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NetBSD 6.1.5 - man page for raid (netbsd section 4)

RAID(4) 			   BSD Kernel Interfaces Manual 			  RAID(4)

NAME
     raid -- RAIDframe disk driver

SYNOPSIS
     options RAID_AUTOCONFIG
     options RAID_DIAGNOSTIC
     options RF_ACC_TRACE=n
     options RF_DEBUG_MAP=n
     options RF_DEBUG_PSS=n
     options RF_DEBUG_QUEUE=n
     options RF_DEBUG_QUIESCE=n
     options RF_DEBUG_RECON=n
     options RF_DEBUG_STRIPELOCK=n
     options RF_DEBUG_VALIDATE_DAG=n
     options RF_DEBUG_VERIFYPARITY=n
     options RF_INCLUDE_CHAINDECLUSTER=n
     options RF_INCLUDE_EVENODD=n
     options RF_INCLUDE_INTERDECLUSTER=n
     options RF_INCLUDE_PARITY_DECLUSTERING=n
     options RF_INCLUDE_PARITY_DECLUSTERING_DS=n
     options RF_INCLUDE_PARITYLOGGING=n
     options RF_INCLUDE_RAID5_RS=n

     pseudo-device raid [count]

DESCRIPTION
     The raid driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to NetBSD.  This docu-
     ment 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.  The configuration
     file options to enable them are briefly outlined at the end of this section.

     Depending on the parity level configured, the device driver can support the failure of com-
     ponent 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 num-
     ber, the row and column of that component in the RAID set, and whether the data (and parity)
     on the component is 'clean'.  The component label currently lives at the half-way point of
     the 'reserved section' located at the beginning of each component.  This 'reserved section'
     is RF_PROTECTED_SECTORS in length (64 blocks or 32Kbytes) and the component label is cur-
     rently 1Kbyte in size.

     If the driver determines that the component 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 its assigned place in the set (e.g. the component label claims the component
     should be the 3rd one in 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 opera-
     tion 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 autoconfiguration of RAID
     sets.  A RAID set can be flagged as autoconfigurable, 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, amd64, i386, pmax, sparc, sparc64, and vax architectures) for booting a ker-
     nel 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 par-
     tition, after which any autoconfiguring RAID set can be used for the root file system.  See
     raidctl(8) for more information on autoconfiguration of RAID sets.  Note that with autocon-
     figuration of RAID sets, it is no longer necessary to hard-code SCSI IDs of drives.  The
     autoconfiguration code will correctly configure a device even after any number of the compo-
     nents have had their device IDs changed or device names changed.

     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
     swappable, 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 recon-
     structed 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 repeat-
     ing 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).

     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   raid   4	  # RAIDframe disk device

     to the kernel configuration file.	The 'count' argument ('4', in this case), specifies the
     number of RAIDframe drivers to configure.	To turn on component auto-detection and autocon-
     figuration 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 autoconfiguration 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.

     Additional internal consistency checking can be enabled by specifying:

	   options RAID_DIAGNOSTIC

     These assertions are disabled by default in order to improve performance.

     RAIDframe supports an access tracing facility for tracking both requests made and perfor-
     mance of various parts of the RAID systems as the request is processed.  To enable this
     tracing the following option may be specified:

	   options RF_ACC_TRACE=1

     For extensive debugging there are a number of kernel options which will aid in performing
     extra diagnosis of various parts of the RAIDframe sub-systems.  Note that in order to make
     full use of these options it is often necessary to enable one or more debugging options as
     listed in src/sys/dev/raidframe/rf_options.h.  As well, these options are also only typi-
     cally useful for people who wish to debug various parts of RAIDframe.  The options include:

     For debugging the code which maps RAID addresses to physical addresses:

	   options RF_DEBUG_MAP=1

     Parity stripe status debugging is enabled with:

	   options RF_DEBUG_PSS=1

     Additional debugging for queuing is enabled with:

	   options RF_DEBUG_QUEUE=1

     Problems with non-quiescent file systems should be easier to debug if the following is
     enabled:

	   options RF_DEBUG_QUIESCE=1

     Stripelock debugging is enabled with:

	   options RF_DEBUG_STRIPELOCK=1

     Additional diagnostic checks during reconstruction are enabled with:

	   options RF_DEBUG_RECON=1

     Validation of the DAGs (Directed Acyclic Graphs) used to describe an I/O access can be per-
     formed when the following is enabled:

	   options RF_DEBUG_VALIDATE_DAG=1

     Additional diagnostics during parity verification are enabled with:

	   options RF_DEBUG_VERIFYPARITY=1

     There are a number of less commonly used RAID levels supported by RAIDframe.  These addi-
     tional RAID types should be considered experimental, and may not be ready for production
     use.  The various types and the options to enable them are shown here:

     For Even-Odd parity:

	   options RF_INCLUDE_EVENODD=1

     For RAID level 5 with rotated sparing:

	   options RF_INCLUDE_RAID5_RS=1

     For Parity Logging (highly experimental):

	   options RF_INCLUDE_PARITYLOGGING=1

     For Chain Declustering:

	   options RF_INCLUDE_CHAINDECLUSTER=1

     For Interleaved Declustering:

	   options RF_INCLUDE_INTERDECLUSTER=1

     For Parity Declustering:

	   options RF_INCLUDE_PARITY_DECLUSTERING=1

     For Parity Declustering with Distributed Spares:

	   options RF_INCLUDE_PARITY_DECLUSTERING_DS=1

     The reader is referred to the RAIDframe documentation mentioned in the HISTORY section for
     more detail on these various RAID configurations.

WARNINGS
     Certain RAID levels (1, 4, 5, 6, and others) can protect against some data loss due to com-
     ponent 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/{,r}raid*  raid device special files.

SEE ALSO
     config(1), sd(4), fsck(8), MAKEDEV(8), mount(8), newfs(8), raidctl(8)

HISTORY
     The raid driver in NetBSD 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 Univer-
     sity (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.

     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 Paral-
     lel Data Laboratory of Carnegie Mellon University.  The raid driver first appeared in
     NetBSD 1.4.

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.

BSD					  August 6, 2007				      BSD


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