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33 .Nd SCSI Direct Access device driver
39 driver provides support for all
41 devices of the direct access class that are attached to the system
45 The direct access class includes disk, magneto-optical,
46 and solid-state devices.
51 adapter must also be separately configured into the system
54 direct access device can be configured.
58 driver allows the disk to have two levels of partitioning.
61 is used to separate the
63 areas of the disk from areas used by other operating systems.
64 The second layer is the native
68 which is used to subdivide the
70 slices into areas for individual file systems and swap spaces.
71 For more information, see
77 If an uninitialized disk is opened, the slice table will be
78 initialized with a fictitious
80 slice spanning the entire disk.
81 Similarly, if an uninitialized
84 slice is opened, its disklabel will be initialized with parameters returned
85 by the drive and a single
87 partition encompassing the entire slice.
89 Many direct access devices are equipped with read and/or write caches.
90 Parameters affecting the device's cache are stored in mode page 8,
91 the caching control page.
92 Mode pages can be examined and modified via the
96 The read cache is used to store data from device-initiated read ahead
97 operations as well as frequently used data.
98 The read cache is transparent
99 to the user and can be enabled without any adverse effect.
101 with a read cache come from the factory with it enabled.
102 The read cache can be disabled by setting the
104 (Read Cache Disable) bit in the caching control mode page.
106 The write cache can greatly decrease the latency of write operations
107 and allows the device to reorganize writes to increase efficiency and
109 This performance gain comes at a price.
111 lose power while its cache contains uncommitted write operations, these
113 The effect of a loss of write transactions on
114 a file system is non-deterministic and can cause corruption.
116 devices age write transactions to limit vulnerability to a few transactions
117 recently reported as complete, but it is none-the-less recommended that
118 systems with write cache enabled devices reside on an Uninterruptible
122 device driver ensures that the cache and media are synchronized upon
123 final close of the device or an unexpected shutdown (panic) event.
124 This ensures that it is safe to disconnect power once the operating system
125 has reported that it has halted.
126 The write cache can be enabled by setting the
128 (Write Cache Enable) bit in the caching control mode page.
132 device driver will take full advantage of the SCSI feature known as tagged
134 Tagged queueing allows the device to process multiple transactions
135 concurrently, often re-ordering them to reduce the number and length of
137 To ensure that transactions to distant portions of the media,
138 which may be deferred indefinitely by servicing requests nearer the current
139 head position, are completed in a timely fashion, an ordered tagged
140 transaction is sent every 15 seconds during continuous device operation.
141 .Sh BAD BLOCK RECOVERY
142 Direct Access devices have the capability of mapping out portions of
144 Media recovery parameters are located in mode page 1,
145 the Read-Write Error Recovery mode page.
146 The most important media
147 remapping features are 'Auto Write Reallocation' and 'Auto Read
148 Reallocation' which can be enabled via the AWRE and ARRE bits,
149 respectively, of the Read-Write Error Recovery page.
150 Many devices do not ship from the factory with these feature enabled.
151 Mode pages can be examined and modified
155 .Sh KERNEL CONFIGURATION
156 It is only necessary to explicitly configure one
158 device; data structures are dynamically allocated as disks are found
167 disks as well as to other disks.
168 They are defined in the header file
169 .In sys/disklabel.h .
171 .Bl -tag -width DIOCSDINFO
173 Usually used to set up a bad-block mapping system on the disk.
175 drives incorporate their own bad-block mapping so this command is not
178 Read, from the kernel, the in-core copy of the disklabel for the
180 This may be a fictitious disklabel if the drive has never
181 been initialized, in which case it will contain information read
186 Give the driver a new disklabel to use.
190 disklabel to the disk.
192 Enable or disable the driver's software
193 write protect of the disklabel on the disk.
195 Give the driver a new disklabel to use.
198 write the new disklabel to the disk.
201 The following variables are available as both
207 .It kern.cam.da.retry_count
209 This variable determines how many times the
211 driver will retry a READ or WRITE command.
212 This does not affect the number of retries used during probe time or for
216 This value currently defaults to 4.
217 .It kern.cam.da.default_timeout
219 This variable determines how long the
221 driver will wait before timing out an outstanding command.
222 The units for this value are seconds, and the default is currently 60
224 .It kern.cam.da.%d.minimum_cmd_size
226 This variable determines what the minimum READ/WRITE CDB size is for a
230 (The %d above denotes the unit number of the
232 driver instance, e.g.\& 1, 2, 4, 8, etc.)
233 Valid minimum command size values are 6, 10, 12 and 16 bytes.
234 The default is 6 bytes.
238 driver issues a CAM Path Inquiry CCB at probe time to determine whether the
239 protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
243 driver will default to using at least 10 byte CDBs.
244 If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
246 driver will then increase the default CDB size for the device to 10 bytes and
249 chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
250 command size, and the LBA and length of the READ or WRITE in question.
251 (e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
254 If a device becomes invalidated (media is removed, device becomes unresponsive)
255 the disklabel and information held within the kernel about the device will
257 To avoid corruption of a newly inserted piece of media or
258 a replacement device, all accesses to the device will be discarded until
259 the last file descriptor referencing the old device is closed.
260 During this period, all new open attempts will be rejected.
262 .Bl -tag -width /dev/rsdXXXXX -compact
264 .It Pa /dev/da Ar u Pa s Ar n
272 accessed as an unpartitioned device
273 .It Pa /dev/da Ns Ar u Ns Ar p
309 driver was written for the
313 .An Justin T. Gibbs .
314 Many ideas were gleaned from the
316 device driver written and ported from
320 .An Julian Elischer .
321 Support for slices was written by