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RAID(4)                FreeBSD Kernel Interfaces Manual                RAID(4)

NAME
     raid - RAIDframe disk driver

SYNOPSIS
     device raidframe

DESCRIPTION
     The raid driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to
     FreeBSD.  This document assumes that the reader has at least some
     familiarity with RAID and RAID concepts.  The reader is also assumed to
     know how to configure disks and pseudo-devices into kernels, how to
     generate kernels, and how to partition disks.

     RAIDframe provides a number of different RAID levels including:

     RAID 0  provides simple data striping across the components.

     RAID 1  provides mirroring.

     RAID 4  provides data striping across the components, with parity stored
             on a dedicated drive (in this case, the last component).

     RAID 5  provides data striping across the components, with parity
             distributed across all the components.

     There are a wide variety of other RAID levels supported by RAIDframe,
     including Even-Odd parity, RAID level 5 with rotated sparing, Chained
     declustering,  and Interleaved declustering.  The reader is referred to
     the RAIDframe documentation mentioned in the HISTORY section for more
     detail on these various RAID configurations.

     Depending on the parity level configured, the device driver can support
     the failure of component drives.  The number of failures allowed depends
     on the parity level selected.  If the driver is able to handle drive
     failures, and a drive does fail, then the system is operating in
     "degraded mode".  In this mode, all missing data must be reconstructed
     from the data and parity present on the other components.  This results
     in much slower data accesses, but does mean that a failure need not bring
     the system to a complete halt.

     The RAID driver supports and enforces the use of `component labels'.  A
     `component label' contains important information about the component,
     including a user-specified serial number, the row and column of that
     component in the RAID set, and whether the data (and parity) on the
     component is `clean'.  If the driver determines that the labels are very
     inconsistent with respect to each other (e.g. two or more serial numbers
     do not match) or that the component label is not consistent with it's
     assigned place in the set (e.g. the component label claims the component
     should be the 3rd one a 6-disk set, but the RAID set has it as the 3rd
     component in a 5-disk set) then the device will fail to configure.  If
     the driver determines that exactly one component label seems to be
     incorrect, and the RAID set is being configured as a set that supports a
     single failure, then the RAID set will be allowed to configure, but the
     incorrectly labeled component will be marked as `failed', and the RAID
     set will begin operation in degraded mode.  If all of the components are
     consistent among themselves, the RAID set will configure normally.

     Component labels are also used to support the auto-detection and auto-
     configuration of RAID sets.  A RAID set can be flagged as auto-
     configurable, in which case it will be configured automatically during
     the kernel boot process.  RAID file systems which are automatically
     configured are also eligible to be the root file system.  There is
     currently only limited support (alpha and pmax architectures) for booting
     a kernel directly from a RAID 1 set, and no support for booting from any
     other RAID sets.  To use a RAID set as the root file system, a kernel is
     usually obtained from a small non-RAID partition, after which any auto-
     configuring RAID set can be used for the root file system.  See
     raidctl(8) for more information on auto-configuration of RAID sets.

     The driver supports `hot spares', disks which are on-line, but are not
     actively used in an existing file system.  Should a disk fail, the driver
     is capable of reconstructing the failed disk onto a hot spare or back
     onto a replacement drive.  If the components are hot swapable, the failed
     disk can then be removed, a new disk put in its place, and a copyback
     operation performed.  The copyback operation, as its name indicates, will
     copy the reconstructed data from the hot spare to the previously failed
     (and now replaced) disk.  Hot spares can also be hot-added using
     raidctl(8).

     If a component cannot be detected when the RAID device is configured,
     that component will be simply marked as 'failed'.

     The user-land utility for doing all raid configuration and other
     operations is raidctl(8).  Most importantly, raidctl(8) must be used with
     the -i option to initialize all RAID sets.  In particular, this
     initialization includes re-building the parity data.  This rebuilding of
     parity data is also required when either a) a new RAID device is brought
     up for the first time or b) after an un-clean shutdown of a RAID device.
     By using the -P option to raidctl(8), and performing this on-demand
     recomputation of all parity before doing a fsck(8) or a newfs(8), file
     system integrity and parity integrity can be ensured.  It bears repeating
     again that parity recomputation is required before any file systems are
     created or used on the RAID device.  If the parity is not correct, then
     missing data cannot be correctly recovered.

     RAID levels may be combined in a hierarchical fashion.  For example, a
     RAID 0 device can be constructed out of a number of RAID 5 devices
     (which, in turn, may be constructed out of the physical disks, or of
     other RAID devices).

