Before You Start
Specially built hardware-based RAID disk controllers are available for both IDE and SCSI drives. They usually have their own BIOS, so you can configure them right after your system's power on self test (POST). Hardware-based RAID is transparent to your operating system; the hardware does all the work.
If hardware RAID isn't available, then you should be aware of these basic guidelines to follow when setting up software RAID.
IDE Drives
To save costs, many small business systems will probably use IDE disks, but they do have some limitations:
The total length of an IDE cable can be only a few feet long, which generally limits IDE drives to small home systems. IDE drives do not hot swap. You cannot replace them while your system is running. Only two devices can be attached per controller. The performance of the IDE bus can be degraded by the presence of a second device on the cable. The failure of one drive on an IDE bus often causes the malfunctioning of the second device. This can be fatal if you have two IDE drives of the same RAID set attached to the same cable.
For these reasons, I recommend you use only one IDE drive per controller when using RAID, especially in a corporate environment. In a home or SOHO setting, IDE-based software RAID may be adequate.
Serial ATA Drives
Serial ATA type drives are rapidly replacing IDE, or ultra ATA, drives as the preferred entry-level disk storage option because of a number of advantages:
The drive data cable can be as long as one meter in length versus IDE's 18 inches. Serial ATA has better error checking than IDE. There is only one drive per cable, which makes hot swapping or the ability to replace components while the system is still running, possible without the fear of affecting other devices on the data cable. There are no jumpers to set on Serial ATA drives to make it a master or slave, which makes them easier to configure. IDE drives have a 133Mbytes/s data rate whereas the Serial ATA specification starts at 150 Mbytes/sec with a goal of reaching 600 Mbytes/s over the expected ten year life of the specification.
If you can't afford more expensive and faster SCSI drives, Serial ATA would be the preferred device for software and hardware RAID.
SCSI Drives
SCSI hard disks have a number of features that make them more attractive for RAID use than either IDE or Serial ATA drives:
SCSI controllers are more tolerant of disk failures. The failure of a single drive is less likely to disrupt the remaining drives on the bus. SCSI cables can be up to 25 meters long, making them suitable for data center applications. Much more than two devices may be connected to a SCSI cable bus. It can accommodate 7 (single-ended SCSI) or 15 (all other SCSI types) devices. Some models of SCSI devices support hot swapping, which allows you to replace them while the system is running. SCSI currently supports data rates of up to 640 Mbytes/s making them highly desirable for installations where rapid data access is imperative.
SCSI drives tend to be more expensive than IDE drives, however, which may make them less attractive for home use.
Should I Use Software RAID Partitions or Entire Disks?
It is generally not a good idea to share RAID-configured partitions with non-RAID partitions. The reason for this is obvious: A disk failure could still incapacitate a system.
If you decide to use RAID, all the partitions on each RAID disk should be part of a RAID set. Many people simplify this problem by filling each disk of a RAID set with only one partition.
Back up Your System First
Software RAID creates the equivalent of a single RAID virtual disk drive made up of all the underlying regular partitions used to create it. You have to format this new RAID device before your Linux system can store files on it. Formatting, however, causes all the old data on the underlying RAID partitions to be lost. It is best to back up the data on these and any other partitions on the disk drive on which you want implement RAID. A mistake could unintentionally corrupt valid data.
Configure RAID in Single-User Mode
As you will be modifying the disk structure of your system, you should also consider configuring RAID while your system is running in single-user mode from the VGA console. This makes sure that most applications and networking are shutdown and that no other users can access the system, reducing the risk of data corruption during the exercise.
[root@bigboy tmp]# init 1
Once finished, issue the exit command, and your system will boot in the default runlevel provided in the /etc/inittab file.
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