xref: /freebsd/share/man/man4/da.4 (revision 6990ffd8a95caaba6858ad44ff1b3157d1efba8f)
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2.\"	Julian Elischer <julian@FreeBSD.org>.  All rights reserved.
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26.\" $FreeBSD$
27.\"
28.Dd October 15, 1998
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.Cd device da1 at scbus0 target 4 unit 0
37.Sh DESCRIPTION
38The
39.Nm
40driver provides support for all
41.Tn SCSI
42devices of the direct access class that are attached to the system
43through a supported
44.Tn SCSI
45Host Adapter.
46The direct access class includes disk, magneto-optical,
47and solid-state devices.
48.Pp
49A
50.Tn SCSI
51Host
52adapter must also be separately configured into the system
53before a
54.Tn SCSI
55direct access device can be configured.
56.Sh PARTITIONING
57The
58.Nm
59driver allows the disk to have two levels of partitioning.
60One layer, called the
61.Dq slice layer ,
62is used to separate the
63.Fx
64areas of the disk from areas used by other operating systems.
65The second layer is the native
66.Bx 4.4
67partitioning scheme,
68.Xr disklabel 5 ,
69which is used to subdivide the
70.Fx
71slices into areas for individual filesystems and swap spaces.
72For more information, see
73.Xr fdisk 8
74and
75.Xr disklabel 8 ,
76respectively.
77.Pp
78If an uninitialized disk is opened, the slice table will be
79initialized with a fictitious
80.Fx
81slice spanning the entire disk.  Similarly, if an uninitialized
82(or
83.No non- Ns Fx )
84slice is opened, its disklabel will be initialized with parameters returned
85by the drive and a single
86.Sq Li c
87partition encompassing the entire slice.
88.Sh CACHE EFFECTS
89Many direct access devices are equipped with read and/or write caches.
90Parameters affecting the device's cache are stored in mode page 8,
91the caching control page.  Mode pages can be examined and modified
92via the
93.Xr camcontrol 8
94utility.
95.Pp
96The read cache is used to store data from device-initiated read ahead
97operations as well as frequently used data.  The read cache is transparent
98to the user and can be enabled without any adverse effect.  Most devices
99with a read cache come from the factory with it enabled.  The read cache
100can be disabled by setting the
101.Tn RCD
102(Read Cache Disable) bit in the caching control mode page.
103.Pp
104The write cache can greatly decrease the latency of write operations
105and allows the device to reorganize writes to increase efficiency and
106performance.  This performance gain comes at a price.  Should the device
107lose power while its cache contains uncommitted write operations, these
108writes will be lost.  The effect of a loss of write transactions on
109a file system is non-deterministic and can cause corruption.  Most
110devices age write transactions to limit vulnerability to a few transactions
111recently reported as complete, but it is none-the-less recommended that
112systems with write cache enabled devices reside on an Uninterruptible
113Power Supply (UPS).  The
114.Nm
115device driver ensures that the cache and media are synchronized upon
116final close of the device or an unexpected shutdown (panic) event.  This
117ensures that it is safe to disconnect power once the operating system
118has reported that it has halted.  The write cache can be enabled by
119setting the
120.Tn WCE
121(Write Cache Enable) bit in the caching control mode page.
122.Sh TAGGED QUEUING
123The
124.Nm
125device driver will take full advantage of the SCSI feature known as tagged
126queueing.  Tagged queueing allows the device to process multiple transactions
127concurrently, often re-ordering them to reduce the number and length of
128seeks.  To ensure that transactions to distant portions of the media,
129which may be deferred indefinitely by servicing requests nearer the current
130head position, are completed in a timely fashion, an ordered tagged
131transaction is sent every 15 seconds during continuous device operation.
132.Sh BAD BLOCK RECOVERY
133Direct Access devices have the capability of mapping out portions of
134defective media.  Media recovery parameters are located in mode page 1,
135the Read-Write Error Recovery mode page.  The most important media
136remapping features are 'Auto Write Reallocation' and 'Auto Read
137Reallocation' which can be enabled via the AWRE and ARRE bits,
138respectively, of the Read-Write Error Recovery page.
139Many devices do not ship from the factory with these feature enabled.
