path: root/share/man/man4/raid.4

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.\" Copyright (c) 1995 Carnegie-Mellon University.
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.\" Author: Mark Holland
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.\"  School of Computer Science
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.Dd November 9, 1998
.Dt RAID 4
.Os
.Sh NAME
.Nm raid
.Nd RAIDframe Disk Driver
.Sh SYNOPSIS
.Cd "pseudo-device raid" Op Ar count
.Sh DESCRIPTION
The
.Nm
driver provides RAID 0, 1, 4, and 5 (and more!) capabilities.  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.
.Pp
RAIDframe provides a number of different RAID levels including:
.Bl -tag -width indent
.It RAID 0
provides simple data striping across the components.
.It RAID 1
provides mirroring.
.It RAID 4
provides data striping across the components, with parity
stored on a dedicated drive (in this case, the last component).
.It RAID 5
provides data striping across the components, with parity
distributed across all the components.
.El
.Pp
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
.Sx HISTORY
section for more detail on these various RAID configurations.
.Pp
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.
.Pp
The RAID driver supports and enforces the use of
.Sq component labels .
A
.Sq 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
.Sq 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
.Sq 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.
.Pp
The driver supports
.Sq 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
.Xr raidctl 8 .
.Pp
If a component cannot be detected when the RAID device is configured,
that component will be simply marked as 'failed'.
.Pp
The user-land utility for doing all
.Nm
configuration and other operations
is
.Xr raidctl 8 .
For any of the RAID flavours which have parity data,
.Xr raidctl 8
must be used with the
.Fl i
option to re-write the data 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 performing this on-demand recomputation of all parity
before doing a
.Xr fsck 8
or a
.Xr newfs 8
filesystem integrity and parity integrity can be ensured.  It bears
repeating again that parity recomputation is
.Em 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.
.Pp
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).
.Pp
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.  For normal SCSI drives, for example, the following can be
used to fix the device addresses:
.Bd -unfilled -offset indent
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
.Ed
.Pp
See
.Xr 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.
.Pp
The first step to using the
.Nm
driver is to ensure that it is suitably configured in the kernel.  This is
done by adding a line similar to:
.Bd -unfilled -offset indent
pseudo-device   raid   4       # RAIDframe disk device
.Ed
.Pp
to the kernel configuration file.  The
.Sq count
argument
.Pf ( Sq 4 ,
in this case), specifies the number of RAIDframe drivers to configure.
At the time of this writing, 4 is the MAXIMUM of
.Nm
devices which are supported.  This will change as soon as kernel threads
are available.
.Pp
In all cases the
.Sq raw
partitions of the disks
.Pa must not
be combined.  Rather, each component partition should be offset by at least one
cylinder from the beginning of that component disk.  This ensures that
the disklabels for the component disks do not conflict with the
disklabel for the
.Nm
device.
As well, all component partitions must be of the type
.Dv FS_BSDFFS .
.Pp
A more detailed treatment of actually using a
.Nm
device is found in
.Xr raidctl 8 .
It is highly recommended that the steps to reconstruct, copyback, and
re-compute parity are well understood by the system administrator(s)
.Em before
a component failure.  Doing the wrong thing when a component fails may
result in data loss.
.Pp
.Sh 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 filesystems on that RAID
device being lost.
RAID is
.Em not
a substitute for good backup practices.
.Pp
Recomputation of parity
.Em 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.
.Pp
.Sh FILES
.Bl -tag -width /dev/XXrXraidX -compact
.It Pa /dev/{,r}raid*
.Nm
device special files.
.El
.Pp
.Sh SEE ALSO
.Xr MAKEDEV 8 ,
.Xr config 8 ,
.Xr fsck 8 ,
.Xr mount 8 ,
.Xr newfs 8 ,
.Xr sd 4 ,
.Xr raidctl 8
.Sh HISTORY
The
.Nm
driver in
.Ox
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
.Nm
driver is a kernelized version of RAIDframe v1.1.
.Pp
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
.Nm
driver first appeared in
.Nx 1.4
from where it was ported to
.Ox 2.5 .
.Sh COPYRIGHT
.Bd -unfilled

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.

.Ed