RAID(4) OpenBSD 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
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 its
assigned 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
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 filesystems which are automatically
configured 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
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),
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
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/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
addresses:
sd0 at scsibus0 target 0 # SCSI disk drives
sd1 at scsibus0 target 1 # SCSI disk drives
sd2 at scsibus0 target 2 # SCSI disk drives
sd3 at scsibus0 target 3 # SCSI disk drives
sd4 at scsibus0 target 4 # SCSI disk drives
sd5 at scsibus0 target 5 # SCSI disk drives
sd6 at scsibus0 target 6 # SCSI disk drives
See sd(4) for more information. 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/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
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 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
RAIDframe 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
partitions.
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 debug information can be sent to the console by specifying:
option RAIDDEBUG
FILES
/dev/{,r}raid* raid device special files.
SEE ALSOccd(4), sd(4), wd(4), config(8), fsck(8), MAKEDEV(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
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 from where it was ported to OpenBSD 2.5.
CAVEATS
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 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
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.
OpenBSD 4.9 April 1, 2010 OpenBSD 4.9