140Mode pages can be examined and modifie
141via the
142.Xr camcontrol 8
143utility.
144.Sh KERNEL CONFIGURATION
145It is only necessary to explicitly configure one
146.Nm
147device; data structures are dynamically allocated as disks are found
148on the
149.Tn SCSI
150bus.
151.Sh IOCTLS
152The following
153.Xr ioctl 2
154calls apply to
155.Tn SCSI
156disks as well as to other disks.  They are defined in the header file
157.Aq Pa sys/disklabel.h .
158.Pp
159.Bl -tag -width DIOCSDINFO
160.It Dv DIOCSBAD
161Usually used to set up a bad-block mapping system on the disk.
162.Tn SCSI
163drives incorporate their own bad-block mapping so this command is not
164implemented.
165.It Dv DIOCGDINFO
166Read, from the kernel, the in-core copy of the disklabel for the
167drive.
168This may be a fictitious disklabel if the drive has never
169been initialized, in which case it will contain information read
170from the
171.Tn SCSI
172inquiry commands.
173.It Dv DIOCSDINFO
174Give the driver a new disklabel to use.
175The driver
176.Em will not
177write the new
178disklabel to the disk.
179.It Dv DIOCWLABEL
180Enable or disable the driver's software
181write protect of the disklabel on the disk.
182.It Dv DIOCWDINFO
183Give the driver a new disklabel to use.
184The driver
185.Em will
186write the new disklabel to the disk.
187.El
188.Sh NOTES
189If a device becomes invalidated (media is removed, device becomes unresponsive)
190the disklabel and information held within the kernel about the device will
191be invalidated.  To avoid corruption of a newly inserted piece of media or
192a replacement device, all accesses to the device will be discarded until
193the last file descriptor referencing the old device is closed.  During this
194period, all new open attempts will be rejected.
195.Sh FILES
196.Bl -tag -width /dev/rsdXXXXX -compact
197.It Pa /dev/rda Ns Ar u
198raw mode
199.Tn SCSI
200disk unit
201.Ar u ,
202accessed as an unpartitioned device
203.Sm off
204.It Pa /dev/da Ar u Pa s Ar n
205.Sm on
206block mode
207.Tn SCSI
208disk unit
209.Ar u ,
210slice
211.Ar n ,
212accessed as an unpartitioned device
213.Sm off
214.It Pa /dev/rda Ar u Pa s Ar n
215.Sm on
216raw mode
217.Tn SCSI
218disk unit
219.Ar u ,
220slice
221.Ar n ,
222accessed as an unpartitioned device
223.It Pa /dev/da Ns Ar u Ns Ar p
224block mode
225.Tn SCSI
226disk unit
227.Ar u ,
228first
229.Fx
230slice, partition
231.Ar p
232.It Pa /dev/rda Ns Ar u Ns Ar p
233raw mode
234.Tn SCSI
235disk unit
236.Ar u ,
237first
238.Fx
239slice, partition
240.Ar p
241.Sm off
242.It Xo
243.Pa /dev/da
244.Ar u
245.Pa s
246.Ar n
247.Ar p
248.Xc
249.Sm on
250block mode
251.Tn SCSI
252disk unit
253.Ar u ,
254.Ar n Ns th
255slice, partition
256.Ar p
257.Sm off
258.It Xo
259.Pa /dev/rda
260.Ar u
261.Pa s
262.Ar n
263.Ar p
264.Xc
265.Sm on
266raw mode
267.Tn SCSI
268disk unit
269.Ar u ,
270.Ar n Ns th
271slice, partition
272.Ar p
273.El
274.Sh DIAGNOSTICS
275None.
276.Sh SEE ALSO
277.Xr ad 4 ,
278.Xr disklabel 5 ,
279.Xr disklabel 8 ,
280.Xr fdisk 8
281.Sh HISTORY
282The
283.Nm
284driver was written for the
285.Tn CAM
286.Tn SCSI
287subsystem by
288.An Justin T. Gibbs .
289Many ideas were gleaned from the
290.Nm sd
291device driver written and ported from
292.Tn Mach
2932.5
294by
295.An Julian Elischer .
296Support for slices was written by
297.An Bruce Evans .
298