gather
220segments specified. If the maximum number allowed for the SCSI adapter
221is smaller than the number of segments used in phases 1 and 2,
222extending the buffer will always fail.
223
224
225EOM BEHAVIOUR WHEN WRITING
226
227When the end of medium early warning is encountered, the current write
228is finished and the number of bytes is returned. The next write
229returns -1 and errno is set to ENOSPC. To enable writing a trailer,
230the next write is allowed to proceed and, if successful, the number of
231bytes is returned. After this, -1 and the number of bytes are
232alternately returned until the physical end of medium (or some other
233error) is encountered.
234
235
236MODULE PARAMETERS
237
238The buffer size, write threshold, and the maximum number of allocated buffers
239are configurable when the driver is loaded as a module. The keywords are:
240
241buffer_kbs=xxx the buffer size for fixed block mode is set
242               to xxx kilobytes
243write_threshold_kbs=xxx the write threshold in kilobytes set to xxx
244max_sg_segs=xxx the maximum number of scatter/gather
245               segments
246try_direct_io=x try direct transfer between user buffer and
247               tape drive if this is non-zero
248
249Note that if the buffer size is changed but the write threshold is not
250set, the write threshold is set to the new buffer size - 2 kB.
251
252
253BOOT TIME CONFIGURATION
254
255If the driver is compiled into the kernel, the same parameters can be
256also set using, e.g., the LILO command line. The preferred syntax is
257to use the same keyword used when loading as module but prepended
258with 'st.'. For instance, to set the maximum number of scatter/gather
259segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
260number of scatter/gather segments).
261
262For compatibility, the old syntax from early 2.5 and 2.4 kernel
263versions is supported. The same keywords can be used as when loading
264the driver as module. If several parameters are set, the keyword-value
265pairs are separated with a comma (no spaces allowed). A colon can be
266used instead of the equal mark. The definition is prepended by the
267string st=. Here is an example:
268
269    st=buffer_kbs:64,write_threshold_kbs:60
270
271The following syntax used by the old kernel versions is also supported:
272
273           st=aa[,bb[,dd]]
274
275where
276  aa is the buffer size for fixed block mode in 1024 byte units
277  bb is the write threshold in 1024 byte units
278  dd is the maximum number of scatter/gather segments
279
280
281IOCTLS
282
283The tape is positioned and the drive parameters are set with ioctls
284defined in mtio.h The tape control program 'mt' uses these ioctls. Try
285to find an mt that supports all of the Linux SCSI tape ioctls and
286opens the device for writing if the tape contents will be modified
287(look for a package mt-st* from the Linux ftp sites; the GNU mt does
288not open for writing for, e.g., erase).
289
290The supported ioctls are:
291
292The following use the structure mtop:
293
294MTFSF Space forward over count filemarks. Tape positioned after filemark.
295MTFSFM As above but tape positioned before filemark.
296MTBSF Space backward over count filemarks. Tape positioned before
297        filemark.
298MTBSFM As above but ape positioned after filemark.
299MTFSR Space forward over count records.
300MTBSR Space backward over count records.
301MTFSS Space forward over count setmarks.
302MTBSS Space backward over count setmarks.
303MTWEOF Write count filemarks.
304MTWSM Write count setmarks.
305MTREW Rewind tape.
306MTOFFL Set device off line (often rewind plus eject).
307MTNOP Do nothing except flush the buffers.
308MTRETEN Re-tension tape.
309MTEOM Space to end of recorded data.
310MTERASE Erase tape. If the argument is zero, the short erase command
311    is used. The long erase command is used with all other values
312    of the argument.
313MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA)
314        for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
315    block numbers in the status are not valid after a seek.
316MTSETBLK Set the drive block size. Setting to zero sets the drive into
317        variable block mode (if applicable).
318MTSETDENSITY Sets the drive density code to arg. See drive
319        documentation for available codes.
320MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door.
321MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the
322    command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
323    MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
324    drive with the command and it selects the tape slot to use of
325    HP C1553A changer.
326MTCOMPRESSION Sets compressing or uncompressing drive mode using the
327    SCSI mode page 15. Note that some drives other methods for
328    control of compression. Some drives (like the Exabytes) use
329    density codes for compression control. Some drives use another
330    mode page but this page has not been implemented in the
331    driver. Some drives without compression capability will accept
332    any compression mode without error.
