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 Aug 29 18:25:47 2010 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 88Writing filemarks without the immediate bit set in the SCSI command block acts 89as a synchronization point, i.e., all remaining data form the drive buffers is 90written to tape before the command returns. This makes sure that write errors 91are caught at that point, but this takes time. In some applications, several 92consecutive files must be written fast. The MTWEOFI operation can be used to 93write the filemarks without flushing the drive buffer. Writing filemark at 94close() is always flushing the drive buffers. However, if the previous 95operation is MTWEOFI, close() does not write a filemark. This can be used if 96the program wants to close/open the tape device between files and wants to 97skip waiting. 98 99If rewind, offline, bsf, or seek is done and previous tape operation was 100write, a filemark is written before moving tape. 101 102The compile options are defined in the file linux/drivers/scsi/st_options.h. 103 1044. If the open option O_NONBLOCK is used, open succeeds even if the 105drive is not ready. If O_NONBLOCK is not used, the driver waits for 106the drive to become ready. If this does not happen in ST_BLOCK_SECONDS 107seconds, open fails with the errno value EIO. With O_NONBLOCK the 108device can be opened for writing even if there is a write protected 109tape in the drive (commands trying to write something return error if 110attempted). 111 112 113MINOR NUMBERS 114 115The tape driver currently supports 128 drives by default. This number 116can be increased by editing st.h and recompiling the driver if 117necessary. The upper limit is 2^17 drives if 4 modes for each drive 118are used. 119 120The minor numbers consist of the following bit fields: 121 122dev_upper non-rew mode dev-lower 123 20 - 8 7 6 5 4 0 124The non-rewind bit is always bit 7 (the uppermost bit in the lowermost 125byte). The bits defining the mode are below the non-rewind bit. The 126remaining bits define the tape device number. This numbering is 127backward compatible with the numbering used when the minor number was 128only 8 bits wide. 129 130 131SYSFS SUPPORT 132 133The driver creates the directory /sys/class/scsi_tape and populates it with 134directories corresponding to the existing tape devices. There are autorewind 135and non-rewind entries for each mode. The names are stxy and nstxy, where x 136is the tape number and y a character corresponding to the mode (none, l, m, 137a). For example, the directories for the first tape device are (assuming four 138modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a. 139 140Each directory contains the entries: default_blksize default_compression 141default_density defined dev device driver. The file 'defined' contains 1 142if the mode is defined and zero if not defined. The files 'default_*' contain 143the defaults set by the user. The value -1 means the default is not set. The 144file 'dev' contains the device numbers corresponding to this device. The links 145'device' and 'driver' point to the SCSI device and driver entries. 146 147Each directory also contains the entry 'options' which shows the currently 148enabled driver and mode options. The value in the file is a bit mask where the 149bit definitions are the same as those used with MTSETDRVBUFFER in setting the 150options. 151 152A link named 'tape' is made from the SCSI device directory to the class 153directory corresponding to the mode 0 auto-rewind device (e.g., st0). 154 155 156BSD AND SYS V SEMANTICS 157 158The user can choose between these two behaviours of the tape driver by 159defining the value of the symbol ST_SYSV. The semantics differ when a 160file being read is closed. The BSD semantics leaves the tape where it 161currently is whereas the SYS V semantics moves the tape past the next 162filemark unless the filemark has just been crossed. 163 164The default is BSD semantics. 165 166 167BUFFERING 168 169The driver tries to do transfers directly to/from user space. If this 170is not possible, a driver buffer allocated at run-time is used. If 171direct i/o is not possible for the whole transfer, the driver buffer 172is used (i.e., bounce buffers for individual pages are not 173used). 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 180The size of the driver buffers is always at least one tape block. In fixed 181block mode, the minimum buffer size is defined (in 1024 byte units) by 182ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of 183several blocks and using one SCSI read or write to transfer all of the 184blocks. Buffering of data across write calls in fixed block mode is 185allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used. 186Buffer allocation uses chunks of memory having sizes 2^n * (page 187size). Because of this the actual buffer size may be larger than the 188minimum allowable buffer size. 189 190NOTE that if direct i/o is used, the small writes are not buffered. This may 191cause 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 1932.6. Direct i/o can be turned off to solve this problem but a better solution 194is to use bigger write() byte counts (e.g., tar -b 64). 195 196Asynchronous writing. Writing the buffer contents to the tape is 197started and the write call returns immediately. The status is checked 198at the next tape operation. Asynchronous writes are not done with 199direct i/o and not in fixed block mode. 200 201Buffered writes and asynchronous writes may in some rare cases cause 202problems in multivolume operations if there is not enough space on the 203tape after the early-warning mark to flush the driver buffer. 204 205Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is 206attempted even if the user does not want to get all of the data at 207this read command. Should be disabled for those drives that don't like 208a filemark to truncate a read request or that don't like backspacing. 209 210Scatter/gather buffers (buffers that consist of chunks non-contiguous 211in the physical memory) are used if contiguous buffers can't be 212allocated. To support all SCSI adapters (including those not 213supporting scatter/gather), buffer allocation is using the following 214three kinds of chunks: 2151. The initial segment that is used for all SCSI adapters including 216those 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 218this size (and it is not larger than the buffer size specified by 219ST_BUFFER_BLOCKS). If this size is not available, the driver halves 220the size and tries again until the size of one page. The default 221settings in st_options.h make the driver to try to allocate all of the 222buffer as one chunk. 