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.\" $NetBSD: raid.4,v 1.35 2008/05/02 18:11:05 martin Exp $
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.\" $NetBSD: raid.4,v 1.36 2009/05/04 20:37:07 wiz Exp $
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.\"
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.\" Copyright (c) 1998 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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@ -87,7 +87,8 @@ driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to
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.Nx .
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This
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document assumes that the reader has at least some familiarity with RAID
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and RAID concepts. The reader is also assumed to know how to configure
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and RAID concepts.
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The reader is also assumed to know how to configure
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disks and pseudo-devices into kernels, how to generate kernels, and how
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to partition disks.
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.Pp
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@ -110,12 +111,16 @@ The configuration file options to enable them are briefly outlined
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at the end of this section.
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.Pp
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Depending on the parity level configured, the device driver can
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support the failure of component drives. The number of failures
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allowed depends on the parity level selected. If the driver is able
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support the failure of component drives.
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The number of failures
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allowed depends on the parity level selected.
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If the driver is able
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to handle drive failures, and a drive does fail, then the system is
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operating in "degraded mode". In this mode, all missing data must be
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operating in "degraded mode".
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In this mode, all missing data must be
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reconstructed from the data and parity present on the other
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components. This results in much slower data accesses, but
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components.
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This results in much slower data accesses, but
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does mean that a failure need not bring the system to a complete halt.
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.Pp
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The RAID driver supports and enforces the use of
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@ -139,7 +144,8 @@ respect to each other (e.g. two or more serial numbers do not match)
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or that the component label is not consistent with its assigned place
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in the set (e.g. the component label claims the component should be
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the 3rd one in a 6-disk set, but the RAID set has it as the 3rd component
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in a 5-disk set) then the device will fail to configure. If the
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in a 5-disk set) then the device will fail to configure.
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If the
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driver determines that exactly one component label seems to be
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incorrect, and the RAID set is being configured as a set that supports
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a single failure, then the RAID set will be allowed to configure, but
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@ -150,17 +156,21 @@ If all of the components are consistent among themselves, the RAID set
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will configure normally.
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.Pp
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Component labels are also used to support the auto-detection and
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autoconfiguration of RAID sets. A RAID set can be flagged as
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autoconfiguration of RAID sets.
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A RAID set can be flagged as
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autoconfigurable, in which case it will be configured automatically
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during the kernel boot process. RAID file systems which are
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during the kernel boot process.
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RAID file systems which are
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automatically configured are also eligible to be the root file system.
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There is currently only limited support (alpha, amd64, i386, pmax,
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sparc, sparc64, and vax architectures)
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for booting a kernel directly from a RAID 1 set, and no support for
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booting from any other RAID sets. To use a RAID set as the root
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booting from any other RAID sets.
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To use a RAID set as the root
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file system, a kernel is usually obtained from a small non-RAID
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partition, after which any autoconfiguring RAID set can be used for the
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root file system. See
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root file system.
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See
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.Xr raidctl 8
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for more information on autoconfiguration of RAID sets.
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Note that with autoconfiguration of RAID sets, it is no longer
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@ -172,14 +182,17 @@ have had their device IDs changed or device names changed.
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The driver supports
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.Sq hot spares ,
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disks which are on-line, but are not
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actively used in an existing file system. Should a disk fail, the
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actively used in an existing file system.
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Should a disk fail, the
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driver is capable of reconstructing the failed disk onto a hot spare
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or back onto a replacement drive.
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If the components are hot swappable, the failed disk can then be
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removed, a new disk put in its place, and a copyback operation
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performed. The copyback operation, as its name indicates, will copy
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performed.
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The copyback operation, as its name indicates, will copy
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the reconstructed data from the hot spare to the previously failed
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(and now replaced) disk. Hot spares can also be hot-added using
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(and now replaced) disk.
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Hot spares can also be hot-added using
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.Xr raidctl 8 .
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.Pp
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If a component cannot be detected when the RAID device is configured,
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@ -194,11 +207,14 @@ Most importantly,
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.Xr raidctl 8
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must be used with the
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.Fl i
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option to initialize all RAID sets. In particular, this
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initialization includes re-building the parity data. This rebuilding
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option to initialize all RAID sets.
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In particular, this
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initialization includes re-building the parity data.
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This rebuilding
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of parity data is also required when either a) a new RAID device is
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brought up for the first time or b) after an un-clean shutdown of a
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RAID device. By using the
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RAID device.