333MTSETPART Moves the tape to the partition given by the argument at the
334    next tape operation. The block at which the tape is positioned
335    is the block where the tape was previously positioned in the
336    new active partition unless the next tape operation is
337    MTSEEK. In this case the tape is moved directly to the block
338    specified by MTSEEK. MTSETPART is inactive unless
339    MT_ST_CAN_PARTITIONS set.
340MTMKPART Formats the tape with one partition (argument zero) or two
341    partitions (the argument gives in megabytes the size of
342    partition 1 that is physically the first partition of the
343    tape). The drive has to support partitions with size specified
344    by the initiator. Inactive unless MT_ST_CAN_PARTITIONS set.
345MTSETDRVBUFFER
346    Is used for several purposes. The command is obtained from count
347        with mask MT_SET_OPTIONS, the low order bits are used as argument.
348    This command is only allowed for the superuser (root). The
349    subcommands are:
350    0
351           The drive buffer option is set to the argument. Zero means
352           no buffering.
353        MT_ST_BOOLEANS
354           Sets the buffering options. The bits are the new states
355           (enabled/disabled) the following options (in the
356       parenthesis is specified whether the option is global or
357       can be specified differently for each mode):
358         MT_ST_BUFFER_WRITES write buffering (mode)
359         MT_ST_ASYNC_WRITES asynchronous writes (mode)
360             MT_ST_READ_AHEAD read ahead (mode)
361             MT_ST_TWO_FM writing of two filemarks (global)
362         MT_ST_FAST_EOM using the SCSI spacing to EOD (global)
363         MT_ST_AUTO_LOCK automatic locking of the drive door (global)
364             MT_ST_DEF_WRITES the defaults are meant only for writes (mode)
365         MT_ST_CAN_BSR backspacing over more than one records can
366        be used for repositioning the tape (global)
367         MT_ST_NO_BLKLIMS the driver does not ask the block limits
368        from the drive (block size can be changed only to
369        variable) (global)
370         MT_ST_CAN_PARTITIONS enables support for partitioned
371        tapes (global)
372         MT_ST_SCSI2LOGICAL the logical block number is used in
373        the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
374        the device dependent address. It is recommended to set
375        this flag unless there are tapes using the device
376        dependent (from the old times) (global)
377         MT_ST_SYSV sets the SYSV semantics (mode)
378         MT_ST_NOWAIT enables immediate mode (i.e., don't wait for
379            the command to finish) for some commands (e.g., rewind)
380         MT_ST_SILI enables setting the SILI bit in SCSI commands when
381        reading in variable block mode to enhance performance when
382        reading blocks shorter than the byte count; set this only
383        if you are sure that the drive supports SILI and the HBA
384        correctly returns transfer residuals
385         MT_ST_DEBUGGING debugging (global; debugging must be
386        compiled into the driver)
387    MT_ST_SETBOOLEANS
388    MT_ST_CLEARBOOLEANS
389       Sets or clears the option bits.
390        MT_ST_WRITE_THRESHOLD
391           Sets the write threshold for this device to kilobytes
392           specified by the lowest bits.
393    MT_ST_DEF_BLKSIZE
394       Defines the default block size set automatically. Value
395       0xffffff means that the default is not used any more.
396    MT_ST_DEF_DENSITY
397    MT_ST_DEF_DRVBUFFER
398       Used to set or clear the density (8 bits), and drive buffer
399       state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
400       (0xfffff) the default will not be used any more. Otherwise
401       the lowermost bits of the value contain the new value of
402       the parameter.
403    MT_ST_DEF_COMPRESSION
404       The compression default will not be used if the value of
405       the lowermost byte is 0xff. Otherwise the lowermost bit
406       contains the new default. If the bits 8-15 are set to a
407       non-zero number, and this number is not 0xff, the number is
408       used as the compression algorithm. The value
409       MT_ST_CLEAR_DEFAULT can be used to clear the compression
410       default.
