Dell
DELL PERC H700 and H800 Technical Guide 21
5 RAID Overview
5.1 About RAID
RAID is a way of storing data on multiple independent physical disks for the purpose of enhanced
performance and/or fault tolerance. The physical disks combine to make up what is called a virtual
disk. This virtual disk appears to the host system as a single logical unit or drive. For example, if you
have physical disk 1 and physical disk 2 forming a RAID virtual disk, those two disks appear to the
host system as one disk.
Virtual Disks are sometimes called volumes, containers, or arrays.
There are several different RAID types or levels, which determine how the data is placed in the
virtual disk. Each RAID level has specific data protection and system performance characteristics.
The following are commonly used RAID levels:
• RAID 0: Striping without parity, improved performance, additional storage, no fault tolerance
• RAID 1: Mirroring without parity, fault tolerance for disk errors, and single disk failures
• RAID 5: Striping with distributed parity, improved performance, fault tolerance for disk
errors, and single disk failures
• RAID 6: Striping with dual parity, fault tolerance for dual drive failures
• RAID 10: Mirroring combined with striping, better performance, fault tolerance for disk
errors, and multiple drive failure (one drive failure per mirror set)
• RAID 50: Combines multiple RAID 5 sets with striping, improved performance, fault disk
errors, and multiple drive failures (one drive failure per span)
• RAID 60: Combines multiple RAID 6 sets with striping, improved performance, fault disk
errors, and multiple drive failures (two drive failures per span)
These RAID levels are discussed in more detail later in this document. You can manage RAID virtual
disks with a RAID controller (hardware RAID) or with software (software RAID).
5.2 Advantages of RAID
Depending on how you implement RAID, the benefits include one or both of the following:
• Faster performance—In RAID 0, 10, 50, or 60 virtual disks, the host system can access
simultaneously. This improves performance because each disk in a virtual disk has to handle
the request. For example, in a two-disk virtual disk, each disk needs to provide only its
requested data.
• Data protection—In RAID 1, 10, 5, 6, 50, and 60 virtual disks, the data is backed up on disk
(mirror). In the RAID 5, 50, 6, or 60 virtual disks, the data is also parity protected. RAID 10,
50, and 60 also allow the host to access disks simultaneously.
5.3 Supported RAID Levels
Dell servers that use RAID controllers may support RAID 0, 1, 5, 6, 10, 50, and 60 depending upon the
controller. The following is a brief explanation of these levels.
5.3.1 RAID 0 (Striped Virtual Disk without Fault Tolerance)
RAID 0, also known as striping, maps data across the physical drives to create a large virtual disk.
The data is divided into consecutive segments or stripes that are written sequentially across the
drives in the virtual disk. See Figure 4. Each stripe has a defined size or depth in blocks.