Commit | Line | Data |
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1da177e4 | 1 | This file contains brief information about the SCSI tape driver. |
be2a608b | 2 | The driver is currently maintained by Kai Mäkisara (email |
1da177e4 LT |
3 | Kai.Makisara@kolumbus.fi) |
4 | ||
3e51d3c9 | 5 | Last modified: Sun Aug 29 18:25:47 2010 by kai.makisara |
1da177e4 LT |
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 | ||
3e51d3c9 KM |
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 | ||
1da177e4 LT |
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 128 drives by default. This number | |
116 | can be increased by editing st.h and recompiling the driver if | |
117 | necessary. The upper limit is 2^17 drives if 4 modes for each drive | |
118 | are used. | |
119 | ||
120 | The minor numbers consist of the following bit fields: | |
121 | ||
122 | dev_upper non-rew mode dev-lower | |
123 | 20 - 8 7 6 5 4 0 | |
124 | The non-rewind bit is always bit 7 (the uppermost bit in the lowermost | |
125 | byte). The bits defining the mode are below the non-rewind bit. The | |
126 | remaining bits define the tape device number. This numbering is | |
127 | backward compatible with the numbering used when the minor number was | |
128 | only 8 bits wide. | |
129 | ||
130 | ||
131 | SYSFS SUPPORT | |
132 | ||
133 | The driver creates the directory /sys/class/scsi_tape and populates it with | |
134 | directories corresponding to the existing tape devices. There are autorewind | |
135 | and non-rewind entries for each mode. The names are stxy and nstxy, where x | |
136 | is the tape number and y a character corresponding to the mode (none, l, m, | |
137 | a). For example, the directories for the first tape device are (assuming four | |
138 | modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a. | |
139 | ||
140 | Each directory contains the entries: default_blksize default_compression | |
141 | default_density defined dev device driver. The file 'defined' contains 1 | |
142 | if the mode is defined and zero if not defined. The files 'default_*' contain | |
143 | the defaults set by the user. The value -1 means the default is not set. The | |
144 | file 'dev' contains the device numbers corresponding to this device. The links | |
145 | 'device' and 'driver' point to the SCSI device and driver entries. | |
146 | ||
b174be02 KM |
147 | Each directory also contains the entry 'options' which shows the currently |
148 | enabled driver and mode options. The value in the file is a bit mask where the | |
149 | bit definitions are the same as those used with MTSETDRVBUFFER in setting the | |
150 | options. | |
151 | ||
1da177e4 LT |
152 | A link named 'tape' is made from the SCSI device directory to the class |
153 | directory corresponding to the mode 0 auto-rewind device (e.g., st0). | |
154 | ||
155 | ||
156 | BSD AND SYS V SEMANTICS | |
157 | ||
158 | The user can choose between these two behaviours of the tape driver by | |
159 | defining the value of the symbol ST_SYSV. The semantics differ when a | |
160 | file being read is closed. The BSD semantics leaves the tape where it | |
161 | currently is whereas the SYS V semantics moves the tape past the next | |
162 | filemark unless the filemark has just been crossed. | |
163 | ||
164 | The default is BSD semantics. | |
165 | ||
166 | ||
167 | BUFFERING | |
168 | ||
169 | The driver tries to do transfers directly to/from user space. If this | |
170 | is not possible, a driver buffer allocated at run-time is used. If | |
171 | direct i/o is not possible for the whole transfer, the driver buffer | |
172 | is used (i.e., bounce buffers for individual pages are not | |
173 | used). 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 | ||
180 | The size of the driver buffers is always at least one tape block. In fixed | |
181 | block mode, the minimum buffer size is defined (in 1024 byte units) by | |
182 | ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of | |
183 | several blocks and using one SCSI read or write to transfer all of the | |
184 | blocks. Buffering of data across write calls in fixed block mode is | |
185 | allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used. | |
186 | Buffer allocation uses chunks of memory having sizes 2^n * (page | |
187 | size). Because of this the actual buffer size may be larger than the | |
188 | minimum allowable buffer size. | |
189 | ||
190 | NOTE that if direct i/o is used, the small writes are not buffered. This may | |
191 | cause 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 | |
193 | 2.6. Direct i/o can be turned off to solve this problem but a better solution | |
194 | is to use bigger write() byte counts (e.g., tar -b 64). | |
195 | ||
196 | Asynchronous writing. Writing the buffer contents to the tape is | |
197 | started and the write call returns immediately. The status is checked | |
198 | at the next tape operation. Asynchronous writes are not done with | |
199 | direct i/o and not in fixed block mode. | |
200 | ||
201 | Buffered writes and asynchronous writes may in some rare cases cause | |
