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26 .\" $FreeBSD: src/share/man/man4/da.4,v 1.22.2.7 2001/10/01 13:07:23 dd Exp $
27 .\" $DragonFly: src/share/man/man4/da.4,v 1.5 2007/05/17 08:19:01 swildner Exp $
34 .Nd SCSI Direct Access device driver
37 .Cd device da1 at scbus0 target 4 unit 0
41 driver provides support for all
43 devices of the direct access class that are attached to the system
47 The direct access class includes disk, magneto-optical,
48 and solid-state devices.
53 adapter must also be separately configured into the system
56 direct access device can be configured.
60 driver allows the disk to have two levels of partitioning.
63 is used to separate the
65 areas of the disk from areas used by other operating systems.
66 The second layer is the native
70 which is used to subdivide the
72 slices into areas for individual filesystems and swap spaces.
73 For more information, see
79 If an uninitialized disk is opened, the slice table will be
80 initialized with a fictitious
82 slice spanning the entire disk. Similarly, if an uninitialized
85 slice is opened, its disklabel will be initialized with parameters returned
86 by the drive and a single
88 partition encompassing the entire slice.
90 Many direct access devices are equipped with read and/or write caches.
91 Parameters affecting the device's cache are stored in mode page 8,
92 the caching control page. Mode pages can be examined and modified
97 The read cache is used to store data from device-initiated read ahead
98 operations as well as frequently used data. The read cache is transparent
99 to the user and can be enabled without any adverse effect. Most devices
100 with a read cache come from the factory with it enabled. The read cache
101 can be disabled by setting the
103 (Read Cache Disable) bit in the caching control mode page.
105 The write cache can greatly decrease the latency of write operations
106 and allows the device to reorganize writes to increase efficiency and
107 performance. This performance gain comes at a price. Should the device
108 lose power while its cache contains uncommitted write operations, these
109 writes will be lost. The effect of a loss of write transactions on
110 a file system is non-deterministic and can cause corruption. Most
111 devices age write transactions to limit vulnerability to a few transactions
112 recently reported as complete, but it is none-the-less recommended that
113 systems with write cache enabled devices reside on an Uninterruptible
114 Power Supply (UPS). The
116 device driver ensures that the cache and media are synchronized upon
117 final close of the device or an unexpected shutdown (panic) event. This
118 ensures that it is safe to disconnect power once the operating system
119 has reported that it has halted. The write cache can be enabled by
122 (Write Cache Enable) bit in the caching control mode page.
126 device driver will take full advantage of the SCSI feature known as tagged
127 queueing. Tagged queueing allows the device to process multiple transactions
128 concurrently, often re-ordering them to reduce the number and length of
129 seeks. To ensure that transactions to distant portions of the media,
130 which may be deferred indefinitely by servicing requests nearer the current
131 head position, are completed in a timely fashion, an ordered tagged
132 transaction is sent every 15 seconds during continuous device operation.
133 .Sh BAD BLOCK RECOVERY
134 Direct Access devices have the capability of mapping out portions of
135 defective media. Media recovery parameters are located in mode page 1,
136 the Read-Write Error Recovery mode page. The most important media
137 remapping features are 'Auto Write Reallocation' and 'Auto Read
138 Reallocation' which can be enabled via the AWRE and ARRE bits,
139 respectively, of the Read-Write Error Recovery page.
140 Many devices do not ship from the factory with these feature enabled.
141 Mode pages can be examined and modified via the
144 .Sh KERNEL CONFIGURATION
145 It is only necessary to explicitly configure one
147 device; data structures are dynamically allocated as disks are found
156 disks as well as to other disks. They are defined in the header file
157 .In sys/disklabel.h .
159 .Bl -tag -width DIOCSDINFO
161 Usually used to set up a bad-block mapping system on the disk.
163 drives incorporate their own bad-block mapping so this command is not
166 Read, from the kernel, the in-core copy of the disklabel for the
168 This may be a fictitious disklabel if the drive has never
169 been initialized, in which case it will contain information read
174 Give the driver a new disklabel to use.
178 disklabel to the disk.
180 Enable or disable the driver's software
181 write protect of the disklabel on the disk.
183 Give the driver a new disklabel to use.
186 write the new disklabel to the disk.
189 If a device becomes invalidated (media is removed, device becomes unresponsive)
190 the disklabel and information held within the kernel about the device will
191 be invalidated. To avoid corruption of a newly inserted piece of media or
192 a replacement device, all accesses to the device will be discarded until
193 the last file descriptor referencing the old device is closed. During this
194 period, all new open attempts will be rejected.
196 .Bl -tag -width /dev/rsdXXXXX -compact
197 .It Pa /dev/rda Ns Ar u
202 accessed as an unpartitioned device
204 .It Pa /dev/da Ar u Pa s Ar n
212 accessed as an unpartitioned device
214 .It Pa /dev/rda Ar u Pa s Ar n
222 accessed as an unpartitioned device
223 .It Pa /dev/da Ns Ar u Ns Ar p
232 .It Pa /dev/rda Ns Ar u Ns Ar p
284 driver was written for the
288 .An Justin T. Gibbs .
289 Many ideas were gleaned from the
291 device driver written and ported from
295 .An Julian Elischer .
296 Support for slices was written by