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 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.  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 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 com-
     ponent is 'clean'.  The component label currently lives at the half-way point of the 'reserved section' located at the beginning of each com-
     ponent.  This 'reserved section' is RF_PROTECTED_SECTORS in length (64 blocks or 32Kbytes) and the component label is currently 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 sin-
     gle 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 autoconfiguration of RAID sets.  A RAID set can be flagged as autoconfig-
     urable, in which case it will be configured automatically during the kernel boot process.	RAID file systems which are automatically config-
     ured 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 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 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 autoconfiguration 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 components 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 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 par-
     ity 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).

     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 autoconfiguration 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 performance 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 typically 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 performed 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 additional 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 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/{,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 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.

     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 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