Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 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[AT]kolumbus[DOT]fi) 4 5 Last modified: Mon Mar 7 21:14:44 2005 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 If rewind, offline, bsf, or seek is done and previous tape operation was 89 write, a filemark is written before moving tape. 90 91 The compile options are defined in the file linux/drivers/scsi/st_options.h. 92 93 4. If the open option O_NONBLOCK is used, open succeeds even if the 94 drive is not ready. If O_NONBLOCK is not used, the driver waits for 95 the drive to become ready. If this does not happen in ST_BLOCK_SECONDS 96 seconds, open fails with the errno value EIO. With O_NONBLOCK the 97 device can be opened for writing even if there is a write protected 98 tape in the drive (commands trying to write something return error if 99 attempted). 100 101 102 MINOR NUMBERS 103 104 The tape driver currently supports 128 drives by default. This number 105 can be increased by editing st.h and recompiling the driver if 106 necessary. The upper limit is 2^17 drives if 4 modes for each drive 107 are used. 108 109 The minor numbers consist of the following bit fields: 110 111 dev_upper non-rew mode dev-lower 112 20 - 8 7 6 5 4 0 113 The non-rewind bit is always bit 7 (the uppermost bit in the lowermost 114 byte). The bits defining the mode are below the non-rewind bit. The 115 remaining bits define the tape device number. This numbering is 116 backward compatible with the numbering used when the minor number was 117 only 8 bits wide. 118 119 120 SYSFS SUPPORT 121 122 The driver creates the directory /sys/class/scsi_tape and populates it with 123 directories corresponding to the existing tape devices. There are autorewind 124 and non-rewind entries for each mode. The names are stxy and nstxy, where x 125 is the tape number and y a character corresponding to the mode (none, l, m, 126 a). For example, the directories for the first tape device are (assuming four 127 modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a. 128 129 Each directory contains the entries: default_blksize default_compression 130 default_density defined dev device driver. The file 'defined' contains 1 131 if the mode is defined and zero if not defined. The files 'default_*' contain 132 the defaults set by the user. The value -1 means the default is not set. The 133 file 'dev' contains the device numbers corresponding to this device. The links 134 'device' and 'driver' point to the SCSI device and driver entries. 135 136 A link named 'tape' is made from the SCSI device directory to the class 137 directory corresponding to the mode 0 auto-rewind device (e.g., st0). 138 139 140 BSD AND SYS V SEMANTICS 141 142 The user can choose between these two behaviours of the tape driver by 143 defining the value of the symbol ST_SYSV. The semantics differ when a 144 file being read is closed. The BSD semantics leaves the tape where it 145 currently is whereas the SYS V semantics moves the tape past the next 146 filemark unless the filemark has just been crossed. 147 148 The default is BSD semantics. 149 150 151 BUFFERING 152 153 The driver tries to do transfers directly to/from user space. If this 154 is not possible, a driver buffer allocated at run-time is used. If 155 direct i/o is not possible for the whole transfer, the driver buffer 156 is used (i.e., bounce buffers for individual pages are not 157 used). Direct i/o can be impossible because of several reasons, e.g.: 158 - one or more pages are at addresses not reachable by the HBA 159 - the number of pages in the transfer exceeds the number of 160 scatter/gather segments permitted by the HBA 161 - one or more pages can't be locked into memory (should not happen in 162 any reasonable situation) 163 164 The size of the driver buffers is always at least one tape block. In fixed 165 block mode, the minimum buffer size is defined (in 1024 byte units) by 166 ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of 167 several blocks and using one SCSI read or write to transfer all of the 168 blocks. Buffering of data across write calls in fixed block mode is 169 allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used. 170 Buffer allocation uses chunks of memory having sizes 2^n * (page 171 size). Because of this the actual buffer size may be larger than the 172 minimum allowable buffer size. 173 174 NOTE that if direct i/o is used, the small writes are not buffered. This may 175 cause a surprise when moving from 2.4. There small writes (e.g., tar without 176 -b option) may have had good throughput but this is not true any more with 177 2.6. Direct i/o can be turned off to solve this problem but a better solution 178 is to use bigger write() byte counts (e.g., tar -b 64). 179 180 Asynchronous writing. Writing the buffer contents to the tape is 181 started and the write call returns immediately. The status is checked 182 at the next tape operation. Asynchronous writes are not done with 183 direct i/o and not in fixed block mode. 184 185 Buffered writes and asynchronous writes may in some rare cases cause 186 problems in multivolume operations if there is not enough space on the 187 tape after the early-warning mark to flush the driver buffer. 