The basic idea behind RAID is to combine multiple small, inexpensive disk
drives into an array which yields performance exceeding
that of one large and expensive drive. This array of drives will appear to
the computer as a single logical storage unit or drive.
RAID is a method in which information is spread across several disks, using
techniques such as disk striping (RAID Level 0) and
disk mirroring (RAID level 1) to achieve
redundancy, lower latency and/or higher bandwidth for reading and/or
writing to disks, and maximize recoverability from hard-disk crashes.
The underlying concept in RAID is that data may be distributed across each
drive in the array in a consistent manner. To do this, the data much
first be broken into consistently-sized "chunks" (often 32K or 64K in
size, although different sizes can be used). Each chunk is then written
to each drive in turn. When the data is to be read, the process is
reversed, giving the illusion that multiple drives are actually one large
drive.
Those of you who need to keep large quantities of data on hand (such as
an average administrator) would benefit by using RAID
technology. Primary reasons to use RAID include:
enhanced speed
increased storage capacity (and more economical)
greater efficiency in recovering from a disk failure
The hardware-based system manages the RAID subsystem independently
from the host and presents to the host only a single disk per RAID
array.
An example of a hardware RAID device would be one that connects to a
SCSI controller and presents the RAID arrays as a single SCSI
drive. An external RAID system moves all RAID handling "intelligence"
into a controller located in the external disk subsystem. The whole
subsystem is connected to the host via a normal SCSI controller and
appears to the host as a single disk.
RAID controllers also come in the form of cards that
act like a SCSI controller to the operating
system, but handle all of the actual drive communications
themselves. In these cases, you plug the drives into the RAID
controller just like you would a SCSI controller, but then you add
them to the RAID controller's configuration, and the operating system
never knows the difference.
Software RAID implements the various RAID levels in the kernel disk
(block device) code. It also offers the cheapest possible solution:
Expensive disk controller cards or hot-swap chassis
[1]
are not required, and software RAID works with cheaper IDE disks as
well as SCSI disks. With today's fast CPUs, software RAID performance
can excel against hardware RAID.
The MD driver in the Linux kernel is an example of a RAID solution
that is completely hardware independent. The performance of a
software-based array is dependent on the server CPU performance
and load.
RAID also offers levels 0, 1, 4, 5, and linear support. These RAID
types act as follows:
Level 0 -- RAID level 0, often called
"striping," is a performance- oriented striped data mapping
technique. That means the data being written to the array is
broken down into strips and written across the member disks of the
array. This allows high I/O performance at low inherent cost but
provides no redundancy. Storage capacity of the array is equal to
the total capacity of the member disks.
Level 1 -- RAID level 1, or "mirroring," has
been used longer than any other form of RAID. Level 1 provides
redundancy by writing identical data to each member disk of the
array, leaving a "mirrored" copy on each disk. Mirroring remains
popular due to its simplicity and high level of data availability.
Level 1 operates with two or more disks that may use parallel
access for high data-transfer rates when reading, but more
commonly operate independently to provide high I/O transaction
rates. Level 1 provides very good data reliability and improves
performance for read-intensive applications but at a relatively
high cost[2]. Array capacity is equal to the capacity of one member
disk if you use identical disk drives.
Level 4 -- Level 4 uses parity[3]
concentrated on a single disk drive to protect data. It's better
suited to transaction I/O rather than large file
transfers. Because the dedicated parity disk represents an
inherent bottleneck, level 4 is seldom used without accompanying
technologies such as write-back caching. Although RAID level 4 is
an option in some RAID partitioning schemes, it is not an option
allowed in Red Hat Linux RAID installations[4]. Array capacity is equal to the capacity of member disks,
minus capacity of one member disk if you use identical disk drives.
