6028082f68
on per-device basis. - While adding support for per-device sysctls, merge from graid branch support for ADA_TEST_FAILURE kernel option, which opens few more sysctl, allowing to simulate read and write errors for testing purposes.
154 lines
5.3 KiB
Groff
154 lines
5.3 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 April 8, 2011
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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 determines 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 in runtime take effect only after device reset.
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The global default is currently enabled.
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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 ad 4 ,
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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 mav@FreeBSD.org
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