Documentation/scsi/st.txt

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