st.rst 29 KB

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639640641642643644645646647648649650651652653654655656657658659660661662663664665666667668669670671672673
  1. .. SPDX-License-Identifier: GPL-2.0
  2. ====================
  3. The SCSI Tape Driver
  4. ====================
  5. This file contains brief information about the SCSI tape driver.
  6. The driver is currently maintained by Kai Mäkisara (email
  7. [email protected])
  8. Last modified: Tue Feb 9 21:54:16 2016 by kai.makisara
  9. Basics
  10. ======
  11. The driver is generic, i.e., it does not contain any code tailored
  12. to any specific tape drive. The tape parameters can be specified with
  13. 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.
  19. 2. The system manager (root) can define default values for some tape
  20. parameters, like block size and density using the MTSETDRVBUFFER ioctl.
  21. These parameters can be programmed to come into effect either when a
  22. new tape is loaded into the drive or if writing begins at the
  23. beginning of the tape. The second method is applicable if the tape
  24. drive performs auto-detection of the tape format well (like some
  25. QIC-drives). The result is that any tape can be read, writing can be
  26. continued using existing format, and the default format is used if
  27. the tape is rewritten from the beginning (or a new tape is written
  28. for the first time). The first method is applicable if the drive
  29. does not perform auto-detection well enough and there is a single
  30. "sensible" mode for the device. An example is a DAT drive that is
  31. used only in variable block mode (I don't know if this is sensible
  32. or not :-).
  33. The user can override the parameters defined by the system
  34. manager. The changes persist until the defaults again come into
  35. effect.
  36. 3. By default, up to four modes can be defined and selected using the minor
  37. number (bits 5 and 6). The number of modes can be changed by changing
  38. ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed
  39. above. Additional modes are dormant until they are defined by the
  40. system manager (root). When specification of a new mode is started,
  41. the configuration of mode 0 is used to provide a starting point for
  42. definition of the new mode.
  43. Using the modes allows the system manager to give the users choices
  44. over some of the buffering parameters not directly accessible to the
  45. users (buffered and asynchronous writes). The modes also allow choices
  46. between formats in multi-tape operations (the explicitly overridden
  47. parameters are reset when a new tape is loaded).
  48. If more than one mode is used, all modes should contain definitions
  49. for the same set of parameters.
  50. Many Unices contain internal tables that associate different modes to
  51. supported devices. The Linux SCSI tape driver does not contain such
  52. tables (and will not do that in future). Instead of that, a utility
  53. program can be made that fetches the inquiry data sent by the device,
  54. scans its database, and sets up the modes using the ioctls. Another
  55. alternative is to make a small script that uses mt to set the defaults
  56. tailored to the system.
  57. The driver supports fixed and variable block size (within buffer
  58. limits). Both the auto-rewind (minor equals device number) and
  59. non-rewind devices (minor is 128 + device number) are implemented.
  60. In variable block mode, the byte count in write() determines the size
  61. of the physical block on tape. When reading, the drive reads the next
  62. tape block and returns to the user the data if the read() byte count
  63. is at least the block size. Otherwise, error ENOMEM is returned.
  64. In fixed block mode, the data transfer between the drive and the
  65. driver is in multiples of the block size. The write() byte count must
  66. be a multiple of the block size. This is not required when reading but
  67. may be advisable for portability.
  68. Support is provided for changing the tape partition and partitioning
  69. of the tape with one or two partitions. By default support for
  70. partitioned tape is disabled for each driver and it can be enabled
  71. with the ioctl MTSETDRVBUFFER.
  72. By default the driver writes one filemark when the device is closed after
  73. writing and the last operation has been a write. Two filemarks can be
  74. optionally written. In both cases end of data is signified by
  75. returning zero bytes for two consecutive reads.
  76. Writing filemarks without the immediate bit set in the SCSI command block acts
  77. as a synchronization point, i.e., all remaining data form the drive buffers is
  78. written to tape before the command returns. This makes sure that write errors
  79. are caught at that point, but this takes time. In some applications, several
  80. consecutive files must be written fast. The MTWEOFI operation can be used to
  81. write the filemarks without flushing the drive buffer. Writing filemark at
  82. close() is always flushing the drive buffers. However, if the previous
  83. operation is MTWEOFI, close() does not write a filemark. This can be used if
  84. the program wants to close/open the tape device between files and wants to
  85. skip waiting.
