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 September 10, 2022 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 Va kern.cam.da.default_timeout 137This variable determines how long the 138.Nm 139driver will wait before timing out an outstanding command. 140The units for this value are seconds, and the default is currently 60 141seconds. 142.It Va kern.cam.da.disable_wp_protection 143Disable detection of write-protected disks. 144Default is disabled. 145.Po 146detection of write-protected disks is enabled 147.Pc . 148.It Va kern.cam.da.enable_biospeedup 149Enable 150.Dv BIO_SPEEDUP 151processing. 152Default is enabled. 153.It Va kern.cam.da.enable_uma_ccbs 154Use UMA for CCBs. 155Default is enabled. 156.It Va kern.cam.da.poll_period 157Media polling period in seconds. 158Default is 3 seconds. 159.It Va kern.cam.da.retry_count 160This variable determines how many times the 161.Nm 162driver will retry a READ or WRITE command. 163This does not affect the number of retries used during probe time or for 164the 165.Nm 166driver dump routine. 167This value currently defaults to 4. 168.It Va kern.cam.da.send_ordered 169Send Ordered Tags. 170On shutdown, step through all the 171.Nm 172peripheral drivers, and if the device is still open, 173sync the disk to physical media. 174Default is enabled. 175.It Va kern.cam.sort_io_queue 176.It Va kern.cam.da . Ns Ar X Ns Va .sort_io_queue 177These variables determine whether request queue should be sorted trying 178to optimize head seeks. 179Set to 1 to enable sorting, 0 to disable, -1 to leave it as-is. 180The default is sorting enabled for HDDs and disabled for SSDs. 181.It Va kern.cam.da . Ns Ar X Ns Va .delete_method 182This variable specifies method to handle BIO_DELETE requests: 183.Bl -tag -width "ATA_TRIM" 184.It ATA_TRIM 185ATA TRIM via ATA COMMAND PASS THROUGH command, 186.It UNMAP 187UNMAP command, 188.It WS16 189WRITE SAME(16) command with UNMAP flag, 190.It WS10 191WRITE SAME(10) command with UNMAP flag, 192.It ZERO 193WRITE SAME(10) command without UNMAP flag, 194.It DISABLE 195disable BIO_DELETE support. 196.El 197.It Va kern.cam.da . Ns Ar X Ns Va .minimum_cmd_size 198This variable determines what the minimum READ/WRITE CDB size is for a 199given 200.Nm 201unit. 202Valid minimum command size values are 6, 10, 12 and 16 bytes. 203The default is 6 bytes. 204.Pp 205The 206.Nm 207driver issues a CAM Path Inquiry CCB at probe time to determine whether the 208protocol the device in question speaks (e.g.\& ATAPI) typically does not allow 2096 byte commands. 210If it does not, the 211.Nm 212driver will default to using at least 10 byte CDBs. 213If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the 214.Nm 215driver will then increase the default CDB size for the device to 10 bytes and 216retry the command. 217CDB size is always 218chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum 219command size, and the LBA and length of the READ or WRITE in question. 220(e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.) 221.El 222.Sh NOTES 223If a device becomes invalidated (media is removed, device becomes unresponsive) 224the disklabel and information held within the kernel about the device will 225be invalidated. 226To avoid corruption of a newly inserted piece of media or 227a replacement device, all accesses to the device will be discarded until 228the last file descriptor referencing the old device is closed. 229During this period, all new open attempts will be rejected. 230.Sh FILES 231.Bl -tag -width ".Pa /dev/da*" -compact 232.It Pa /dev/da* 233SCSI disk device nodes 234.El 235.Sh DIAGNOSTICS 236None. 237.Sh SEE ALSO 238.Xr ada 4 , 239.Xr cam 4 , 240.Xr geom 4 , 241.Xr nda 4 , 242.Xr gpart 8 243.Sh HISTORY 244The 245.Nm 246driver was written for the 247.Tn CAM 248.Tn SCSI 249subsystem by 250.An Justin T. Gibbs . 251Many ideas were gleaned from the 252.Nm sd 253device driver written and ported from 254.Tn Mach 2552.5 256by 257.An Julian Elischer . 258