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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.
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 kern.cam.da.retry_count
138 This variable determines how many times the
140 driver will retry a READ or WRITE command.
141 This does not affect the number of retries used during probe time or for
145 This value currently defaults to 4.
146 .It kern.cam.da.default_timeout
148 This variable determines how long the
150 driver will wait before timing out an outstanding command.
151 The units for this value are seconds, and the default is currently 60
153 .It Va kern.cam.sort_io_queue
154 .It Va kern.cam.da. Ns Ar X Ns Va .sort_io_queue
156 These variables determine whether request queue should be sorted trying
157 to optimize head seeks.
158 Set to 1 to enable sorting, 0 to disable, -1 to leave it as-is.
159 The default is sorting enabled for HDDs and disabled for SSDs.
160 .It kern.cam.da.%d.minimum_cmd_size
162 This variable determines what the minimum READ/WRITE CDB size is for a
166 (The %d above denotes the unit number of the
168 driver instance, e.g.\& 1, 2, 4, 8, etc.)
169 Valid minimum command size values are 6, 10, 12 and 16 bytes.
170 The default is 6 bytes.
174 driver issues a CAM Path Inquiry CCB at probe time to determine whether the
175 protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
179 driver will default to using at least 10 byte CDBs.
180 If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
182 driver will then increase the default CDB size for the device to 10 bytes and
185 chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
186 command size, and the LBA and length of the READ or WRITE in question.
187 (e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
190 If a device becomes invalidated (media is removed, device becomes unresponsive)
191 the disklabel and information held within the kernel about the device will
193 To avoid corruption of a newly inserted piece of media or
194 a replacement device, all accesses to the device will be discarded until
195 the last file descriptor referencing the old device is closed.
196 During this period, all new open attempts will be rejected.
198 .Bl -tag -width ".Pa /dev/da*" -compact
200 SCSI disk device nodes
213 driver was written for the
217 .An Justin T. Gibbs .
218 Many ideas were gleaned from the
220 device driver written and ported from
224 .An Julian Elischer .