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33 .Nd SCSI Sequential Access Device Driver
36 .Cd device sa1 at scbus0 target 4 unit 0
40 driver provides support for all
42 devices of the sequential access class that are attached to the system
46 The sequential access class includes tape and other linear access devices.
51 adapter must also be separately configured into the system
54 sequential access device can be configured.
58 driver is based around the concept of a
59 .Dq Em mount session ,
60 which is defined as the period between the time that a tape is
61 mounted, and the time when it is unmounted. Any parameters set during
62 a mount session remain in effect for the remainder of the session or
63 until replaced. The tape can be unmounted, bringing the session to a
64 close in several ways. These include:
67 Closing a `rewind device',
68 referred to as sub-mode 00 below. An example is
73 command, reachable through the
79 It should be noted that tape devices are exclusive open devices, except in
80 the case where a control mode device is opened. In the latter case, exclusive
81 access is only sought when needed (e.g., to set parameters).
83 Bits 0 and 1 of the minor number are interpreted as
85 The sub-modes differ in the action taken when the device is closed:
88 A close will rewind the device; if the tape has been
89 written, then a file mark will be written before the rewind is requested.
90 The device is unmounted.
92 A close will leave the tape mounted.
93 If the tape was written to, a file mark will be written.
94 No other head positioning takes place.
95 Any further reads or writes will occur directly after the
96 last read, or the written file mark.
98 A close will rewind the device. If the tape has been
99 written, then a file mark will be written before the rewind is requested.
100 On completion of the rewind an unload command will be issued.
101 The device is unmounted.
104 tapes may run in either
108 block-size modes. Most
110 devices run in fixed block-size mode, where most nine-track tapes and
111 many new cartridge formats allow variable block-size. The difference
112 between the two is as follows:
114 .It Variable block-size:
115 Each write made to the device results in a single logical record
116 written to the tape. One can never read or write
118 of a record from tape (though you may request a larger block and read
119 a smaller record); nor can one read multiple blocks. Data from a
120 single write is therefore read by a single read. The block size used
121 may be any value supported by the device, the
123 adapter and the system (usually between 1 byte and 64 Kbytes,
126 When reading a variable record/block from the tape, the head is
127 logically considered to be immediately after the last item read,
128 and before the next item after that. If the next item is a file mark,
129 but it was never read, then the next
130 process to read will immediately hit the file mark and receive an end-of-file notification.
131 .It Fixed block-size:
132 Data written by the user is passed to the tape as a succession of
133 fixed size blocks. It may be contiguous in memory, but it is
134 considered to be a series of independent blocks. One may never write
135 an amount of data that is not an exact multiple of the blocksize. One
136 may read and write the same data as a different set of records, In
137 other words, blocks that were written together may be read separately,
140 If one requests more blocks than remain in the file, the drive will
141 encounter the file mark. Because there is some data to return (unless
142 there were no records before the file mark), the read will succeed,
143 returning that data, The next read will return immediately with a value
144 of 0. (As above, if the file mark is never read, it remains for the next
145 process to read if in no-rewind mode.)
147 .Sh FILE MARK HANDLING
148 The handling of file marks on write is automatic. If the user has
149 written to the tape, and has not done a read since the last write,
150 then a file mark will be written to the tape when the device is
151 closed. If a rewind is requested after a write, then the driver
152 assumes that the last file on the tape has been written, and ensures
153 that there are two file marks written to the tape. The exception to
154 this is that there seems to be a standard (which we follow, but don't
155 understand why) that certain types of tape do not actually write two
156 file marks to tape, but when read, report a `phantom' file mark when the
157 last file is read. These devices include the QIC family of devices.
158 (It might be that this set of devices is the same set as that of fixed
159 block devices. This has not been determined yet, and they are treated
160 as separate behaviors by the driver at this time.)
164 driver supports all of the ioctls of
167 .Bl -tag -width /dev/[n][e]rsa[0-9] -compact
168 .It Pa /dev/[n][e]rsa[0-9]
175 Eject on close (if capable)
177 Control mode device (to examine state while another program is
178 accessing the device, e.g.).
180 This driver lacks many of the hacks required to deal with older devices.
183 devices may not work properly with this driver yet.
185 Additionally, certain
186 tapes (QIC tapes mostly) that were written under FreeBSD 2.X
187 aren't automatically read correctly with this driver: you may need to
188 explicitly set variable block mode or set to the blocksize that works best
189 for your device in order to read tapes written under FreeBSD 2.X.
191 Fine grained density and compression mode support that is bound to specific
192 device names needs to be added.
194 Support for fast indexing by use of partitions is missing.
203 driver was written for the
206 subsystem by Justin T. Gibbs and Kenneth Merry.
207 Many ideas were gleaned from the
209 device driver written and ported from
214 The current owner of record is Matthew Jacob who has suffered too many
215 years of breaking tape drivers.
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