  86. If rewind, offline, bsf, or seek is done and previous tape operation was
  87. write, a filemark is written before moving tape.
  88. The compile options are defined in the file linux/drivers/scsi/st_options.h.
  89. 4. If the open option O_NONBLOCK is used, open succeeds even if the
  90. drive is not ready. If O_NONBLOCK is not used, the driver waits for
  91. the drive to become ready. If this does not happen in ST_BLOCK_SECONDS
  92. seconds, open fails with the errno value EIO. With O_NONBLOCK the
  93. device can be opened for writing even if there is a write protected
  94. tape in the drive (commands trying to write something return error if
  95. attempted).
  96. Minor Numbers
  97. =============
  98. The tape driver currently supports up to 2^17 drives if 4 modes for
  99. each drive are used.
  100. The minor numbers consist of the following bit fields::
  101. dev_upper non-rew mode dev-lower
  102. 20 - 8 7 6 5 4 0
  103. The non-rewind bit is always bit 7 (the uppermost bit in the lowermost
  104. byte). The bits defining the mode are below the non-rewind bit. The
  105. remaining bits define the tape device number. This numbering is
  106. backward compatible with the numbering used when the minor number was
  107. only 8 bits wide.
  108. Sysfs Support
  109. =============
  110. The driver creates the directory /sys/class/scsi_tape and populates it with
  111. directories corresponding to the existing tape devices. There are autorewind
  112. and non-rewind entries for each mode. The names are stxy and nstxy, where x
  113. is the tape number and y a character corresponding to the mode (none, l, m,
  114. a). For example, the directories for the first tape device are (assuming four
  115. modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a.
  116. Each directory contains the entries: default_blksize default_compression
  117. default_density defined dev device driver. The file 'defined' contains 1
  118. if the mode is defined and zero if not defined. The files 'default_*' contain
  119. the defaults set by the user. The value -1 means the default is not set. The
  120. file 'dev' contains the device numbers corresponding to this device. The links
  121. 'device' and 'driver' point to the SCSI device and driver entries.
  122. Each directory also contains the entry 'options' which shows the currently
  123. enabled driver and mode options. The value in the file is a bit mask where the
  124. bit definitions are the same as those used with MTSETDRVBUFFER in setting the
  125. options.
  126. A link named 'tape' is made from the SCSI device directory to the class
  127. directory corresponding to the mode 0 auto-rewind device (e.g., st0).
  128. Sysfs and Statistics for Tape Devices
  129. =====================================
  130. The st driver maintains statistics for tape drives inside the sysfs filesystem.
  131. The following method can be used to locate the statistics that are
  132. available (assuming that sysfs is mounted at /sys):
  133. 1. Use opendir(3) on the directory /sys/class/scsi_tape
  134. 2. Use readdir(3) to read the directory contents
  135. 3. Use regcomp(3)/regexec(3) to match directory entries to the extended
  136. regular expression "^st[0-9]+$"
  137. 4. Access the statistics from the /sys/class/scsi_tape/<match>/stats
  138. directory (where <match> is a directory entry from /sys/class/scsi_tape
  139. that matched the extended regular expression)
  140. The reason for using this approach is that all the character devices
  141. pointing to the same tape drive use the same statistics. That means
  142. that st0 would have the same statistics as nst0.
  143. The directory contains the following statistics files:
  144. 1. in_flight
  145. - The number of I/Os currently outstanding to this device.
  146. 2. io_ns
  147. - The amount of time spent waiting (in nanoseconds) for all I/O
  148. to complete (including read and write). This includes tape movement
  149. commands such as seeking between file or set marks and implicit tape
  150. movement such as when rewind on close tape devices are used.
