How Raid Works; Data Storage Methods; Data Striping; Data Mirroring - Accusys ExaSAN User Manual

5. RAID Overview
This chapter gives an overview of RAID storage system within the context of recommended
setting of ExaSAN based SAN environment to provide fast, shared storage to client computers.

5.1 How RAID Works

RAID, or Redundant Array of Independent Disks, is a data-storage technology that spreads data
across multiple drives. This technology provides several benefits over a single large hard disk,
including:

Data redundancy for protection and availability

Higher performance as a result of reading or writing on several drives simultaneously

Scalability for expansion of storage
The ExaSAN-family RAID systems use a hardware controller, which manages up to 48 HDDs.
By segmenting and writing or reading data on multiple drives simultaneously, the RAID
controller achieves fast and highly efficient storage and access.
The way the controller stores and retrieves data on the RAID system is determined by the RAID
level and storage method you choose. For ExaSAN applications, the recommended setting is
mainly RAID 1 and RAID 5, which will be discussed in details later in this chapter.
Once you have defined a group of drive modules as a RAID set, the controller groups those
drives into "logical disks." On the ExaSAN RAID system, each logical disk appears to the client
system as one Logical Unit (LUN), regardless of the number of actual drives in that logical unit.
5.2

Data Storage Methods

The controller stores and retrieves data on a RAID system using techniques such as "data
striping", "data mirroring", and "data parity".

5.2.1 Data Striping

Multiple hard disk drives in a RAID group, referred to as a "set" or "array," are divided
(partitioned) into stripes. The controller spreads stripes across the disks in alternating sections
on each drive.
In data-intensive environments such as digital video editing, performance is optimized by writing
data across small stripes, so that each record spans all drives. This method ensures that access
to large records is very fast because data is transferred in parallel across multiple drives.

5.2.2 Data Mirroring

To mirror data, the RAID controller duplicates all data on two different disks. One disk is the
primary; the other is the mirrored disk. The primary and mirrored disks are synchronized; that is,
anything written to one disk is also written to the other. Mirrored data is very secure because if
one disk fails, the data is available from the other disk.
User Guide
RAID
5-1