/share/man/man4/da.4

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 26.\" $FreeBSD$
 27.\"
 28.Dd February 8, 2012
 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
 37The
 38.Nm
 39driver provides support for all
 40.Tn SCSI
 41devices of the direct access class that are attached to the system
 42through a supported
 43.Tn SCSI
 44Host Adapter.
 45The direct access class includes disk, magneto-optical,
 46and solid-state devices.
 47.Pp
 48A
 49.Tn SCSI
 50Host
 51adapter must also be separately configured into the system
 52before a
 53.Tn SCSI
 54direct access device can be configured.
 55.Sh CACHE EFFECTS
 56Many direct access devices are equipped with read and/or write caches.
 57Parameters affecting the device's cache are stored in mode page 8,
 58the caching control page.
 59Mode pages can be examined and modified via the
 60.Xr camcontrol 8
 61utility.
 62.Pp
 63The read cache is used to store data from device-initiated read ahead
 64operations as well as frequently used data.
 65The read cache is transparent
 66to the user and can be enabled without any adverse effect.
 67Most devices
 68with a read cache come from the factory with it enabled.
 69The read cache can be disabled by setting the
 70.Tn RCD
 71(Read Cache Disable) bit in the caching control mode page.
 72.Pp
 73The write cache can greatly decrease the latency of write operations
 74and allows the device to reorganize writes to increase efficiency and
 75performance.
 76This performance gain comes at a price.
 77Should the device
 78lose power while its cache contains uncommitted write operations, these
 79writes will be lost.
 80The effect of a loss of write transactions on
 81a file system is non-deterministic and can cause corruption.
 82Most
 83devices age write transactions to limit vulnerability to a few transactions
 84recently reported as complete, but it is none-the-less recommended that
 85systems with write cache enabled devices reside on an Uninterruptible
 86Power Supply (UPS).
 87The
 88.Nm
 89device driver ensures that the cache and media are synchronized upon
 90final close of the device or an unexpected shutdown (panic) event.
 91This ensures that it is safe to disconnect power once the operating system
 92has reported that it has halted.
 93The 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
 97The
 98.Nm
 99device driver will take full advantage of the SCSI feature known as tagged
100queueing.
101Tagged queueing allows the device to process multiple transactions
102concurrently, often re-ordering them to reduce the number and length of
103seeks.
104To ensure that transactions to distant portions of the media,
105which may be deferred indefinitely by servicing requests nearer the current
106head position, are completed in a timely fashion, an ordered tagged
107transaction is sent every 15 seconds during continuous device operation.
108.Sh BAD BLOCK RECOVERY
109Direct Access devices have the capability of mapping out portions of
110defective media.
111Media recovery parameters are located in mode page 1,
112the Read-Write Error Recovery mode page.
113The most important media
114remapping features are 'Auto Write Reallocation' and 'Auto Read
115Reallocation' which can be enabled via the AWRE and ARRE bits,
116respectively, of the Read-Write Error Recovery page.
117Many devices do not ship from the factory with these feature enabled.
118Mode pages can be examined and modified
119via the
120.Xr camcontrol 8
121utility.
122.Sh KERNEL CONFIGURATION
123It is only necessary to explicitly configure one
124.Nm
125device; data structures are dynamically allocated as disks are found
126on the
127.Tn SCSI
128bus.
129.Sh SYSCTL VARIABLES
130The following variables are available as both
131.Xr sysctl 8
132variables and
133.Xr loader 8
134tunables:
135.Bl -tag -width 12
136.It kern.cam.da.retry_count
137.Pp
138This variable determines how many times the
139.Nm
140driver will retry a READ or WRITE command.
141This does not affect the number of retries used during probe time or for
142the
143.Nm
144driver dump routine.
145This value currently defaults to 4.
146.It kern.cam.da.default_timeout
147.Pp
148This variable determines how long the
149.Nm
150driver will wait before timing out an outstanding command.
151The units for this value are seconds, and the default is currently 60
152seconds.
153.It kern.cam.da.%d.minimum_cmd_size
154.Pp
155This variable determines what the minimum READ/WRITE CDB size is for a
156given
157.Nm
158unit.
159(The %d above denotes the unit number of the
160.Nm
161driver instance, e.g.\& 1, 2, 4, 8, etc.)
162Valid minimum command size values are 6, 10, 12 and 16 bytes.
163The default is 6 bytes.
164.Pp
165The
166.Nm
167driver issues a CAM Path Inquiry CCB at probe time to determine whether the
168protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
1696 byte commands.
170If it does not, the
171.Nm
172driver will default to using at least 10 byte CDBs.
173If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
174.Nm
175driver will then increase the default CDB size for the device to 10 bytes and
176retry the command.
177CDB size is always
178chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
179command 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
183If a device becomes invalidated (media is removed, device becomes unresponsive)
184the disklabel and information held within the kernel about the device will
185be invalidated.
186To avoid corruption of a newly inserted piece of media or
187a replacement device, all accesses to the device will be discarded until
188the last file descriptor referencing the old device is closed.
189During 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*
193SCSI disk device nodes
194.El
195.Sh DIAGNOSTICS
196None.
197.Sh SEE ALSO
198.Xr ada 4 ,
199.Xr cam 4 ,
200.Xr geom 4 ,
201.Xr bsdlabel 8 ,
202.Xr fdisk 8
203.Sh HISTORY
204The
205.Nm
206driver was written for the
207.Tn CAM
208.Tn SCSI
209subsystem by
210.An Justin T. Gibbs .
211Many ideas were gleaned from the
212.Nm sd
213device driver written and ported from
214.Tn Mach
2152.5
216by
217.An Julian Elischer .