  151. 3. other_cnt
  152. - The number of I/Os issued to the tape drive other than read or
  153. write commands. The time taken to complete these commands uses the
  154. following calculation io_ms-read_ms-write_ms.
  155. 4. read_byte_cnt
  156. - The number of bytes read from the tape drive.
  157. 5. read_cnt
  158. - The number of read requests issued to the tape drive.
  159. 6. read_ns
  160. - The amount of time (in nanoseconds) spent waiting for read
  161. requests to complete.
  162. 7. write_byte_cnt
  163. - The number of bytes written to the tape drive.
  164. 8. write_cnt
  165. - The number of write requests issued to the tape drive.
  166. 9. write_ns
  167. - The amount of time (in nanoseconds) spent waiting for write
  168. requests to complete.
  169. 10. resid_cnt
  170. - The number of times during a read or write we found
  171. the residual amount to be non-zero. This should mean that a program
  172. is issuing a read larger thean the block size on tape. For write
  173. not all data made it to tape.
  174. .. Note::
  175. The in_flight value is incremented when an I/O starts the I/O
  176. itself is not added to the statistics until it completes.
  177. The total of read_cnt, write_cnt, and other_cnt may not total to the same
  178. value as iodone_cnt at the device level. The tape statistics only count
  179. I/O issued via the st module.
  180. When read the statistics may not be temporally consistent while I/O is in
  181. progress. The individual values are read and written to atomically however
  182. when reading them back via sysfs they may be in the process of being
  183. updated when starting an I/O or when it is completed.
  184. The value shown in in_flight is incremented before any statstics are
  185. updated and decremented when an I/O completes after updating statistics.
  186. The value of in_flight is 0 when there are no I/Os outstanding that are
  187. issued by the st driver. Tape statistics do not take into account any
  188. I/O performed via the sg device.
  189. BSD and Sys V Semantics
  190. =======================
  191. The user can choose between these two behaviours of the tape driver by
  192. defining the value of the symbol ST_SYSV. The semantics differ when a
  193. file being read is closed. The BSD semantics leaves the tape where it
  194. currently is whereas the SYS V semantics moves the tape past the next
  195. filemark unless the filemark has just been crossed.
  196. The default is BSD semantics.
  197. Buffering
  198. =========
  199. The driver tries to do transfers directly to/from user space. If this
  200. is not possible, a driver buffer allocated at run-time is used. If
  201. direct i/o is not possible for the whole transfer, the driver buffer
  202. is used (i.e., bounce buffers for individual pages are not
  203. used). Direct i/o can be impossible because of several reasons, e.g.:
  204. - one or more pages are at addresses not reachable by the HBA
  205. - the number of pages in the transfer exceeds the number of
  206. scatter/gather segments permitted by the HBA
  207. - one or more pages can't be locked into memory (should not happen in
  208. any reasonable situation)
  209. The size of the driver buffers is always at least one tape block. In fixed
  210. block mode, the minimum buffer size is defined (in 1024 byte units) by
  211. ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of
  212. several blocks and using one SCSI read or write to transfer all of the
  213. blocks. Buffering of data across write calls in fixed block mode is
  214. allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used.
  215. Buffer allocation uses chunks of memory having sizes 2^n * (page
  216. size). Because of this the actual buffer size may be larger than the
  217. minimum allowable buffer size.
  218. NOTE that if direct i/o is used, the small writes are not buffered. This may
  219. cause a surprise when moving from 2.4. There small writes (e.g., tar without
  220. -b option) may have had good throughput but this is not true any more with
  221. 2.6. Direct i/o can be turned off to solve this problem but a better solution
  222. is to use bigger write() byte counts (e.g., tar -b 64).
  223. Asynchronous writing. Writing the buffer contents to the tape is
  224. started and the write call returns immediately. The status is checked
  225. at the next tape operation. Asynchronous writes are not done with
  226. direct i/o and not in fixed block mode.
  227. Buffered writes and asynchronous writes may in some rare cases cause
  228. problems in multivolume operations if there is not enough space on the
  229. tape after the early-warning mark to flush the driver buffer.