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By using the
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.Fl P
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option to
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.Xr raidctl 8 ,
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@ -207,28 +223,31 @@ before doing a
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.Xr fsck 8
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or a
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.Xr newfs 8 ,
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file system integrity and parity integrity can be ensured. It bears
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repeating again that parity recomputation is
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file system integrity and parity integrity can be ensured.
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It bears repeating again that parity recomputation is
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.Ar required
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before any file systems are created or used on the RAID device. If the
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before any file systems are created or used on the RAID device.
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If the
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parity is not correct, then missing data cannot be correctly recovered.
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.Pp
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RAID levels may be combined in a hierarchical fashion. For example, a RAID 0
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RAID levels may be combined in a hierarchical fashion.
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For example, a RAID 0
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device can be constructed out of a number of RAID 5 devices (which, in turn,
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may be constructed out of the physical disks, or of other RAID devices).
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.Pp
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The first step to using the
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.Nm
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driver is to ensure that it is suitably configured in the kernel. This is
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done by adding a line similar to:
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driver is to ensure that it is suitably configured in the kernel.
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This is done by adding a line similar to:
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.Bd -unfilled -offset indent
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pseudo-device raid 4 # RAIDframe disk device
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.Ed
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.Pp
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to the kernel configuration file. The
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to the kernel configuration file.
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The
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.Sq count
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argument (
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.Sq 4 ,
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argument
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.Sq ( 4 ,
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in this case), specifies the number of RAIDframe drivers to configure.
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To turn on component auto-detection and autoconfiguration of RAID
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sets, simply add:
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@ -243,7 +262,8 @@ All component partitions must be of the type
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(e.g. 4.2BSD) or
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.Dv FS_RAID .
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The use of the latter is strongly encouraged, and is required if
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autoconfiguration of the RAID set is desired. Since RAIDframe leaves
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autoconfiguration of the RAID set is desired.
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Since RAIDframe leaves
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room for disklabels, RAID components can be simply raw disks, or
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partitions which use an entire disk.
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.Pp
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@ -254,7 +274,8 @@ device is found in
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It is highly recommended that the steps to reconstruct, copyback, and
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re-compute parity are well understood by the system administrator(s)
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.Ar before
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a component failure. Doing the wrong thing when a component fails may
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a component failure.
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Doing the wrong thing when a component fails may
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result in data loss.
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.Pp
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Additional internal consistency checking can be enabled by specifying:
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@ -372,7 +393,8 @@ The reader is referred to the RAIDframe documentation mentioned in the
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section for more detail on these various RAID configurations.
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.Sh WARNINGS
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Certain RAID levels (1, 4, 5, 6, and others) can protect against some
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data loss due to component failure. However the loss of two
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data loss due to component failure.
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However the loss of two
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components of a RAID 4 or 5 system, or the loss of a single component
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of a RAID 0 system, will result in the entire file systems on that RAID
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device being lost.
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@ -383,11 +405,14 @@ a substitute for good backup practices.
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Recomputation of parity
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.Ar MUST
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be performed whenever there is a chance that it may have been
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compromised. This includes after system crashes, or before a RAID
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device has been used for the first time. Failure to keep parity
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compromised.
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This includes after system crashes, or before a RAID
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device has been used for the first time.
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Failure to keep parity
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correct will be catastrophic should a component ever fail -- it is
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better to use RAID 0 and get the additional space and speed, than it
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is to use parity, but not keep the parity correct. At least with RAID
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is to use parity, but not keep the parity correct.
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At least with RAID
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0 there is no perception of increased data security.
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.Sh FILES
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.Bl -tag -width /dev/XXrXraidX -compact
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@ -398,8 +423,8 @@ device special files.
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.Sh SEE ALSO
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.Xr config 1 ,
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.Xr sd 4 ,
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.Xr MAKEDEV 8 ,
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.Xr fsck 8 ,
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.Xr MAKEDEV 8 ,
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.Xr mount 8 ,
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.Xr newfs 8 ,
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.Xr raidctl 8
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@ -410,10 +435,12 @@ driver in
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.Nx
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is a port of RAIDframe, a framework for rapid prototyping of RAID
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structures developed by the folks at the Parallel Data Laboratory at
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Carnegie Mellon University (CMU). RAIDframe, as originally distributed
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Carnegie Mellon University (CMU).
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RAIDframe, as originally distributed
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by CMU, provides a RAID simulator for a number of different
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architectures, and a user-level device driver and a kernel device
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driver for Digital Unix. The
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driver for Digital Unix.
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The
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.Nm
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driver is a kernelized version of RAIDframe v1.1.
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.Pp
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