2232. The scatter/gather segments to fill the specified buffer size are 224allocated so that as many segments as possible are used but the number 225of segments does not exceed ST_FIRST_SG. 2263. The remaining segments between ST_MAX_SG (or the module parameter 227max_sg_segs) and the number of segments used in phases 1 and 2 228are used to extend the buffer at run-time if this is necessary. The 229number of scatter/gather segments allowed for the SCSI adapter is not 230exceeded if it is smaller than the maximum number of scatter/gather 231segments specified. If the maximum number allowed for the SCSI adapter 232is smaller than the number of segments used in phases 1 and 2, 233extending the buffer will always fail. 234 235 236EOM BEHAVIOUR WHEN WRITING 237 238When the end of medium early warning is encountered, the current write 239is finished and the number of bytes is returned. The next write 240returns -1 and errno is set to ENOSPC. To enable writing a trailer, 241the next write is allowed to proceed and, if successful, the number of 242bytes is returned. After this, -1 and the number of bytes are 243alternately returned until the physical end of medium (or some other 244error) is encountered. 245 246 247MODULE PARAMETERS 248 249The buffer size, write threshold, and the maximum number of allocated buffers 250are configurable when the driver is loaded as a module. The keywords are: 251 252buffer_kbs=xxx the buffer size for fixed block mode is set 253 to xxx kilobytes 254write_threshold_kbs=xxx the write threshold in kilobytes set to xxx 255max_sg_segs=xxx the maximum number of scatter/gather 256 segments 257try_direct_io=x try direct transfer between user buffer and 258 tape drive if this is non-zero 259 260Note that if the buffer size is changed but the write threshold is not 261set, the write threshold is set to the new buffer size - 2 kB. 262 263 264BOOT TIME CONFIGURATION 265 266If the driver is compiled into the kernel, the same parameters can be 267also set using, e.g., the LILO command line. The preferred syntax is 268to use the same keyword used when loading as module but prepended 269with 'st.'. For instance, to set the maximum number of scatter/gather 270segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the 271number of scatter/gather segments). 272 273For compatibility, the old syntax from early 2.5 and 2.4 kernel 274versions is supported. The same keywords can be used as when loading 275the driver as module. If several parameters are set, the keyword-value 276pairs are separated with a comma (no spaces allowed). A colon can be 277used instead of the equal mark. The definition is prepended by the 278string st=. Here is an example: 279 280 st=buffer_kbs:64,write_threshold_kbs:60 281 282The following syntax used by the old kernel versions is also supported: 283 284 st=aa[,bb[,dd]] 285 286where 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 292IOCTLS 293 294The tape is positioned and the drive parameters are set with ioctls 295defined in mtio.h The tape control program 'mt' uses these ioctls. Try 296to find an mt that supports all of the Linux SCSI tape ioctls and 297opens 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 299not open for writing for, e.g., erase). 300 301The supported ioctls are: 302 303The following use the structure mtop: 304 305MTFSF Space forward over count filemarks. Tape positioned after filemark. 306MTFSFM As above but tape positioned before filemark. 307MTBSF Space backward over count filemarks. Tape positioned before 308 filemark. 309MTBSFM As above but ape positioned after filemark. 310MTFSR Space forward over count records. 311MTBSR Space backward over count records. 312MTFSS Space forward over count setmarks. 313MTBSS Space backward over count setmarks. 314MTWEOF Write count filemarks. 315MTWEOFI Write count filemarks with immediate bit set (i.e., does not 316 wait until data is on tape) 317MTWSM Write count setmarks. 318MTREW Rewind tape. 319MTOFFL Set device off line (often rewind plus eject). 320MTNOP Do nothing except flush the buffers. 321MTRETEN Re-tension tape. 322MTEOM Space to end of recorded data. 323MTERASE 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. 326MTSEEK 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. 329MTSETBLK Set the drive block size. Setting to zero sets the drive into 330 variable block mode (if applicable). 331MTSETDENSITY Sets the drive density code to arg. See drive 332 documentation for available codes. 333MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door. 334MTLOAD 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. 339MTCOMPRESSION 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. 346MTSETPART 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. 353MTMKPART 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. 358MTSETDRVBUFFER 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 458The following ioctl uses the structure mtpos: 459MTIOCPOS 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 463The following ioctl uses the structure mtget to return the status: 464MTIOCGET 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 478MISCELLANEOUS COMPILE OPTIONS 479 480The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL 481is defined. 482 483The maximum number of tape devices is determined by the define 484ST_MAX_TAPES. If more tapes are detected at driver initialization, the 485maximum is adjusted accordingly. 486 487Immediate return from tape positioning SCSI commands can be enabled by 488defining ST_NOWAIT. If this is defined, the user should take care that 489the next tape operation is not started before the previous one has 490finished. The drives and SCSI adapters should handle this condition 491gracefully, but some drive/adapter combinations are known to hang the 492SCSI bus in this case. 493 494The MTEOM command is by default implemented as spacing over 32767 495filemarks. With this method the file number in the status is 496correct. The user can request using direct spacing to EOD by setting 497ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file 498number will be invalid. 499 500When using read ahead or buffered writes the position within the file 501may not be correct after the file is closed (correct position may 502require backspacing over more than one record). The correct position 503within file can be obtained if ST_IN_FILE_POS is defined at compile 504time 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 506user does not request data that far.) 507 508 509DEBUGGING HINTS 510 511To enable debugging messages, edit st.c and #define DEBUG 1. As seen 512above, debugging can be switched off with an ioctl if debugging is 513compiled into the driver. The debugging output is not voluminous. 514 515If the tape seems to hang, I would be very interested to hear where 516the driver is waiting. With the command 'ps -l' you can see the state 517of the process using the tape. If the state is D, the process is 518waiting for something. The field WCHAN tells where the driver is 519waiting. 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 521ps), ps writes the function name in the WCHAN field. If not, you have 522to look up the function from System.map. 523 524Note also that the timeouts are very long compared to most other 525drivers. This means that the Linux driver may appear hung although the 526real reason is that the tape firmware has got confused. 527