  230. Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
  231. attempted even if the user does not want to get all of the data at
  232. this read command. Should be disabled for those drives that don't like
  233. a filemark to truncate a read request or that don't like backspacing.
  234. Scatter/gather buffers (buffers that consist of chunks non-contiguous
  235. in the physical memory) are used if contiguous buffers can't be
  236. allocated. To support all SCSI adapters (including those not
  237. supporting scatter/gather), buffer allocation is using the following
  238. three kinds of chunks:
  239. 1. The initial segment that is used for all SCSI adapters including
  240. those not supporting scatter/gather. The size of this buffer will be
  241. (PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
  242. this size (and it is not larger than the buffer size specified by
  243. ST_BUFFER_BLOCKS). If this size is not available, the driver halves
  244. the size and tries again until the size of one page. The default
  245. settings in st_options.h make the driver to try to allocate all of the
  246. buffer as one chunk.
  247. 2. The scatter/gather segments to fill the specified buffer size are
  248. allocated so that as many segments as possible are used but the number
  249. of segments does not exceed ST_FIRST_SG.
  250. 3. The remaining segments between ST_MAX_SG (or the module parameter
  251. max_sg_segs) and the number of segments used in phases 1 and 2
  252. are used to extend the buffer at run-time if this is necessary. The
  253. number of scatter/gather segments allowed for the SCSI adapter is not
  254. exceeded if it is smaller than the maximum number of scatter/gather
  255. segments specified. If the maximum number allowed for the SCSI adapter
  256. is smaller than the number of segments used in phases 1 and 2,
  257. extending the buffer will always fail.
  258. EOM Behaviour When Writing
  259. ==========================
  260. When the end of medium early warning is encountered, the current write
  261. is finished and the number of bytes is returned. The next write
  262. returns -1 and errno is set to ENOSPC. To enable writing a trailer,
  263. the next write is allowed to proceed and, if successful, the number of
  264. bytes is returned. After this, -1 and the number of bytes are
  265. alternately returned until the physical end of medium (or some other
  266. error) is encountered.
  267. Module Parameters
  268. =================
  269. The buffer size, write threshold, and the maximum number of allocated buffers
  270. are configurable when the driver is loaded as a module. The keywords are:
  271. ========================== ===========================================
  272. buffer_kbs=xxx the buffer size for fixed block mode is set
  273. to xxx kilobytes
  274. write_threshold_kbs=xxx the write threshold in kilobytes set to xxx
  275. max_sg_segs=xxx the maximum number of scatter/gather
  276. segments
  277. try_direct_io=x try direct transfer between user buffer and
  278. tape drive if this is non-zero
  279. ========================== ===========================================
  280. Note that if the buffer size is changed but the write threshold is not
  281. set, the write threshold is set to the new buffer size - 2 kB.
  282. Boot Time Configuration
  283. =======================
  284. If the driver is compiled into the kernel, the same parameters can be
  285. also set using, e.g., the LILO command line. The preferred syntax is
  286. to use the same keyword used when loading as module but prepended
  287. with 'st.'. For instance, to set the maximum number of scatter/gather
  288. segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the
  289. number of scatter/gather segments).
  290. For compatibility, the old syntax from early 2.5 and 2.4 kernel
  291. versions is supported. The same keywords can be used as when loading
  292. the driver as module. If several parameters are set, the keyword-value
  293. pairs are separated with a comma (no spaces allowed). A colon can be
  294. used instead of the equal mark. The definition is prepended by the
  295. string st=. Here is an example::
  296. st=buffer_kbs:64,write_threshold_kbs:60
  297. The following syntax used by the old kernel versions is also supported::
  298. st=aa[,bb[,dd]]
  299. where:
  300. - aa is the buffer size for fixed block mode in 1024 byte units
  301. - bb is the write threshold in 1024 byte units
  302. - dd is the maximum number of scatter/gather segments
  303. IOCTLs
  304. ======
  305. The tape is positioned and the drive parameters are set with ioctls
  306. defined in mtio.h The tape control program 'mt' uses these ioctls. Try
  307. to find an mt that supports all of the Linux SCSI tape ioctls and
  308. opens the device for writing if the tape contents will be modified
  309. (look for a package mt-st* from the Linux ftp sites; the GNU mt does
  310. not open for writing for, e.g., erase).
