2 .\" Julian Elischer <julian@FreeBSD.org>. All rights reserved.
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28 .Dd September 10, 2022
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.
56 Many direct access devices are equipped with read and/or write caches.
57 Parameters affecting the device's cache are stored in mode page 8,
58 the caching control page.
59 Mode pages can be examined and modified via the
63 The read cache is used to store data from device-initiated read ahead
64 operations as well as frequently used data.
65 The read cache is transparent
66 to the user and can be enabled without any adverse effect.
68 with a read cache come from the factory with it enabled.
69 The read cache can be disabled by setting the
71 (Read Cache Disable) bit in the caching control mode page.
73 The write cache can greatly decrease the latency of write operations
74 and allows the device to reorganize writes to increase efficiency and
76 This performance gain comes at a price.
78 lose power while its cache contains uncommitted write operations, these
80 The effect of a loss of write transactions on
81 a file system is non-deterministic and can cause corruption.
83 devices age write transactions to limit vulnerability to a few transactions
84 recently reported as complete, but it is none-the-less recommended that
85 systems with write cache enabled devices reside on an Uninterruptible
89 device driver ensures that the cache and media are synchronized upon
90 final close of the device or an unexpected shutdown (panic) event.
91 This ensures that it is safe to disconnect power once the operating system
92 has reported that it has halted.
93 The write cache can be enabled by setting the
95 (Write Cache Enable) bit in the caching control mode page.
99 device driver will take full advantage of the SCSI feature known as tagged
101 Tagged queueing allows the device to process multiple transactions
102 concurrently, often re-ordering them to reduce the number and length of
104 To ensure that transactions to distant portions of the media,
105 which may be deferred indefinitely by servicing requests nearer the current
106 head position, are completed in a timely fashion, an ordered tagged
107 transaction is sent every 15 seconds during continuous device operation.
108 .Sh BAD BLOCK RECOVERY
109 Direct Access devices have the capability of mapping out portions of
111 Media recovery parameters are located in mode page 1,
112 the Read-Write Error Recovery mode page.
113 The most important media
114 remapping features are 'Auto Write Reallocation' and 'Auto Read
115 Reallocation' which can be enabled via the AWRE and ARRE bits,
116 respectively, of the Read-Write Error Recovery page.
117 Many devices do not ship from the factory with these feature enabled.
118 Mode pages can be examined and modified
122 .Sh KERNEL CONFIGURATION
123 It is only necessary to explicitly configure one
125 device; data structures are dynamically allocated as disks are found
130 The following variables are available as both
136 .It Va kern.cam.da.default_softtimeout
137 Soft I/O timeout (ms). The default is 0.
138 Attempting to set this value to a
139 time span longer than
140 .It kern.cam.da.default_timeout
143 .It Va kern.cam.da.default_timeout
144 This variable determines how long the
146 driver will wait before timing out an outstanding command.
147 The units for this value are seconds, and the default is currently 60
149 .It Va kern.cam.da.disable_wp_protection
150 Disable detection of write-protected disks.
153 detection of write-protected disks is enabled
155 .It Va kern.cam.da.enable_biospeedup
160 .It Va kern.cam.da.enable_uma_ccbs
163 .It Va kern.cam.da.poll_period
164 Media polling period in seconds.
165 Default is 3 seconds.
166 .It Va kern.cam.da.retry_count
167 This variable determines how many times the
169 driver will retry a READ or WRITE command.
170 This does not affect the number of retries used during probe time or for
174 This value currently defaults to 4.
175 .It Va kern.cam.da.send_ordered
177 On shutdown, step through all the
179 peripheral drivers, and if the device is still open,
180 sync the disk to physical media.
182 .It Va kern.cam.sort_io_queue
183 .It Va kern.cam.da . Ns Ar X Ns Va .sort_io_queue
184 These variables determine whether request queue should be sorted trying
185 to optimize head seeks.
186 Set to 1 to enable sorting, 0 to disable, -1 to leave it as-is.
187 The default is sorting enabled for HDDs and disabled for SSDs.
188 .It Va kern.cam.da . Ns Ar X Ns Va .delete_method
189 This variable specifies method to handle BIO_DELETE requests:
190 .Bl -tag -width "ATA_TRIM"
192 ATA TRIM via ATA COMMAND PASS THROUGH command,
196 WRITE SAME(16) command with UNMAP flag,
198 WRITE SAME(10) command with UNMAP flag,
200 WRITE SAME(10) command without UNMAP flag,
202 disable BIO_DELETE support.
204 .It Va kern.cam.da . Ns Ar X Ns Va .minimum_cmd_size
205 This variable determines what the minimum READ/WRITE CDB size is for a
209 Valid minimum command size values are 6, 10, 12 and 16 bytes.
210 The default is 6 bytes.
214 driver issues a CAM Path Inquiry CCB at probe time to determine whether the
215 protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
219 driver will default to using at least 10 byte CDBs.
220 If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
222 driver will then increase the default CDB size for the device to 10 bytes and
225 chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
226 command size, and the LBA and length of the READ or WRITE in question.
227 (e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
230 If a device becomes invalidated (media is removed, device becomes unresponsive)
231 the disklabel and information held within the kernel about the device will
233 To avoid corruption of a newly inserted piece of media or
234 a replacement device, all accesses to the device will be discarded until
235 the last file descriptor referencing the old device is closed.
236 During this period, all new open attempts will be rejected.
238 .Bl -tag -width ".Pa /dev/da*" -compact
240 SCSI disk device nodes
253 driver was written for the
257 .An Justin T. Gibbs .
258 Many ideas were gleaned from the
260 device driver written and ported from
264 .An Julian Elischer .