RAID(4) BSD Kernel Interfaces Manual RAID(4)NAMEraid — 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 famil‐
iarity 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 dis‐
tributed 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 com‐
ponent in the RAID set, and whether the data (and parity) on the compo‐
nent is ‘clean’. The component label currently lives at the half-way
point of the ‘reserved section’ located at the beginning of each compo‐
nent. 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 deter‐
mines 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 autocon‐
figuration 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 kernel directly from a RAID 1 set, and no support for boot‐
ing 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 copy‐
back 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 opera‐
tions is raidctl(8). Most importantly, raidctl(8) must be used with the
-i option to initialize all RAID sets. In particular, this initializa‐
tion 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 recompu‐
tation 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 parti‐
tions 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 admin‐
istrator(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 speci‐
fied:
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 RAID‐
frame. 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 ALSOconfig(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 orig‐
inally distributed by CMU, provides a RAID simulator for a number of dif‐
ferent 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 RAID‐
frame 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 Labora‐
tory 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