  311. The supported ioctls are:
  312. The following use the structure mtop:
  313. MTFSF
  314. Space forward over count filemarks. Tape positioned after filemark.
  315. MTFSFM
  316. As above but tape positioned before filemark.
  317. MTBSF
  318. Space backward over count filemarks. Tape positioned before
  319. filemark.
  320. MTBSFM
  321. As above but ape positioned after filemark.
  322. MTFSR
  323. Space forward over count records.
  324. MTBSR
  325. Space backward over count records.
  326. MTFSS
  327. Space forward over count setmarks.
  328. MTBSS
  329. Space backward over count setmarks.
  330. MTWEOF
  331. Write count filemarks.
  332. MTWEOFI
  333. Write count filemarks with immediate bit set (i.e., does not
  334. wait until data is on tape)
  335. MTWSM
  336. Write count setmarks.
  337. MTREW
  338. Rewind tape.
  339. MTOFFL
  340. Set device off line (often rewind plus eject).
  341. MTNOP
  342. Do nothing except flush the buffers.
  343. MTRETEN
  344. Re-tension tape.
  345. MTEOM
  346. Space to end of recorded data.
  347. MTERASE
  348. Erase tape. If the argument is zero, the short erase command
  349. is used. The long erase command is used with all other values
  350. of the argument.
  351. MTSEEK
  352. Seek to tape block count. Uses Tandberg-compatible seek (QFA)
  353. for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and
  354. block numbers in the status are not valid after a seek.
  355. MTSETBLK
  356. Set the drive block size. Setting to zero sets the drive into
  357. variable block mode (if applicable).
  358. MTSETDENSITY
  359. Sets the drive density code to arg. See drive
  360. documentation for available codes.
  361. MTLOCK and MTUNLOCK
  362. Explicitly lock/unlock the tape drive door.
  363. MTLOAD and MTUNLOAD
  364. Explicitly load and unload the tape. If the
  365. command argument x is between MT_ST_HPLOADER_OFFSET + 1 and
  366. MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the
  367. drive with the command and it selects the tape slot to use of
  368. HP C1553A changer.
  369. MTCOMPRESSION
  370. Sets compressing or uncompressing drive mode using the
  371. SCSI mode page 15. Note that some drives other methods for
  372. control of compression. Some drives (like the Exabytes) use
  373. density codes for compression control. Some drives use another
  374. mode page but this page has not been implemented in the
  375. driver. Some drives without compression capability will accept
  376. any compression mode without error.
  377. MTSETPART
  378. Moves the tape to the partition given by the argument at the
  379. next tape operation. The block at which the tape is positioned
  380. is the block where the tape was previously positioned in the
  381. new active partition unless the next tape operation is
  382. MTSEEK. In this case the tape is moved directly to the block
  383. specified by MTSEEK. MTSETPART is inactive unless
  384. MT_ST_CAN_PARTITIONS set.
  385. MTMKPART
  386. Formats the tape with one partition (argument zero) or two
  387. partitions (argument non-zero). If the argument is positive,
  388. it specifies the size of partition 1 in megabytes. For DDS
  389. drives and several early drives this is the physically first
  390. partition of the tape. If the argument is negative, its absolute
  391. value specifies the size of partition 0 in megabytes. This is
  392. the physically first partition of many later drives, like the
  393. LTO drives from LTO-5 upwards. The drive has to support partitions
  394. with size specified by the initiator. Inactive unless
  395. MT_ST_CAN_PARTITIONS set.
  396. MTSETDRVBUFFER
  397. Is used for several purposes. The command is obtained from count
  398. with mask MT_SET_OPTIONS, the low order bits are used as argument.
  399. This command is only allowed for the superuser (root). The
  400. subcommands are:
  401. * 0
  402. The drive buffer option is set to the argument. Zero means
  403. no buffering.