202 | problems in multivolume operations if there is not enough space on the | |
203 | tape after the early-warning mark to flush the driver buffer. | |
204 | ||
205 | Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is | |
206 | attempted even if the user does not want to get all of the data at | |
207 | this read command. Should be disabled for those drives that don't like | |
208 | a filemark to truncate a read request or that don't like backspacing. | |
209 | ||
210 | Scatter/gather buffers (buffers that consist of chunks non-contiguous | |
211 | in the physical memory) are used if contiguous buffers can't be | |
212 | allocated. To support all SCSI adapters (including those not | |
213 | supporting scatter/gather), buffer allocation is using the following | |
214 | three kinds of chunks: | |
215 | 1. The initial segment that is used for all SCSI adapters including | |
216 | those 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 | |
218 | this size (and it is not larger than the buffer size specified by | |
219 | ST_BUFFER_BLOCKS). If this size is not available, the driver halves | |
220 | the size and tries again until the size of one page. The default | |
221 | settings in st_options.h make the driver to try to allocate all of the | |
222 | buffer as one chunk. | |
223 | 2. The scatter/gather segments to fill the specified buffer size are | |
224 | allocated so that as many segments as possible are used but the number | |
225 | of segments does not exceed ST_FIRST_SG. | |
226 | 3. The remaining segments between ST_MAX_SG (or the module parameter | |
227 | max_sg_segs) and the number of segments used in phases 1 and 2 | |
228 | are used to extend the buffer at run-time if this is necessary. The | |
229 | number of scatter/gather segments allowed for the SCSI adapter is not | |
230 | exceeded if it is smaller than the maximum number of scatter/gather | |
231 | segments specified. If the maximum number allowed for the SCSI adapter | |
232 | is smaller than the number of segments used in phases 1 and 2, | |
233 | extending the buffer will always fail. | |
234 | ||
235 | ||
236 | EOM BEHAVIOUR WHEN WRITING | |
237 | ||
238 | When the end of medium early warning is encountered, the current write | |
239 | is finished and the number of bytes is returned. The next write | |
240 | returns -1 and errno is set to ENOSPC. To enable writing a trailer, | |
241 | the next write is allowed to proceed and, if successful, the number of | |
242 | bytes is returned. After this, -1 and the number of bytes are | |
243 | alternately returned until the physical end of medium (or some other | |
244 | error) is encountered. | |
245 | ||
246 | ||
247 | MODULE PARAMETERS | |
248 | ||
249 | The buffer size, write threshold, and the maximum number of allocated buffers | |
250 | are configurable when the driver is loaded as a module. The keywords are: | |
251 | ||
252 | buffer_kbs=xxx the buffer size for fixed block mode is set | |
253 | to xxx kilobytes | |
254 | write_threshold_kbs=xxx the write threshold in kilobytes set to xxx | |
255 | max_sg_segs=xxx the maximum number of scatter/gather | |
256 | segments | |
257 | try_direct_io=x try direct transfer between user buffer and | |
258 | tape drive if this is non-zero | |
259 | ||
260 | Note that if the buffer size is changed but the write threshold is not | |
261 | set, the write threshold is set to the new buffer size - 2 kB. | |
262 | ||
263 | ||
264 | BOOT TIME CONFIGURATION | |
265 | ||
266 | If the driver is compiled into the kernel, the same parameters can be | |
267 | also set using, e.g., the LILO command line. The preferred syntax is | |
670e9f34 | 268 | to use the same keyword used when loading as module but prepended |
1da177e4 LT |
269 | with 'st.'. For instance, to set the maximum number of scatter/gather |
270 | segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the | |
271 | number of scatter/gather segments). | |
272 | ||
273 | For compatibility, the old syntax from early 2.5 and 2.4 kernel | |
274 | versions is supported. The same keywords can be used as when loading | |
275 | the driver as module. If several parameters are set, the keyword-value | |
276 | pairs are separated with a comma (no spaces allowed). A colon can be | |
277 | used instead of the equal mark. The definition is prepended by the | |
278 | string st=. Here is an example: | |
279 | ||
fa00e7e1 | 280 | st=buffer_kbs:64,write_threshold_kbs:60 |
1da177e4 LT |
281 | |
282 | The following syntax used by the old kernel versions is also supported: | |
283 | ||
284 | st=aa[,bb[,dd]] | |
285 | ||
286 | where | |
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 | ||
292 | IOCTLS | |
293 | ||
294 | The tape is positioned and the drive parameters are set with ioctls | |
295 | defined in mtio.h The tape control program 'mt' uses these ioctls. Try | |
296 | to find an mt that supports all of the Linux SCSI tape ioctls and | |
297 | opens 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 | |
299 | not open for writing for, e.g., erase). | |
300 | ||
301 | The supported ioctls are: | |
302 | ||
303 | The following use the structure mtop: | |
