raid -- RAIDframe disk driver
device raidframe
The raid driver provides RAID 0, 1, 4, and 5 (and more!) capabilities to
FreeBSD. 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 add 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 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 autoconfiguration
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 file systems which are automatically configured
are also eligible to be the root file system. There is currently only
limited support (alpha and pmax 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 auto-configuring
RAID set can be used for the root file system. 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 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 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 userland 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 unclean shutdown of a RAID device. By
using the -P option to raidctl(8), and performing this on-demand recomputation
of all parity before doing an 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).
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/da0c) for well-behaved functioning of the RAID device. This is
true for all types of drives, including IDE, SCSI, etc. For IDE drivers,
use the options ATA_STATIC_ID in your kernel config file. For SCSI, you
should ``wire down'' the devices according to their ID. See cam(4) for
examples of this. The rationale for fixing the device addresses is as
follows: consider a system with three SCSI drives at SCSI IDs 4, 5, and
6, and which map to components /dev/da0e, /dev/da1e, and /dev/da2e of a
RAID 5 set. If the drive with SCSI ID 5 fails, and the system reboots,
the old /dev/da2e will show up as /dev/da1e. 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 the
device raidframe
line to the kernel configuration file. No count argument is required as
the driver will automatically create and configure new device units as
needed. To turn on component auto-detection and auto-configuration 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 auto-configuration of the RAID set is desired. Since RAIDframe leaves
room for disk labels, 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.
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.
/dev/raid* The raid device special files.
config(8), fsck(8), mount(8), newfs(8), raidctl(8)
The raid driver in FreeBSD 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, based on the NetBSD port of RAIDframe by Greg Oster.
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
FreeBSD 4.4.
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 [email protected]
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.
FreeBSD 5.2.1 October 20, 2002 FreeBSD 5.2.1 [ Back ] |