411    MT_ST_SET_TIMEOUT
412       Set the normal timeout in seconds for this device. The
413       default is 900 seconds (15 minutes). The timeout should be
414       long enough for the retries done by the device while
415       reading/writing.
416    MT_ST_SET_LONG_TIMEOUT
417       Set the long timeout that is used for operations that are
418       known to take a long time. The default is 14000 seconds
419       (3.9 hours). For erase this value is further multiplied by
420       eight.
421    MT_ST_SET_CLN
422       Set the cleaning request interpretation parameters using
423       the lowest 24 bits of the argument. The driver can set the
424       generic status bit GMT_CLN if a cleaning request bit pattern
425       is found from the extended sense data. Many drives set one or
426       more bits in the extended sense data when the drive needs
427       cleaning. The bits are device-dependent. The driver is
428       given the number of the sense data byte (the lowest eight
429       bits of the argument; must be >= 18 (values 1 - 17
430       reserved) and <= the maximum requested sense data sixe),
431       a mask to select the relevant bits (the bits 9-16), and the
432       bit pattern (bits 17-23). If the bit pattern is zero, one
433       or more bits under the mask indicate cleaning request. If
434       the pattern is non-zero, the pattern must match the masked
435       sense data byte.
436
437       (The cleaning bit is set if the additional sense code and
438       qualifier 00h 17h are seen regardless of the setting of
439       MT_ST_SET_CLN.)
440
441The following ioctl uses the structure mtpos:
442MTIOCPOS Reads the current position from the drive. Uses
443        Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
444        command for the SCSI-2 drives.
445
446The following ioctl uses the structure mtget to return the status:
447MTIOCGET Returns some status information.
448        The file number and block number within file are returned. The
449        block is -1 when it can't be determined (e.g., after MTBSF).
450        The drive type is either MTISSCSI1 or MTISSCSI2.
451        The number of recovered errors since the previous status call
452        is stored in the lower word of the field mt_erreg.
453        The current block size and the density code are stored in the field
454        mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
455        MT_ST_DENSITY_SHIFT).
456    The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
457    is set if there is no tape in the drive. GMT_EOD means either
458    end of recorded data or end of tape. GMT_EOT means end of tape.
459
460
461MISCELLANEOUS COMPILE OPTIONS
462
463The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
464is defined.
465
466The maximum number of tape devices is determined by the define
467ST_MAX_TAPES. If more tapes are detected at driver initialization, the
468maximum is adjusted accordingly.
469
470Immediate return from tape positioning SCSI commands can be enabled by
471defining ST_NOWAIT. If this is defined, the user should take care that
472the next tape operation is not started before the previous one has
473finished. The drives and SCSI adapters should handle this condition
474gracefully, but some drive/adapter combinations are known to hang the
475SCSI bus in this case.
476
477The MTEOM command is by default implemented as spacing over 32767
478filemarks. With this method the file number in the status is
479correct. The user can request using direct spacing to EOD by setting
480ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
481number will be invalid.
482
483When using read ahead or buffered writes the position within the file
484may not be correct after the file is closed (correct position may
485require backspacing over more than one record). The correct position
486within file can be obtained if ST_IN_FILE_POS is defined at compile
487time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
488(The driver always backs over a filemark crossed by read ahead if the
489user does not request data that far.)
490
491
492DEBUGGING HINTS
493
494To enable debugging messages, edit st.c and #define DEBUG 1. As seen
495above, debugging can be switched off with an ioctl if debugging is
496compiled into the driver. The debugging output is not voluminous.
497
498If the tape seems to hang, I would be very interested to hear where
499the driver is waiting. With the command 'ps -l' you can see the state
500of the process using the tape. If the state is D, the process is
501waiting for something. The field WCHAN tells where the driver is
502waiting. If you have the current System.map in the correct place (in
503/boot for the procps I use) or have updated /etc/psdatabase (for kmem
504ps), ps writes the function name in the WCHAN field. If not, you have
505to look up the function from System.map.
506
507Note also that the timeouts are very long compared to most other
508drivers. This means that the Linux driver may appear hung although the
509real reason is that the tape firmware has got confused.
510

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