  404. * MT_ST_BOOLEANS
  405. Sets the buffering options. The bits are the new states
  406. (enabled/disabled) the following options (in the
  407. parenthesis is specified whether the option is global or
  408. can be specified differently for each mode):
  409. MT_ST_BUFFER_WRITES
  410. write buffering (mode)
  411. MT_ST_ASYNC_WRITES
  412. asynchronous writes (mode)
  413. MT_ST_READ_AHEAD
  414. read ahead (mode)
  415. MT_ST_TWO_FM
  416. writing of two filemarks (global)
  417. MT_ST_FAST_EOM
  418. using the SCSI spacing to EOD (global)
  419. MT_ST_AUTO_LOCK
  420. automatic locking of the drive door (global)
  421. MT_ST_DEF_WRITES
  422. the defaults are meant only for writes (mode)
  423. MT_ST_CAN_BSR
  424. backspacing over more than one records can
  425. be used for repositioning the tape (global)
  426. MT_ST_NO_BLKLIMS
  427. the driver does not ask the block limits
  428. from the drive (block size can be changed only to
  429. variable) (global)
  430. MT_ST_CAN_PARTITIONS
  431. enables support for partitioned
  432. tapes (global)
  433. MT_ST_SCSI2LOGICAL
  434. the logical block number is used in
  435. the MTSEEK and MTIOCPOS for SCSI-2 drives instead of
  436. the device dependent address. It is recommended to set
  437. this flag unless there are tapes using the device
  438. dependent (from the old times) (global)
  439. MT_ST_SYSV
  440. sets the SYSV semantics (mode)
  441. MT_ST_NOWAIT
  442. enables immediate mode (i.e., don't wait for
  443. the command to finish) for some commands (e.g., rewind)
  444. MT_ST_NOWAIT_EOF
  445. enables immediate filemark mode (i.e. when
  446. writing a filemark, don't wait for it to complete). Please
  447. see the BASICS note about MTWEOFI with respect to the
  448. possible dangers of writing immediate filemarks.
  449. MT_ST_SILI
  450. enables setting the SILI bit in SCSI commands when
  451. reading in variable block mode to enhance performance when
  452. reading blocks shorter than the byte count; set this only
  453. if you are sure that the drive supports SILI and the HBA
  454. correctly returns transfer residuals
  455. MT_ST_DEBUGGING
  456. debugging (global; debugging must be
  457. compiled into the driver)
  458. * MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS
  459. Sets or clears the option bits.
  460. * MT_ST_WRITE_THRESHOLD
  461. Sets the write threshold for this device to kilobytes
  462. specified by the lowest bits.
  463. * MT_ST_DEF_BLKSIZE
  464. Defines the default block size set automatically. Value
  465. 0xffffff means that the default is not used any more.
  466. * MT_ST_DEF_DENSITY, MT_ST_DEF_DRVBUFFER
  467. Used to set or clear the density (8 bits), and drive buffer
  468. state (3 bits). If the value is MT_ST_CLEAR_DEFAULT
  469. (0xfffff) the default will not be used any more. Otherwise
  470. the lowermost bits of the value contain the new value of
  471. the parameter.
  472. * MT_ST_DEF_COMPRESSION
  473. The compression default will not be used if the value of
  474. the lowermost byte is 0xff. Otherwise the lowermost bit
  475. contains the new default. If the bits 8-15 are set to a
  476. non-zero number, and this number is not 0xff, the number is
  477. used as the compression algorithm. The value
  478. MT_ST_CLEAR_DEFAULT can be used to clear the compression
  479. default.
  480. * MT_ST_SET_TIMEOUT
  481. Set the normal timeout in seconds for this device. The
  482. default is 900 seconds (15 minutes). The timeout should be
  483. long enough for the retries done by the device while
  484. reading/writing.
  485. * MT_ST_SET_LONG_TIMEOUT
  486. Set the long timeout that is used for operations that are
  487. known to take a long time. The default is 14000 seconds
  488. (3.9 hours). For erase this value is further multiplied by
  489. eight.
