User manual

Cobalt RaQ 4 User Manual 217
Redundant Array of Independent Disks (RAID)
A redundant array of independent disks is a way of storing the same data in
different places (thus, redundantly) on multiple hard disk drives. A RAID
appears to the operating system to be a single virtual disk drive.
Redundancy means that there is protection against the failure of any single
hard disk drive. Redundant data is used by a RAID system in the event of a
failure; this redundant data can either be a mirror copy or parity data used to
reconstruct the actual data.
There are a variety of different types and implementations of RAID, each
with its own advantages and disadvantages.
• RAID-0 combines the separate hard disk drives into one virtual disk
drive and offers the best performance of the three options. However, the
data on the disk drives is not redundant and the system is thus not
fault-tolerant. This option is available on server configurations with two
or more hard disk drives.
• RAID-1, also known as disk mirroring, consists of a primary hard disk
drive and a secondary hard disk drive; the secondary disk drive is an
exact copy or “mirror image” of the primary disk drive. This option is
only available on a configuration with two hard disk drives.
• RAID-5 includes a rotating parity-bit array. All read and write
operations can be overlapped. RAID-5 does not store redundant data but
it does store the parity information which can be used to reconstruct data
in the event of a single hard-disk-drive failure. RAID-5 requires at least
three hard disk drives for the array.
Although RAID-1 and RAID-5 (but not RAID-0) can protect your data in
case of a hard-disk-drive failure, they do not protect against operator and
administrator (human) error, or against loss due to programming bugs.
RAID can be implemented in hardware or in software. Hardware RAID is
always a “disk controller”, that is, a device to which one can cable up the
hard disk drives. Software RAID is a set of kernel modules, together with
management utilities that implement RAID purely in software, and require
no extra hardware.
Root name server
On the Internet, the root name server system is the manner in which an
authoritative master list of all top-level domain names (such as .com, .net,
.org and individual country codes) is maintained and made available.
SCSI
See Small Computer System Interface (SCSI).