188 189 Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is 190 attempted even if the user does not want to get all of the data at 191 this read command. Should be disabled for those drives that don't like 192 a filemark to truncate a read request or that don't like backspacing. 193 194 Scatter/gather buffers (buffers that consist of chunks non-contiguous 195 in the physical memory) are used if contiguous buffers can't be 196 allocated. To support all SCSI adapters (including those not 197 supporting scatter/gather), buffer allocation is using the following 198 three kinds of chunks: 199 1. The initial segment that is used for all SCSI adapters including 200 those not supporting scatter/gather. The size of this buffer will be 201 (PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of 202 this size (and it is not larger than the buffer size specified by 203 ST_BUFFER_BLOCKS). If this size is not available, the driver halves 204 the size and tries again until the size of one page. The default 205 settings in st_options.h make the driver to try to allocate all of the 206 buffer as one chunk. 207 2. The scatter/gather segments to fill the specified buffer size are 208 allocated so that as many segments as possible are used but the number 209 of segments does not exceed ST_FIRST_SG. 210 3. The remaining segments between ST_MAX_SG (or the module parameter 211 max_sg_segs) and the number of segments used in phases 1 and 2 212 are used to extend the buffer at run-time if this is necessary. The 213 number of scatter/gather segments allowed for the SCSI adapter is not 214 exceeded if it is smaller than the maximum number of scatter/gather 215 segments specified. If the maximum number allowed for the SCSI adapter 216 is smaller than the number of segments used in phases 1 and 2, 217 extending the buffer will always fail. 218 219 220 EOM BEHAVIOUR WHEN WRITING 221 222 When the end of medium early warning is encountered, the current write 223 is finished and the number of bytes is returned. The next write 224 returns -1 and errno is set to ENOSPC. To enable writing a trailer, 225 the next write is allowed to proceed and, if successful, the number of 226 bytes is returned. After this, -1 and the number of bytes are 227 alternately returned until the physical end of medium (or some other 228 error) is encountered. 229 230 231 MODULE PARAMETERS 232 233 The buffer size, write threshold, and the maximum number of allocated buffers 234 are configurable when the driver is loaded as a module. The keywords are: 235 236 buffer_kbs=xxx the buffer size for fixed block mode is set 237 to xxx kilobytes 238 write_threshold_kbs=xxx the write threshold in kilobytes set to xxx 239 max_sg_segs=xxx the maximum number of scatter/gather 240 segments 241 try_direct_io=x try direct transfer between user buffer and 242 tape drive if this is non-zero 243 244 Note that if the buffer size is changed but the write threshold is not 245 set, the write threshold is set to the new buffer size - 2 kB. 246 247 248 BOOT TIME CONFIGURATION 249 250 If the driver is compiled into the kernel, the same parameters can be 251 also set using, e.g., the LILO command line. The preferred syntax is 252 to use the same keyword used when loading as module but prepended 253 with 'st.'. For instance, to set the maximum number of scatter/gather 254 segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the 255 number of scatter/gather segments). 256 257 For compatibility, the old syntax from early 2.5 and 2.4 kernel 258 versions is supported. The same keywords can be used as when loading 259 the driver as module. If several parameters are set, the keyword-value 260 pairs are separated with a comma (no spaces allowed). A colon can be 261 used instead of the equal mark. The definition is prepended by the 262 string st=. Here is an example: 263 264 st=buffer_kbs:64,write_threshold_kbs:60 265 266 The following syntax used by the old kernel versions is also supported: 267 268 st=aa[,bb[,dd]] 269 270 where 271 aa is the buffer size for fixed block mode in 1024 byte units 272 bb is the write threshold in 1024 byte units 273 dd is the maximum number of scatter/gather segments 274 275 276 IOCTLS 277 278 The tape is positioned and the drive parameters are set with ioctls 279 defined in mtio.h The tape control program 'mt' uses these ioctls. Try 280 to find an mt that supports all of the Linux SCSI tape ioctls and 281 opens the device for writing if the tape contents will be modified 282 (look for a package mt-st* from the Linux ftp sites; the GNU mt does 283 not open for writing for, e.g., erase). 284 285 The supported ioctls are: 286 287 The following use the structure mtop: 288 289 MTFSF Space forward over count filemarks. Tape positioned after filemark. 290 MTFSFM As above but tape positioned before filemark. 291 MTBSF Space backward over count filemarks. Tape positioned before 292 filemark. 293 MTBSFM As above but ape positioned after filemark. 