Accusys ExaSAN SW-08 - Manual (Page 49)

5. RAID
User Guide Page 5-1
5. RAID Overview
This chapter gives an overview of RAID storage system within the context of recommended setting of
ExaSAN based Xsan 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 A08S-PS RAID systems use a hardware controller, which manages up to eight drive
modules. 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 Compact 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.
5.2.3 Data Parity
The controller can generate “parity” for the ability to protect and rebuild data. Parity protects stored
information without requiring data mirroring. When data is protected by parity, it is still available if a
drive fails. Parity-protected data is reconstructed using the parity formula. You can remove and replace a
failed disk (known as “hot swapping”), and the controller then rebuilds the data using the information on
the remaining drives.