158 lines
5.5 KiB
Groff
158 lines
5.5 KiB
Groff
.\" Copyright (c) 2009 Alexander Motin <mav@FreeBSD.org>
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.\" 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 February 21, 2014
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.Dt ADA 4
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.Os
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.Sh NAME
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.Nm ada
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.Nd ATA Direct Access device driver
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.Sh SYNOPSIS
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.Cd device ada
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.Sh DESCRIPTION
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The
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.Nm
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driver provides support for direct access devices, implementing the
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.Tn ATA
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command protocol, that are attached to the system through a host adapter
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supported by the CAM subsystem.
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.Pp
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The host adapter must also be separately configured into the system before an
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.Tn ATA
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direct access device can be configured.
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.Sh COMMAND QUEUING
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Command 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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.Tn ATA
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defines two types of queueing:
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.Tn TCQ (Tagged Command Queueing, PATA legacy)
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and
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.Tn NCQ (Native Command Queueing, SATA) .
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The
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.Nm
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device driver takes full advantage of NCQ, when supported.
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To ensure that transactions to distant parts of the media,
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which may be deferred indefinitely by servicing requests closer to the current
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head position, are completed in a timely fashion, an ordered
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transaction is sent every 7 seconds during continuous device operation.
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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 reported in device IDENTIFY data
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and 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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.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 the vulnerability to a few transactions
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recently reported as complete, but it is nonetheless 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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.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 Va kern.cam.ada.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 Va kern.cam.ada.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 30
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seconds.
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.It Va kern.cam.ada.spindown_shutdown
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.Pp
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This variable determines whether to spin-down disks when shutting down.
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Set to 1 to enable spin-down, 0 to disable.
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The default is currently enabled.
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.It Va kern.cam.ada.write_cache
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.It Va kern.cam.ada. Ns Ar X Ns Va .write_cache
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.Pp
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These variables determine whether device write cache should be enabled
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globally or per-device or disabled.
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Set to 1 to enable write cache, 0 to disable, -1 to leave it as-is.
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Values modified at runtime take effect only after device reset
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.Pq using the reset subcommand of Xr camcontrol 8 .
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Because of that, this setting should be changed in
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.Pa /boot/loader.conf
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instead of
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.Pa /etc/sysctl.conf .
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The global default is currently 1.
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The per-device default is to leave it as-is (follow global setting).
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.El
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.Sh FILES
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.Bl -tag -width ".Pa /dev/ada*" -compact
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.It Pa /dev/ada*
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ATA device nodes
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.El
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.Sh SEE ALSO
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.Xr ahci 4 ,
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.Xr cam 4 ,
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.Xr da 4 ,
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.Xr siis 4
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.Sh HISTORY
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The
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.Nm
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driver first appeared in
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.Fx 8.0 .
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.Sh AUTHORS
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.An Alexander Motin Aq Mt mav@FreeBSD.org
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