1.\" Copyright (c) 1996 2.\" Julian Elischer <julian@FreeBSD.org>. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 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 . 218