297 lines
8.1 KiB
Groff
297 lines
8.1 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 15, 1998
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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 PARTITIONING
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The
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.Nm
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driver allows the disk to have two levels of partitioning.
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One layer, called the
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.Dq slice layer ,
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is used to separate the
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.Fx
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areas of the disk from areas used by other operating systems.
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The second layer is the native
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.Bx 4.4
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partitioning scheme,
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.Xr disklabel 5 ,
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which is used to subdivide the
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.Fx
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slices into areas for individual filesystems and swap spaces.
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For more information, see
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.Xr fdisk 8
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and
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.Xr disklabel 8 ,
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respectively.
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.Pp
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If an uninitialized disk is opened, the slice table will be
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initialized with a fictitious
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.Fx
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slice spanning the entire disk. Similarly, if an uninitialized
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(or
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.No non- Ns Fx )
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slice is opened, its disklabel will be initialized with parameters returned
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by the drive and a single
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.Sq Li c
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partition encompassing the entire slice.
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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. 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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.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. The read cache is transparent
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to the user and can be enabled without any adverse effect. Most devices
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with a read cache come from the factory with it enabled. The read cache
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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. This performance gain comes at a price. 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. The effect of a loss of write transactions on
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a file system is non-deterministic and can cause corruption. 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). 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. This
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ensures that it is safe to disconnect power once the operating system
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has reported that it has halted. The write cache can be enabled by
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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. 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. 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. Media recovery parameters are located in mode page 1,
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the Read-Write Error Recovery mode page. 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 IOCTLS
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The following
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.Xr ioctl 2
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calls apply to
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.Tn SCSI
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disks as well as to other disks. They are defined in the header file
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.Aq Pa sys/disklabel.h .
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.Pp
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.Bl -tag -width DIOCSDINFO
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.It Dv DIOCSBAD
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Usually used to set up a bad-block mapping system on the disk.
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.Tn SCSI
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drives incorporate their own bad-block mapping so this command is not
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implemented.
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.It Dv DIOCGDINFO
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Read, from the kernel, the in-core copy of the disklabel for the
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drive.
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This may be a fictitious disklabel if the drive has never
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been initialized, in which case it will contain information read
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from the
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.Tn SCSI
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inquiry commands.
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.It Dv DIOCSDINFO
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Give the driver a new disklabel to use.
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The driver
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.Em will not
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write the new
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disklabel to the disk.
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.It Dv DIOCWLABEL
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Enable or disable the driver's software
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write protect of the disklabel on the disk.
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.It Dv DIOCWDINFO
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Give the driver a new disklabel to use.
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The driver
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.Em will
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write the new disklabel to the disk.
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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. 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. During this
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period, all new open attempts will be rejected.
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.Sh FILES
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.Bl -tag -width /dev/rsdXXXXX -compact
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.It Pa /dev/rda Ns Ar u
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raw mode
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.Tn SCSI
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disk unit
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.Ar u ,
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accessed as an unpartitioned device
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.Sm off
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.It Pa /dev/da Ar u Pa s Ar n
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.Sm on
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block mode
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.Tn SCSI
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disk unit
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.Ar u ,
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slice
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.Ar n ,
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accessed as an unpartitioned device
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.Sm off
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.It Pa /dev/rda Ar u Pa s Ar n
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.Sm on
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raw mode
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.Tn SCSI
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disk unit
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.Ar u ,
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slice
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.Ar n ,
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accessed as an unpartitioned device
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.It Pa /dev/da Ns Ar u Ns Ar p
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block mode
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.Tn SCSI
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disk unit
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.Ar u ,
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first
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.Fx
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slice, partition
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.Ar p
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.It Pa /dev/rda Ns Ar u Ns Ar p
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raw mode
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.Tn SCSI
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disk unit
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.Ar u ,
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first
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.Fx
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slice, partition
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.Ar p
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.Sm off
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.It Xo
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.Pa /dev/da
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.Ar u
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.Pa s
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.Ar n
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.Ar p
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.Xc
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.Sm on
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block mode
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.Tn SCSI
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disk unit
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.Ar u ,
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.Ar n Ns th
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slice, partition
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.Ar p
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.Sm off
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.It Xo
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.Pa /dev/rda
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.Ar u
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.Pa s
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.Ar n
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.Ar p
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.Xc
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.Sm on
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raw mode
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.Tn SCSI
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disk unit
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.Ar u ,
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.Ar n Ns th
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slice, partition
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.Ar p
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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 ad 4 ,
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.Xr disklabel 5 ,
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.Xr disklabel 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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Support for slices was written by
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.An Bruce Evans .
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