294 MTFSR Space forward over count records. 295 MTBSR Space backward over count records. 296 MTFSS Space forward over count setmarks. 297 MTBSS Space backward over count setmarks. 298 MTWEOF Write count filemarks. 299 MTWSM Write count setmarks. 300 MTREW Rewind tape. 301 MTOFFL Set device off line (often rewind plus eject). 302 MTNOP Do nothing except flush the buffers. 303 MTRETEN Re-tension tape. 304 MTEOM Space to end of recorded data. 305 MTERASE Erase tape. If the argument is zero, the short erase command 306 is used. The long erase command is used with all other values 307 of the argument. 308 MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA) 309 for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and 310 block numbers in the status are not valid after a seek. 311 MTSETBLK Set the drive block size. Setting to zero sets the drive into 312 variable block mode (if applicable). 313 MTSETDENSITY Sets the drive density code to arg. See drive 314 documentation for available codes. 315 MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door. 316 MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the 317 command argument x is between MT_ST_HPLOADER_OFFSET + 1 and 318 MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the 319 drive with the command and it selects the tape slot to use of 320 HP C1553A changer. 321 MTCOMPRESSION Sets compressing or uncompressing drive mode using the 322 SCSI mode page 15. Note that some drives other methods for 323 control of compression. Some drives (like the Exabytes) use 324 density codes for compression control. Some drives use another 325 mode page but this page has not been implemented in the 326 driver. Some drives without compression capability will accept 327 any compression mode without error. 328 MTSETPART Moves the tape to the partition given by the argument at the 329 next tape operation. The block at which the tape is positioned 330 is the block where the tape was previously positioned in the 331 new active partition unless the next tape operation is 332 MTSEEK. In this case the tape is moved directly to the block 333 specified by MTSEEK. MTSETPART is inactive unless 334 MT_ST_CAN_PARTITIONS set. 335 MTMKPART Formats the tape with one partition (argument zero) or two 336 partitions (the argument gives in megabytes the size of 337 partition 1 that is physically the first partition of the 338 tape). The drive has to support partitions with size specified 339 by the initiator. Inactive unless MT_ST_CAN_PARTITIONS set. 340 MTSETDRVBUFFER 341 Is used for several purposes. The command is obtained from count 342 with mask MT_SET_OPTIONS, the low order bits are used as argument. 343 This command is only allowed for the superuser (root). The 344 subcommands are: 345 0 346 The drive buffer option is set to the argument. Zero means 347 no buffering. 348 MT_ST_BOOLEANS 349 Sets the buffering options. The bits are the new states 350 (enabled/disabled) the following options (in the 351 parenthesis is specified whether the option is global or 352 can be specified differently for each mode): 353 MT_ST_BUFFER_WRITES write buffering (mode) 354 MT_ST_ASYNC_WRITES asynchronous writes (mode) 355 MT_ST_READ_AHEAD read ahead (mode) 356 MT_ST_TWO_FM writing of two filemarks (global) 357 MT_ST_FAST_EOM using the SCSI spacing to EOD (global) 358 MT_ST_AUTO_LOCK automatic locking of the drive door (global) 359 MT_ST_DEF_WRITES the defaults are meant only for writes (mode) 360 MT_ST_CAN_BSR backspacing over more than one records can 361 be used for repositioning the tape (global) 362 MT_ST_NO_BLKLIMS the driver does not ask the block limits 363 from the drive (block size can be changed only to 364 variable) (global) 365 MT_ST_CAN_PARTITIONS enables support for partitioned 366 tapes (global) 367 MT_ST_SCSI2LOGICAL the logical block number is used in 368 the MTSEEK and MTIOCPOS for SCSI-2 drives instead of 369 the device dependent address. It is recommended to set 370 this flag unless there are tapes using the device 371 dependent (from the old times) (global) 372 MT_ST_SYSV sets the SYSV semantics (mode) 373 MT_ST_NOWAIT enables immediate mode (i.e., don't wait for 374 the command to finish) for some commands (e.g., rewind) 375 MT_ST_DEBUGGING debugging (global; debugging must be 376 compiled into the driver) 377 MT_ST_SETBOOLEANS 378 MT_ST_CLEARBOOLEANS 379 Sets or clears the option bits. 380 MT_ST_WRITE_THRESHOLD 381 Sets the write threshold for this device to kilobytes 382 specified by the lowest bits. 383 MT_ST_DEF_BLKSIZE 384 Defines the default block size set automatically. Value 385 0xffffff means that the default is not used any more. 386 MT_ST_DEF_DENSITY 387 MT_ST_DEF_DRVBUFFER 388 Used to set or clear the density (8 bits), and drive buffer 389 state (3 bits). If the value is MT_ST_CLEAR_DEFAULT 390 (0xfffff) the default will not be used any more. Otherwise 391 the lowermost bits of the value contain the new value of 392 the parameter. 