304 | ||
305 | MTFSF Space forward over count filemarks. Tape positioned after filemark. | |
306 | MTFSFM As above but tape positioned before filemark. | |
307 | MTBSF Space backward over count filemarks. Tape positioned before | |
308 | filemark. | |
309 | MTBSFM As above but ape positioned after filemark. | |
310 | MTFSR Space forward over count records. | |
311 | MTBSR Space backward over count records. | |
312 | MTFSS Space forward over count setmarks. | |
313 | MTBSS Space backward over count setmarks. | |
314 | MTWEOF Write count filemarks. | |
3e51d3c9 KM |
315 | MTWEOFI Write count filemarks with immediate bit set (i.e., does not |
316 | wait until data is on tape) | |
1da177e4 LT |
317 | MTWSM Write count setmarks. |
318 | MTREW Rewind tape. | |
319 | MTOFFL Set device off line (often rewind plus eject). | |
320 | MTNOP Do nothing except flush the buffers. | |
321 | MTRETEN Re-tension tape. | |
322 | MTEOM Space to end of recorded data. | |
323 | MTERASE 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. | |
326 | MTSEEK 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. | |
329 | MTSETBLK Set the drive block size. Setting to zero sets the drive into | |
330 | variable block mode (if applicable). | |
331 | MTSETDENSITY Sets the drive density code to arg. See drive | |
332 | documentation for available codes. | |
333 | MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door. | |
334 | MTLOAD 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. | |
339 | MTCOMPRESSION 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. | |
346 | MTSETPART 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. | |
353 | MTMKPART 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. | |
358 | MTSETDRVBUFFER | |
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) | |
53cb4726 | 390 | MT_ST_SYSV sets the SYSV semantics (mode) |
1da177e4 LT |
391 | MT_ST_NOWAIT enables immediate mode (i.e., don't wait for |
392 | the command to finish) for some commands (e.g., rewind) | |
c743e44f LD |
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. | |
40f6b36c KM |
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 | |
1da177e4 LT |
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 | ||
458 | The following ioctl uses the structure mtpos: | |
459 | MTIOCPOS 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 | ||
463 | The following ioctl uses the structure mtget to return the status: | |
464 | MTIOCGET 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 | ||
478 | MISCELLANEOUS COMPILE OPTIONS | |
479 | ||
480 | The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL | |
481 | is defined. | |
482 | ||
483 | The maximum number of tape devices is determined by the define | |
484 | ST_MAX_TAPES. If more tapes are detected at driver initialization, the | |
485 | maximum is adjusted accordingly. | |
486 | ||
487 | Immediate return from tape positioning SCSI commands can be enabled by | |
488 | defining ST_NOWAIT. If this is defined, the user should take care that | |
489 | the next tape operation is not started before the previous one has | |
490 | finished. The drives and SCSI adapters should handle this condition | |
491 | gracefully, but some drive/adapter combinations are known to hang the | |
492 | SCSI bus in this case. | |
493 | ||
494 | The MTEOM command is by default implemented as spacing over 32767 | |
495 | filemarks. With this method the file number in the status is | |
496 | correct. The user can request using direct spacing to EOD by setting | |
497 | ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file | |
498 | number will be invalid. | |
499 | ||
500 | When using read ahead or buffered writes the position within the file | |
501 | may not be correct after the file is closed (correct position may | |
502 | require backspacing over more than one record). The correct position | |
503 | within file can be obtained if ST_IN_FILE_POS is defined at compile | |
504 | time 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 | |
506 | user does not request data that far.) | |
507 | ||
508 | ||
509 | DEBUGGING HINTS | |
510 | ||
511 | To enable debugging messages, edit st.c and #define DEBUG 1. As seen | |
512 | above, debugging can be switched off with an ioctl if debugging is | |
513 | compiled into the driver. The debugging output is not voluminous. | |
514 | ||
515 | If the tape seems to hang, I would be very interested to hear where | |
516 | the driver is waiting. With the command 'ps -l' you can see the state | |
517 | of the process using the tape. If the state is D, the process is | |
518 | waiting for something. The field WCHAN tells where the driver is | |
519 | waiting. 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 | |
521 | ps), ps writes the function name in the WCHAN field. If not, you have | |
522 | to look up the function from System.map. | |
523 | ||
524 | Note also that the timeouts are very long compared to most other | |
525 | drivers. This means that the Linux driver may appear hung although the | |
526 | real reason is that the tape firmware has got confused. |