  490. * MT_ST_SET_CLN
  491. Set the cleaning request interpretation parameters using
  492. the lowest 24 bits of the argument. The driver can set the
  493. generic status bit GMT_CLN if a cleaning request bit pattern
  494. is found from the extended sense data. Many drives set one or
  495. more bits in the extended sense data when the drive needs
  496. cleaning. The bits are device-dependent. The driver is
  497. given the number of the sense data byte (the lowest eight
  498. bits of the argument; must be >= 18 (values 1 - 17
  499. reserved) and <= the maximum requested sense data sixe),
  500. a mask to select the relevant bits (the bits 9-16), and the
  501. bit pattern (bits 17-23). If the bit pattern is zero, one
  502. or more bits under the mask indicate cleaning request. If
  503. the pattern is non-zero, the pattern must match the masked
  504. sense data byte.
  505. (The cleaning bit is set if the additional sense code and
  506. qualifier 00h 17h are seen regardless of the setting of
  507. MT_ST_SET_CLN.)
  508. The following ioctl uses the structure mtpos:
  509. MTIOCPOS
  510. Reads the current position from the drive. Uses
  511. Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2
  512. command for the SCSI-2 drives.
  513. The following ioctl uses the structure mtget to return the status:
  514. MTIOCGET
  515. Returns some status information.
  516. The file number and block number within file are returned. The
  517. block is -1 when it can't be determined (e.g., after MTBSF).
  518. The drive type is either MTISSCSI1 or MTISSCSI2.
  519. The number of recovered errors since the previous status call
  520. is stored in the lower word of the field mt_erreg.
  521. The current block size and the density code are stored in the field
  522. mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and
  523. MT_ST_DENSITY_SHIFT).
  524. The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN
  525. is set if there is no tape in the drive. GMT_EOD means either
  526. end of recorded data or end of tape. GMT_EOT means end of tape.
  527. Miscellaneous Compile Options
  528. =============================
  529. The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL
  530. is defined.
  531. The maximum number of tape devices is determined by the define
  532. ST_MAX_TAPES. If more tapes are detected at driver initialization, the
  533. maximum is adjusted accordingly.
  534. Immediate return from tape positioning SCSI commands can be enabled by
  535. defining ST_NOWAIT. If this is defined, the user should take care that
  536. the next tape operation is not started before the previous one has
  537. finished. The drives and SCSI adapters should handle this condition
  538. gracefully, but some drive/adapter combinations are known to hang the
  539. SCSI bus in this case.
  540. The MTEOM command is by default implemented as spacing over 32767
  541. filemarks. With this method the file number in the status is
  542. correct. The user can request using direct spacing to EOD by setting
  543. ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file
  544. number will be invalid.
  545. When using read ahead or buffered writes the position within the file
  546. may not be correct after the file is closed (correct position may
  547. require backspacing over more than one record). The correct position
  548. within file can be obtained if ST_IN_FILE_POS is defined at compile
  549. time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl.
  550. (The driver always backs over a filemark crossed by read ahead if the
  551. user does not request data that far.)
  552. Debugging Hints
  553. ===============
  554. Debugging code is now compiled in by default but debugging is turned off
  555. with the kernel module parameter debug_flag defaulting to 0. Debugging
  556. can still be switched on and off with an ioctl. To enable debug at
  557. module load time add debug_flag=1 to the module load options, the
  558. debugging output is not voluminous. Debugging can also be enabled
  559. and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
  560. file /sys/bus/scsi/drivers/st/debug_flag.
  561. If the tape seems to hang, I would be very interested to hear where
  562. the driver is waiting. With the command 'ps -l' you can see the state
  563. of the process using the tape. If the state is D, the process is
  564. waiting for something. The field WCHAN tells where the driver is
  565. waiting. If you have the current System.map in the correct place (in
  566. /boot for the procps I use) or have updated /etc/psdatabase (for kmem
  567. ps), ps writes the function name in the WCHAN field. If not, you have
  568. to look up the function from System.map.
  569. Note also that the timeouts are very long compared to most other
  570. drivers. This means that the Linux driver may appear hung although the
  571. real reason is that the tape firmware has got confused.