     It is important that drives be hard-coded at their respective addresses
     (i.e. not left free-floating, where a drive with SCSI ID of 4 can end up
     as /dev/da0c) for well-behaved functioning of the RAID device.  This is
     true for all types of drives, including IDE, SCSI, etc.  For IDE drivers,
     use the option ATAPI_STATIC_ID in your kernel config file.  For SCSI, you
     should 'wire down' the devices according to their ID.  See cam(4) for
     examples of this.  The rationale for fixing the device addresses is as
     follows: Consider a system with three SCSI drives at SCSI ID's 4, 5, and
     6, and which map to components /dev/da0e, /dev/da1e, and /dev/da2e of a
     RAID 5 set.  If the drive with SCSI ID 5 fails, and the system reboots,
     the old /dev/da2e will show up as /dev/da1e.  The RAID driver is able to
     detect that component positions have changed, and will not allow normal
     configuration.  If the device addresses are hard coded, however, the RAID
     driver would detect that the middle component is unavailable, and bring
     the RAID 5 set up in degraded mode.  Note that the auto-detection and
     auto-configuration code does not care about where the components live.
     The auto-configuration code will correctly configure a device even after
     any number of the components have been re-arranged.

     The first step to using the raid driver is to ensure that it is suitably
     configured in the kernel.  This is done by adding a line similar to:

           pseudo-device   raidframe      # RAIDframe disk device

     to the kernel configuration file.  No count argument is required as the
     driver will automatically create and configure new device units as
     needed.  To turn on component auto-detection and auto-configuration of
     RAID sets, simply add:

           options    RAID_AUTOCONFIG

     to the kernel configuration file.

     All component partitions must be of the type FS_BSDFFS (e.g. 4.2BSD) or
     FS_RAID.  The use of the latter is strongly encouraged, and is required
     if auto-configuration of the RAID set is desired.  Since RAIDframe leaves
     room for disklabels, RAID components can be simply raw disks, or
     partitions which use an entire disk.

     A more detailed treatment of actually using a raid device is found in
     raidctl(8).  It is highly recommended that the steps to reconstruct,
     copyback, and re-compute parity are well understood by the system
     administrator(s) before a component failure.  Doing the wrong thing when
     a component fails may result in data loss.

WARNINGS
     Certain RAID levels (1, 4, 5, 6, and others) can protect against some
     data loss due to component failure.  However the loss of two components
     of a RAID 4 or 5 system, or the loss of a single component of a RAID 0
     system, will result in the entire file systems on that RAID device being
     lost.  RAID is NOT a substitute for good backup practices.

     Recomputation of parity MUST be performed whenever there is a chance that
     it may have been compromised.  This includes after system crashes, or
     before a RAID device has been used for the first time.  Failure to keep
     parity correct will be catastrophic should a component ever fail -- it is
     better to use RAID 0 and get the additional space and speed, than it is
     to use parity, but not keep the parity correct.  At least with RAID 0
     there is no perception of increased data security.

FILES
     /dev/raid*      raid device special files.

SEE ALSO
     raidctl(8), config(8), fsck(8), mount(8), newfs(8),

HISTORY
     The raid driver in FreeBSD is a port of RAIDframe, a framework for rapid
     prototyping of RAID structures developed by the folks at the Parallel
     Data Laboratory at Carnegie Mellon University (CMU).  RAIDframe, as
     originally distributed by CMU, provides a RAID simulator for a number of
     different architectures, and a user-level device driver and a kernel
     device driver for Digital Unix.  The raid driver is a kernelized version
     of RAIDframe v1.1, based on the NetBSD port of RAIDframe by Greg Oster.

     A more complete description of the internals and functionality of
     RAIDframe is found in the paper "RAIDframe: A Rapid Prototyping Tool for
     RAID Systems", by William V. Courtright II, Garth Gibson, Mark Holland,
     LeAnn Neal Reilly, and Jim Zelenka, and published by the Parallel Data
     Laboratory of Carnegie Mellon University.  The raid driver first appeared
     in FreeBSD 4.4.

COPYRIGHT
     The RAIDframe Copyright is as follows:

     Copyright (c) 1994-1996 Carnegie-Mellon University.
     All rights reserved.

     Permission to use, copy, modify and distribute this software and
     its documentation is hereby granted, provided that both the copyright
     notice and this permission notice appear in all copies of the
     software, derivative works or modified versions, and any portions
     thereof, and that both notices appear in supporting documentation.

     CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
     CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
     FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.

     Carnegie Mellon requests users of this software to return to

      Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
      School of Computer Science
      Carnegie Mellon University
      Pittsburgh PA 15213-3890

     any improvements or extensions that they make and grant Carnegie the
     rights to redistribute these changes.

FreeBSD 11.0-PRERELEASE        October 20, 2002        FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | WARNINGS | FILES | SEE ALSO | HISTORY | COPYRIGHT

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