393 MT_ST_DEF_COMPRESSION 394 The compression default will not be used if the value of 395 the lowermost byte is 0xff. Otherwise the lowermost bit 396 contains the new default. If the bits 8-15 are set to a 397 non-zero number, and this number is not 0xff, the number is 398 used as the compression algorithm. The value 399 MT_ST_CLEAR_DEFAULT can be used to clear the compression 400 default. 401 MT_ST_SET_TIMEOUT 402 Set the normal timeout in seconds for this device. The 403 default is 900 seconds (15 minutes). The timeout should be 404 long enough for the retries done by the device while 405 reading/writing. 406 MT_ST_SET_LONG_TIMEOUT 407 Set the long timeout that is used for operations that are 408 known to take a long time. The default is 14000 seconds 409 (3.9 hours). For erase this value is further multiplied by 410 eight. 411 MT_ST_SET_CLN 412 Set the cleaning request interpretation parameters using 413 the lowest 24 bits of the argument. The driver can set the 414 generic status bit GMT_CLN if a cleaning request bit pattern 415 is found from the extended sense data. Many drives set one or 416 more bits in the extended sense data when the drive needs 417 cleaning. The bits are device-dependent. The driver is 418 given the number of the sense data byte (the lowest eight 419 bits of the argument; must be >= 18 (values 1 - 17 420 reserved) and <= the maximum requested sense data sixe), 421 a mask to select the relevant bits (the bits 9-16), and the 422 bit pattern (bits 17-23). If the bit pattern is zero, one 423 or more bits under the mask indicate cleaning request. If 424 the pattern is non-zero, the pattern must match the masked 425 sense data byte. 426 427 (The cleaning bit is set if the additional sense code and 428 qualifier 00h 17h are seen regardless of the setting of 429 MT_ST_SET_CLN.) 430 431 The following ioctl uses the structure mtpos: 432 MTIOCPOS Reads the current position from the drive. Uses 433 Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2 434 command for the SCSI-2 drives. 435 436 The following ioctl uses the structure mtget to return the status: 437 MTIOCGET Returns some status information. 438 The file number and block number within file are returned. The 439 block is -1 when it can't be determined (e.g., after MTBSF). 440 The drive type is either MTISSCSI1 or MTISSCSI2. 441 The number of recovered errors since the previous status call 442 is stored in the lower word of the field mt_erreg. 443 The current block size and the density code are stored in the field 444 mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and 445 MT_ST_DENSITY_SHIFT). 446 The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN 447 is set if there is no tape in the drive. GMT_EOD means either 448 end of recorded data or end of tape. GMT_EOT means end of tape. 449 450 451 MISCELLANEOUS COMPILE OPTIONS 452 453 The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL 454 is defined. 455 456 The maximum number of tape devices is determined by the define 457 ST_MAX_TAPES. If more tapes are detected at driver initialization, the 458 maximum is adjusted accordingly. 459 460 Immediate return from tape positioning SCSI commands can be enabled by 461 defining ST_NOWAIT. If this is defined, the user should take care that 462 the next tape operation is not started before the previous one has 463 finished. The drives and SCSI adapters should handle this condition 464 gracefully, but some drive/adapter combinations are known to hang the 465 SCSI bus in this case. 466 467 The MTEOM command is by default implemented as spacing over 32767 468 filemarks. With this method the file number in the status is 469 correct. The user can request using direct spacing to EOD by setting 470 ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file 471 number will be invalid. 472 473 When using read ahead or buffered writes the position within the file 474 may not be correct after the file is closed (correct position may 475 require backspacing over more than one record). The correct position 476 within file can be obtained if ST_IN_FILE_POS is defined at compile 477 time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl. 478 (The driver always backs over a filemark crossed by read ahead if the 479 user does not request data that far.) 480 481 482 DEBUGGING HINTS 483 484 To enable debugging messages, edit st.c and #define DEBUG 1. As seen 485 above, debugging can be switched off with an ioctl if debugging is 486 compiled into the driver. The debugging output is not voluminous. 487 488 If the tape seems to hang, I would be very interested to hear where 489 the driver is waiting. With the command 'ps -l' you can see the state 490 of the process using the tape. If the state is D, the process is 491 waiting for something. The field WCHAN tells where the driver is 492 waiting. If you have the current System.map in the correct place (in 493 /boot for the procps I use) or have updated /etc/psdatabase (for kmem 494 ps), ps writes the function name in the WCHAN field. If not, you have 495 to look up the function from System.map. 496 497 Note also that the timeouts are very long compared to most other 498 drivers. This means that the Linux driver may appear hung although the 499 real reason is that the tape firmware has got confused.