xref: /dragonfly/share/man/man4/da.4 (revision e1acdbad) 
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2.\"	Julian Elischer <julian@FreeBSD.org>.  All rights reserved.
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29.Dd October 15, 1998
30.Dt DA 4
31.Os
32.Sh NAME
33.Nm da
34.Nd SCSI Direct Access device driver
35.Sh SYNOPSIS
36.Cd device da
37.Cd device da1 at scbus0 target 4 unit 0
38.Sh DESCRIPTION
39The
40.Nm
41driver provides support for all
42.Tn SCSI
43devices of the direct access class that are attached to the system
44through a supported
45.Tn SCSI
46Host Adapter.
47The direct access class includes disk, magneto-optical,
48and solid-state devices.
49.Pp
50A
51.Tn SCSI
52Host
53adapter must also be separately configured into the system
54before a
55.Tn SCSI
56direct access device can be configured.
57.Sh PARTITIONING
58The
59.Nm
60driver allows the disk to have two levels of partitioning.
61One layer, called the
62.Dq slice layer ,
63is used to separate the
64.Dx
65areas of the disk from areas used by other operating systems.
66The second layer is the native
67.Bx 4.4
68partitioning scheme,
69.Xr disklabel 5 ,
70which is used to subdivide the
71.Dx
72slices into areas for individual filesystems and swap spaces.
73For more information, see
74.Xr fdisk 8
75and
76.Xr disklabel 8 ,
77respectively.
78.Pp
79If an uninitialized disk is opened, the slice table will be
80initialized with a fictitious
81.Dx
82slice spanning the entire disk.  Similarly, if an uninitialized
83(or
84.No non- Ns Dx )
85slice is opened, its disklabel will be initialized with parameters returned
86by the drive and a single
87.Sq Li c
88partition encompassing the entire slice.
89.Sh CACHE EFFECTS
90Many direct access devices are equipped with read and/or write caches.
91Parameters affecting the device's cache are stored in mode page 8,
92the caching control page.  Mode pages can be examined and modified
93via the
94.Xr camcontrol 8
95utility.
96.Pp
97The read cache is used to store data from device-initiated read ahead
98operations as well as frequently used data.  The read cache is transparent
99to the user and can be enabled without any adverse effect.  Most devices
100with a read cache come from the factory with it enabled.  The read cache
101can be disabled by setting the
102.Tn RCD
103(Read Cache Disable) bit in the caching control mode page.
104.Pp
105The write cache can greatly decrease the latency of write operations
106and allows the device to reorganize writes to increase efficiency and
107performance.  This performance gain comes at a price.  Should the device
108lose power while its cache contains uncommitted write operations, these
109writes will be lost.  The effect of a loss of write transactions on
110a file system is non-deterministic and can cause corruption.  Most
111devices age write transactions to limit vulnerability to a few transactions
112recently reported as complete, but it is none-the-less recommended that
113systems with write cache enabled devices reside on an Uninterruptible
114Power Supply (UPS).  The
115.Nm
116device driver ensures that the cache and media are synchronized upon
117final close of the device or an unexpected shutdown (panic) event.  This
118ensures that it is safe to disconnect power once the operating system
119has reported that it has halted.  The write cache can be enabled by
120setting the
121.Tn WCE
122(Write Cache Enable) bit in the caching control mode page.
123.Sh TAGGED QUEUING
124The
125.Nm
126device driver will take full advantage of the SCSI feature known as tagged
127queueing.  Tagged queueing allows the device to process multiple transactions
128concurrently, often re-ordering them to reduce the number and length of
129seeks.  To ensure that transactions to distant portions of the media,
130which may be deferred indefinitely by servicing requests nearer the current
131head position, are completed in a timely fashion, an ordered tagged
132transaction is sent every 15 seconds during continuous device operation.
133.Sh BAD BLOCK RECOVERY
134Direct Access devices have the capability of mapping out portions of
135defective media.  Media recovery parameters are located in mode page 1,
136the Read-Write Error Recovery mode page.  The most important media
137remapping features are 'Auto Write Reallocation' and 'Auto Read
138Reallocation' which can be enabled via the AWRE and ARRE bits,
139respectively, of the Read-Write Error Recovery page.
