APC RAID Subsystem SCSI-SATA II User Manual download pdf (Page 34)

SCSI to SATA II RAID Subsystem

User Manual

Chapter 4 RAID Concepts

4.1 RAID Fundamentals

The basic idea of RAID (Redundant Array of Independent Disks) is to combine multiple

inexpensive disk drives into an array of disk drives to obtain performance, capacity and

reliability that exceeds that of a single large drive. The array of drives appears to the

host computer as a single logical drive.

Five types of array architectures, RAID 1 through RAID 6, were originally defined; each

provides disk fault-tolerance with different compromises in features and performance. In

addition to these five redundant array architectures, it has become popular to refer to a

non-redundant array of disk drives as a RAID 0 arrays.

Disk Striping

Fundamental to RAID technology is striping. This is a method of combining multiple

drives into one logical storage unit. Striping partitions the storage space of each drive

into stripes, which can be as small as one sector (512 bytes) or as large as several

megabytes. These stripes are then interleaved in a rotating sequence, so that the

combined space is composed alternately of stripes from each drive. The specific type of

operating environment determines whether large or small stripes should be used.

Most operating systems today support concurrent disk I/O operations across multiple

drives. However, in order to maximize throughput for the disk subsystem, the I/O load

must be balanced across all the drives so that each drive can be kept busy as much as

possible. In a multiple drive system without striping, the disk I/O load is never perfectly

balanced. Some drives will contain data files that are frequently accessed and some

drives will rarely be accessed.

By striping the drives in the array with stripes large enough so that each record falls

entirely within one stripe, most records can be evenly distributed across all drives. This

keeps all drives in the array busy during heavy load situations. This situation allows all

drives to work concurrently on different I/O operations, and thus maximize the number

of simultaneous I/O operations that can be performed by the array.

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APC RAID Subsystem SCSI-SATA II User Manual Page 34