225 lines
7.4 KiB
Groff
225 lines
7.4 KiB
Groff
.\" Copyright (c) 1996
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.\" Julian Elischer <julian@FreeBSD.org>. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\"
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 22, 2014
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.Dt DA 4
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.Os
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.Sh NAME
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.Nm da
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.Nd SCSI Direct Access device driver
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.Sh SYNOPSIS
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.Cd device da
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.Sh DESCRIPTION
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The
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.Nm
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driver provides support for all
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.Tn SCSI
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devices of the direct access class that are attached to the system
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through a supported
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.Tn SCSI
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Host Adapter.
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The direct access class includes disk, magneto-optical,
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and solid-state devices.
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.Pp
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A
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.Tn SCSI
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Host
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adapter must also be separately configured into the system
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before a
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.Tn SCSI
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direct access device can be configured.
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.Sh CACHE EFFECTS
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Many direct access devices are equipped with read and/or write caches.
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Parameters affecting the device's cache are stored in mode page 8,
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the caching control page.
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Mode pages can be examined and modified via the
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.Xr camcontrol 8
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utility.
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.Pp
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The read cache is used to store data from device-initiated read ahead
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operations as well as frequently used data.
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The read cache is transparent
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to the user and can be enabled without any adverse effect.
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Most devices
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with a read cache come from the factory with it enabled.
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The read cache can be disabled by setting the
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.Tn RCD
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(Read Cache Disable) bit in the caching control mode page.
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.Pp
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The write cache can greatly decrease the latency of write operations
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and allows the device to reorganize writes to increase efficiency and
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performance.
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This performance gain comes at a price.
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Should the device
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lose power while its cache contains uncommitted write operations, these
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writes will be lost.
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The effect of a loss of write transactions on
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a file system is non-deterministic and can cause corruption.
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Most
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devices age write transactions to limit vulnerability to a few transactions
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recently reported as complete, but it is none-the-less recommended that
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systems with write cache enabled devices reside on an Uninterruptible
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Power Supply (UPS).
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The
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.Nm
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device driver ensures that the cache and media are synchronized upon
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final close of the device or an unexpected shutdown (panic) event.
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This ensures that it is safe to disconnect power once the operating system
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has reported that it has halted.
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The write cache can be enabled by setting the
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.Tn WCE
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(Write Cache Enable) bit in the caching control mode page.
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.Sh TAGGED QUEUING
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The
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.Nm
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device driver will take full advantage of the SCSI feature known as tagged
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queueing.
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Tagged queueing allows the device to process multiple transactions
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concurrently, often re-ordering them to reduce the number and length of
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seeks.
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To ensure that transactions to distant portions of the media,
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which may be deferred indefinitely by servicing requests nearer the current
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head position, are completed in a timely fashion, an ordered tagged
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transaction is sent every 15 seconds during continuous device operation.
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.Sh BAD BLOCK RECOVERY
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Direct Access devices have the capability of mapping out portions of
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defective media.
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Media recovery parameters are located in mode page 1,
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the Read-Write Error Recovery mode page.
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The most important media
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remapping features are 'Auto Write Reallocation' and 'Auto Read
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Reallocation' which can be enabled via the AWRE and ARRE bits,
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respectively, of the Read-Write Error Recovery page.
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Many devices do not ship from the factory with these feature enabled.
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Mode pages can be examined and modified
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via the
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.Xr camcontrol 8
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utility.
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.Sh KERNEL CONFIGURATION
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It is only necessary to explicitly configure one
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.Nm
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device; data structures are dynamically allocated as disks are found
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on the
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.Tn SCSI
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bus.
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.Sh SYSCTL VARIABLES
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The following variables are available as both
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.Xr sysctl 8
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variables and
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.Xr loader 8
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tunables:
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.Bl -tag -width 12
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.It kern.cam.da.retry_count
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.Pp
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This variable determines how many times the
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.Nm
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driver will retry a READ or WRITE command.
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This does not affect the number of retries used during probe time or for
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the
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.Nm
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driver dump routine.
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This value currently defaults to 4.
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.It kern.cam.da.default_timeout
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.Pp
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This variable determines how long the
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.Nm
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driver will wait before timing out an outstanding command.
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The units for this value are seconds, and the default is currently 60
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seconds.
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.It Va kern.cam.sort_io_queue
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.It Va kern.cam.da. Ns Ar X Ns Va .sort_io_queue
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.Pp
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These variables determine whether request queue should be sorted trying
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to optimize head seeks.
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Set to 1 to enable sorting, 0 to disable, -1 to leave it as-is.
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The default is sorting enabled for HDDs and disabled for SSDs.
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.It kern.cam.da.%d.minimum_cmd_size
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.Pp
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This variable determines what the minimum READ/WRITE CDB size is for a
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given
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.Nm
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unit.
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(The %d above denotes the unit number of the
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.Nm
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driver instance, e.g.\& 1, 2, 4, 8, etc.)
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Valid minimum command size values are 6, 10, 12 and 16 bytes.
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The default is 6 bytes.
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.Pp
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The
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.Nm
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driver issues a CAM Path Inquiry CCB at probe time to determine whether the
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protocol the device in question speaks (e.g.\& ATAPI) typically does not allow
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6 byte commands.
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If it does not, the
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.Nm
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driver will default to using at least 10 byte CDBs.
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If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
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.Nm
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driver will then increase the default CDB size for the device to 10 bytes and
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retry the command.
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CDB size is always
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chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
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command size, and the LBA and length of the READ or WRITE in question.
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(e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
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.El
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.Sh NOTES
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If a device becomes invalidated (media is removed, device becomes unresponsive)
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the disklabel and information held within the kernel about the device will
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be invalidated.
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To avoid corruption of a newly inserted piece of media or
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a replacement device, all accesses to the device will be discarded until
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the last file descriptor referencing the old device is closed.
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During this period, all new open attempts will be rejected.
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.Sh FILES
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.Bl -tag -width ".Pa /dev/da*" -compact
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.It Pa /dev/da*
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SCSI disk device nodes
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.El
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.Sh DIAGNOSTICS
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None.
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.Sh SEE ALSO
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.Xr ada 4 ,
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.Xr cam 4 ,
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.Xr geom 4 ,
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.Xr bsdlabel 8 ,
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.Xr fdisk 8
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.Sh HISTORY
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The
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.Nm
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driver was written for the
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.Tn CAM
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.Tn SCSI
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subsystem by
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.An Justin T. Gibbs .
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Many ideas were gleaned from the
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.Nm sd
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device driver written and ported from
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.Tn Mach
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2.5
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by
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.An Julian Elischer .
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