Level 5 -- The most common type of RAID. By
distributing parity across some or all of an array's member disk
drives, RAID level 5 eliminates the write bottleneck inherent in
level 4. The only bottleneck is the parity calculation
process. With modern CPUs and software RAID, that isn't a very big
bottleneck. As with level 4, the result is asymmetrical
performance, with reads substantially outperforming writes. Level
5 is often used with write-back caching to reduce the
asymmetry. Array capacity is equal to the capacity of member
disks, minus capacity of one member disk if you use identical disk
drives.
Linear RAID -- Linear RAID is a simple
grouping of drives to create a larger virtual drive. In linear
RAID, the chunks are allocated sequentially from one member drive,
going to the next drive only when the first is completely filled.
This grouping provides no performance benefit, as it is unlikely
that any I/O operations will be split between member drives.
Linear RAID also offers no redundancy, and in fact decreases
reliability -- if any one member drive fails, the entire array
cannot be used. The capacity is total of all member disks.
RAID is available in both the GUI and kickstart installation modes. You
can use fdisk or Disk
Druid to create your RAID configuration, but these
instructions will focus mainly on using Disk
Druid to complete this task.
Before you can create a RAID device, you must first create RAID
partitions, using the following step-by-step instructions.
Tip: If You Use fdisk
If you are using fdisk to create a RAID
partition, bear in mind that instead of creating a partition as type
83, which is Linux native, you must create the
partition as type fd (Linux RAID) and that
partitions within a given RAID array should span identical cylinders
on drives for best performance.
Create a partition. In Disk Druid, choose
Add to create a new partition (see Figure E-1).
Figure E-1. Creating a New RAID Partition
You will not be able to enter a mount point (you will be able to do
that once you've created your RAID device).
Enter the size that you want the partition to be.
Select Grow to fill disk if you want the
partition to grow to fill all available space on the hard disk. In this
case, the partition's size will expand and contract as other partitions
are modified. If you make more than one partition grow-able, the partitions
will compete for the available free space on the disk.
Enter the partition type as RAID.
Finally, for Allowable Drives, select the
drive that RAID will be created on. If you have multiple drives, all
drives will be selected here and you must deselect those drives
which will not have RAID array on it.
Continue these steps to create as many partitions as needed for your
RAID setup.
Figure E-2. RAID Partitions
Once you have all of your partitions created as RAID partitions, select
the Make RAID Device button on the
Disk Druid main partitioning screen (see
Figure E-2).
Next, Figure E-3 will appear which will allow you to
make a RAID device.
Figure E-3. Making a RAID Device
First, enter a mount point.
Next, make sure the partition type is set as Linux
Native (which will be the default).
Choose your RAID device. You should choose md0
for your first device, md1 for your second
device, and so on, unless you have a specific reason to make it
something else. Raid devices range from md0 to md7, and each may
only be used once.
Choose your RAID type. You can choose from RAID
0, RAID 1, and RAID
5.
Please Note
If you are making a RAID partition of
/boot, you must choose RAID level 1 and it
must use one of the first two drives (IDE first, SCSI
second). If you are not creating a RAID partition of
/boot, and are making a RAID partition of
/, it must be RAID level 1 and it must use
one of the first two drives (IDE first, SCSI second).
Finally, select which partitions will go into this RAID array (as in
Figure E-4) and then click
Next.
Figure E-4. Creating a RAID Array
From here, you can continue with your installation process. Refer back to
the Official Red Hat Linux Installation Guide for further instructions.
RAID level 1 is at a high cost because you write the same
information to all of the disks in the array, which wastes
drive space. For example, you have RAID level 1 set up so that
your "/" (root) partition spans across two 4G drives. You have
8G total but are only able to access 4G of that 8G. The other
4G acts like a mirror of the first 4G.
Parity information is calculated based on the contents of the
rest of the member disks in the array. This information can
then be used to reconstruct data when a disk in the array
fails. The reconstructed data can then be used to satisfy I/O
requests to the failed disk, and to repopulate the failed disk
after it has been repaired or replaced.
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