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1 .\" Copyright (c) 1996
2 .\" Julian Elischer <julian@FreeBSD.org>. All rights reserved.
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26 .\" $FreeBSD$
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28 .Dd September 2, 2003
29 .Dt DA 4
30 .Os
31 .Sh NAME
32 .Nm da
33 .Nd SCSI Direct Access device driver
34 .Sh SYNOPSIS
35 .Cd device da
36 .Sh DESCRIPTION
37 The
38 .Nm
39 driver provides support for all
40 .Tn SCSI
41 devices of the direct access class that are attached to the system
42 through a supported
43 .Tn SCSI
44 Host Adapter.
45 The direct access class includes disk, magneto-optical,
46 and solid-state devices.
47 .Pp
48 A
49 .Tn SCSI
50 Host
51 adapter must also be separately configured into the system
52 before a
53 .Tn SCSI
54 direct access device can be configured.
55 .Sh CACHE EFFECTS
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
60 .Xr camcontrol 8
61 utility.
62 .Pp
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.
67 Most devices
68 with a read cache come from the factory with it enabled.
69 The read cache can be disabled by setting the
70 .Tn RCD
71 (Read Cache Disable) bit in the caching control mode page.
72 .Pp
73 The write cache can greatly decrease the latency of write operations
74 and allows the device to reorganize writes to increase efficiency and
75 performance.
76 This performance gain comes at a price.
77 Should the device
78 lose power while its cache contains uncommitted write operations, these
79 writes will be lost.
80 The effect of a loss of write transactions on
81 a file system is non-deterministic and can cause corruption.
82 Most
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
86 Power Supply (UPS).
87 The
88 .Nm
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
94 .Tn WCE
95 (Write Cache Enable) bit in the caching control mode page.
96 .Sh TAGGED QUEUING
97 The
98 .Nm
99 device driver will take full advantage of the SCSI feature known as tagged
100 queueing.
101 Tagged queueing allows the device to process multiple transactions
102 concurrently, often re-ordering them to reduce the number and length of
103 seeks.
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
110 defective media.
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
119 via the
120 .Xr camcontrol 8
121 utility.
122 .Sh KERNEL CONFIGURATION
123 It is only necessary to explicitly configure one
124 .Nm
125 device; data structures are dynamically allocated as disks are found
126 on the
127 .Tn SCSI
128 bus.
129 .Sh SYSCTL VARIABLES
130 The following variables are available as both
131 .Xr sysctl 8
132 variables and
133 .Xr loader 8
134 tunables:
135 .Bl -tag -width 12
136 .It kern.cam.da.retry_count
137 .Pp
138 This variable determines how many times the
139 .Nm
140 driver will retry a READ or WRITE command.
141 This does not affect the number of retries used during probe time or for
142 the
143 .Nm
144 driver dump routine.
145 This value currently defaults to 4.
146 .It kern.cam.da.default_timeout
147 .Pp
148 This variable determines how long the
149 .Nm
150 driver will wait before timing out an outstanding command.
151 The units for this value are seconds, and the default is currently 60
152 seconds.
153 .It kern.cam.da.%d.minimum_cmd_size
154 .Pp
155 This variable determines what the minimum READ/WRITE CDB size is for a
156 given
157 .Nm
158 unit.
159 (The %d above denotes the unit number of the
160 .Nm
161 driver instance, e.g.\& 1, 2, 4, 8, etc.)
162 Valid minimum command size values are 6, 10, 12 and 16 bytes.
163 The default is 6 bytes.
164 .Pp
165 The
166 .Nm
167 driver issues a CAM Path Inquiry CCB at probe time to determine whether the
168 protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
169 6 byte commands.
170 If it does not, the
171 .Nm
172 driver will default to using at least 10 byte CDBs.
173 If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
174 .Nm
175 driver will then increase the default CDB size for the device to 10 bytes and
176 retry the command.
177 CDB size is always
178 chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
179 command size, and the LBA and length of the READ or WRITE in question.
180 (e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
181 .El
182 .Sh NOTES
183 If a device becomes invalidated (media is removed, device becomes unresponsive)
184 the disklabel and information held within the kernel about the device will
185 be invalidated.
186 To avoid corruption of a newly inserted piece of media or
187 a replacement device, all accesses to the device will be discarded until
188 the last file descriptor referencing the old device is closed.
189 During this period, all new open attempts will be rejected.
190 .Sh FILES
191 .Bl -tag -width ".Pa /dev/da*" -compact
192 .It Pa /dev/da*
193 SCSI disk device nodes
194 .El
195 .Sh DIAGNOSTICS
196 None.
197 .Sh SEE ALSO
198 .Xr ad 4 ,
199 .Xr geom 4 ,
200 .Xr bsdlabel 8 ,
201 .Xr fdisk 8
202 .Sh HISTORY
203 The
204 .Nm
205 driver was written for the
206 .Tn CAM
207 .Tn SCSI
208 subsystem by
209 .An Justin T. Gibbs .
210 Many ideas were gleaned from the
211 .Nm sd
212 device driver written and ported from
213 .Tn Mach
214 2.5
215 by
216 .An Julian Elischer .