140Many devices do not ship from the factory with these feature enabled.
141Mode pages can be examined and modifie
142via the
143.Xr camcontrol 8
144utility.
145.Sh KERNEL CONFIGURATION
146It is only necessary to explicitly configure one
147.Nm
148device; data structures are dynamically allocated as disks are found
149on the
150.Tn SCSI
151bus.
152.Sh IOCTLS
153The following
154.Xr ioctl 2
155calls apply to
156.Tn SCSI
157disks as well as to other disks.  They are defined in the header file
158.Aq Pa sys/disklabel.h .
159.Pp
160.Bl -tag -width DIOCSDINFO
161.It Dv DIOCSBAD
162Usually used to set up a bad-block mapping system on the disk.
163.Tn SCSI
164drives incorporate their own bad-block mapping so this command is not
165implemented.
166.It Dv DIOCGDINFO
167Read, from the kernel, the in-core copy of the disklabel for the
168drive.
169This may be a fictitious disklabel if the drive has never
170been initialized, in which case it will contain information read
171from the
172.Tn SCSI
173inquiry commands.
174.It Dv DIOCSDINFO
175Give the driver a new disklabel to use.
176The driver
177.Em will not
178write the new
179disklabel to the disk.
180.It Dv DIOCWLABEL
181Enable or disable the driver's software
182write protect of the disklabel on the disk.
183.It Dv DIOCWDINFO
184Give the driver a new disklabel to use.
185The driver
186.Em will
187write the new disklabel to the disk.
188.El
189.Sh NOTES
190If a device becomes invalidated (media is removed, device becomes unresponsive)
191the disklabel and information held within the kernel about the device will
192be invalidated.  To avoid corruption of a newly inserted piece of media or
193a replacement device, all accesses to the device will be discarded until
194the last file descriptor referencing the old device is closed.  During this
195period, all new open attempts will be rejected.
196.Sh FILES
197.Bl -tag -width /dev/rsdXXXXX -compact
198.It Pa /dev/rda Ns Ar u
199raw mode
200.Tn SCSI
201disk unit
202.Ar u ,
203accessed as an unpartitioned device
204.Sm off
205.It Pa /dev/da Ar u Pa s Ar n
206.Sm on
207block mode
208.Tn SCSI
209disk unit
210.Ar u ,
211slice
212.Ar n ,
213accessed as an unpartitioned device
214.Sm off
215.It Pa /dev/rda Ar u Pa s Ar n
216.Sm on
217raw mode
218.Tn SCSI
219disk unit
220.Ar u ,
221slice
222.Ar n ,
223accessed as an unpartitioned device
224.It Pa /dev/da Ns Ar u Ns Ar p
225block mode
226.Tn SCSI
227disk unit
228.Ar u ,
229first
230.Dx
231slice, partition
232.Ar p
233.It Pa /dev/rda Ns Ar u Ns Ar p
234raw mode
235.Tn SCSI
236disk unit
237.Ar u ,
238first
239.Dx
240slice, partition
241.Ar p
242.Sm off
243.It Xo
244.Pa /dev/da
245.Ar u
246.Pa s
247.Ar n
248.Ar p
249.Xc
250.Sm on
251block mode
252.Tn SCSI
253disk unit
254.Ar u ,
255.Ar n Ns th
256slice, partition
257.Ar p
258.Sm off
259.It Xo
260.Pa /dev/rda
261.Ar u
262.Pa s
263.Ar n
264.Ar p
265.Xc
266.Sm on
267raw mode
268.Tn SCSI
269disk unit
270.Ar u ,
271.Ar n Ns th
272slice, partition
273.Ar p
274.El
275.Sh DIAGNOSTICS
276None.
277.Sh SEE ALSO
278.Xr ad 4 ,
279.Xr disklabel 5 ,
280.Xr disklabel 8 ,
281.Xr fdisk 8
282.Sh HISTORY
283The
284.Nm
285driver was written for the
286.Tn CAM
287.Tn SCSI
288subsystem by
289.An Justin T. Gibbs .
290Many ideas were gleaned from the
291.Nm sd
292device driver written and ported from
293.Tn Mach
2942.5
295by
296.An Julian Elischer .
297Support for slices was written by
298.An Bruce Evans .
299