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33 .Nd SCSI Sequential Access device driver
39 driver provides support for all
41 devices of the sequential access class that are attached to the system
45 The sequential access class includes tape and other linear access devices.
50 adapter must also be separately configured into the system
53 sequential access device can be configured.
57 driver is based around the concept of a
58 .Dq Em mount session ,
59 which is defined as the period between the time that a tape is
60 mounted, and the time when it is unmounted.
61 Any parameters set during
62 a mount session remain in effect for the remainder of the session or
64 The tape can be unmounted, bringing the session to a
65 close in several ways.
69 Closing a `rewind device',
70 referred to as sub-mode 00 below.
76 command, reachable through the
82 It should be noted that tape devices are exclusive open devices, except in
83 the case where a control mode device is opened.
84 In the latter case, exclusive
85 access is only sought when needed (e.g., to set parameters).
87 Bits 0 and 1 of the minor number are interpreted as
89 The sub-modes differ in the action taken when the device is closed:
92 A close will rewind the device; if the tape has been
93 written, then a file mark will be written before the rewind is requested.
94 The device is unmounted.
96 A close will leave the tape mounted.
97 If the tape was written to, a file mark will be written.
98 No other head positioning takes place.
99 Any further reads or writes will occur directly after the
100 last read, or the written file mark.
102 A close will rewind the device.
104 written, then a file mark will be written before the rewind is requested.
105 On completion of the rewind an unload command will be issued.
106 The device is unmounted.
110 tapes may run in either
117 devices run in fixed block-size mode, where most nine-track tapes and
118 many new cartridge formats allow variable block-size.
119 The difference between the two is as follows:
121 .It Variable block-size:
122 Each write made to the device results in a single logical record
124 One can never read or write
126 of a record from tape (though you may request a larger block and read
127 a smaller record); nor can one read multiple blocks.
128 Data from a single write is therefore read by a single read.
130 may be any value supported by the device, the
132 adapter and the system (usually between 1 byte and 64 Kbytes,
135 When reading a variable record/block from the tape, the head is
136 logically considered to be immediately after the last item read,
137 and before the next item after that.
138 If the next item is a file mark,
139 but it was never read, then the next
140 process to read will immediately hit the file mark and receive an end-of-file notification.
141 .It Fixed block-size:
142 Data written by the user is passed to the tape as a succession of
144 It may be contiguous in memory, but it is
145 considered to be a series of independent blocks.
147 an amount of data that is not an exact multiple of the blocksize.
148 One may read and write the same data as a different set of records.
149 In other words, blocks that were written together may be read separately,
152 If one requests more blocks than remain in the file, the drive will
153 encounter the file mark.
154 As there is some data to return (unless
155 there were no records before the file mark), the read will succeed,
157 The next read will return immediately with a value
159 (As above, if the file mark is never read, it remains for the next
160 process to read if in no-rewind mode.)
163 By default, the driver will NOT accept reads or writes to a tape device that
164 are larger than may be written to or read from the mounted tape using a single
165 write or read request.
166 Because of this, the application author may have confidence that his wishes
167 are respected in terms of the block size written to tape.
168 For example, if the user tries to write a 256KB block to the tape, but the
169 controller can handle no more than 128KB, the write will fail.
175 was to break up large reads or writes into smaller blocks when going to the
177 The problem with that behavior, though, is that it hides the actual on-tape
178 block size from the application writer, at least in variable block mode.
180 If the user would like his large reads and writes broken up into separate
181 pieces, he may set the following loader tunables.
182 Note that these tunables WILL GO AWAY in
184 They are provided for transition purposes only.
186 .It kern.cam.sa.allow_io_split
188 This variable, when set to 1, will configure all
190 devices to split large buffers into smaller pieces when needed.
191 .It kern.cam.sa.%d.allow_io_split
193 This variable, when set to 1, will configure the given
195 unit to split large buffers into multiple pieces.
196 This will override the global setting, if it exists.
201 variables available to view block handling parameters:
203 .It kern.cam.sa.%d.allow_io_split
205 This variable allows the user to see, but not modify, the current I/O split
207 The user is not permitted to modify this setting so that there is no chance
208 of behavior changing for the application while a tape is mounted.
209 .It kern.cam.sa.%d.maxio
211 This variable shows the maximum I/O size in bytes that is allowed by the
212 combination of kernel tuning parameters (MAXPHYS, DFLTPHYS) and the
213 capabilities of the controller that is attached to the tape drive.
214 Applications may look at this value for a guide on how large an I/O may be
215 permitted, but should keep in mind that the actual maximum may be
216 restricted further by the tape drive via the
218 READ BLOCK LIMITS command.
219 .It kern.cam.sa.%d.cpi_maxio
221 This variable shows the maximum I/O size supported by the controller, in
222 bytes, that is reported via the CAM Path Inquiry CCB (XPT_PATH_INQ).
223 If this is 0, that means that the controller has not reported a maximum I/O
226 .Sh FILE MARK HANDLING
227 The handling of file marks on write is automatic.
229 written to the tape, and has not done a read since the last write,
230 then a file mark will be written to the tape when the device is
232 If a rewind is requested after a write, then the driver
233 assumes that the last file on the tape has been written, and ensures
234 that there are two file marks written to the tape.
236 this is that there seems to be a standard (which we follow, but do not
237 understand why) that certain types of tape do not actually write two
238 file marks to tape, but when read, report a `phantom' file mark when the
240 These devices include the QIC family of devices.
241 (It might be that this set of devices is the same set as that of fixed
243 This has not been determined yet, and they are treated
244 as separate behaviors by the driver at this time.)
248 driver supports all of the ioctls of
251 .Bl -tag -width /dev/[n][e]sa[0-9] -compact
252 .It Pa /dev/[n][e]sa[0-9]
259 Eject on close (if capable)
261 Control mode device (to examine state while another program is
262 accessing the device, e.g.).
273 driver was written for the
280 Many ideas were gleaned from the
282 device driver written and ported from
286 .An Julian Elischer .
288 The owner of record for many years was
290 The current maintainer is
293 This driver lacks many of the hacks required to deal with older devices.
296 devices may not work properly with this driver yet.
298 Additionally, certain
299 tapes (QIC tapes mostly) that were written under
302 are not automatically read correctly with this driver: you may need to
303 explicitly set variable block mode or set to the blocksize that works best
304 for your device in order to read tapes written under
308 Partitions are only supported for status information and location.
309 It would be nice to add support for creating and editing tape partitions.
projects / FreeBSD / stable / 10.git / blob