RAID(4) BSD Programmer's Manual RAID(4)NAMEraid - RAIDframe disk driver
SYNOPSIS
pseudo-device raid [count]
DESCRIPTION
The raid driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to
OpenBSD. This document assumes that the reader has at least some fami-
liarity 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 distri-
buted across all the components.
There are a wide variety of other RAID levels supported by RAIDframe, in-
cluding Even-Odd parity, RAID level 5 with rotated sparing, Chained de-
clustering, 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 "degrad-
ed 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, in-
cluding 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 com-
ponent is 'clean'. If the driver determines that the labels are very in-
consistent 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 as-
signed place in the set (e.g., the component label claims the component
should be the 3rd one of 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 in-
correctly labeled component will be marked as 'failed', and the RAID set
will begin operation in degraded mode. If all of the components are con-
sistent 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 filesystems which are automatically config-
ured are also eligible to be the root filesystem. There is currently no
support for booting a kernel directly from a RAID set. To use a RAID set
as the root filesystem, a kernel is usually obtained from a small non-
RAID partition, after which any auto-configuring RAID set can be used for
the root filesystem. 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 filesystem. 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 opera-
tions 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), filesystem integrity
and parity integrity can be ensured. It bears repeating again that parity
recomputation is required before any filesystems 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 oth-
er 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/sd0c) for well-behaved functioning of the RAID device. This is
true for all types of drives, including IDE, HP-IB, etc. For normal SCSI
drives, for example, the following can be used to fix the device ad-
dresses:
sd0 at scsibus0 target 0 lun ? # SCSI disk drives
sd1 at scsibus0 target 1 lun ? # SCSI disk drives
sd2 at scsibus0 target 2 lun ? # SCSI disk drives
sd3 at scsibus0 target 3 lun ? # SCSI disk drives
sd4 at scsibus0 target 4 lun ? # SCSI disk drives
sd5 at scsibus0 target 5 lun ? # SCSI disk drives
sd6 at scsibus0 target 6 lun ? # SCSI disk drives
See sd(4) for more information. The rationale for fixing the device ad-
dresses is as follows: Consider a system with three SCSI drives at SCSI
ID's 4, 5, and 6, and which map to components /dev/sd0e, /dev/sd1e, and
/dev/sd2e of a RAID 5 set. If the drive with SCSI ID 5 fails, and the
system reboots, the old /dev/sd2e will show up as /dev/sd1e. 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 una-
vailable, 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 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 auto-configuration of RAID sets, simply add:
option RAID_AUTOCONFIG
to the kernel configuration file.
All component partitions must be of the type FS_BSDFFS (e.g., 4.2BSD) or
FS_RAID (e.g., RAID). The use of the latter is strongly encouraged, and
is required if auto-configuration of the RAID set is desired. Since RAID-
frame leaves room for disklabels, RAID components can be simply raw
disks, or partitions which use an entire disk. Note that some platforms
(such as SUN) do not allow using the FS_RAID partition type. On these
platforms, the raid driver can still auto-configure from FS_BSDFFS parti-
tions.
A more detailed treatment of actually using a raid device is found in
raidctl(8). It is highly recommended that the steps to reconstruct, copy-
back, 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 debug information can be sent to the console by specifying:
option RAIDDEBUG
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 sys-
tem, will result in the entire filesystems 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 be-
fore a RAID device has been used for the first time. Failure to keep par-
ity 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 ALSOccd(4), sd(4), wd(4), MAKEDEV(8), config(8), fsck(8), mount(8), newfs(8),
raidctl(8)HISTORY
The raid driver in OpenBSD 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 origi-
nally 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 RAID-
frame 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 from where it was ported to OpenBSD 2.5.
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.
MirOS BSD #10-current November 9, 1998 3
