Sans Digital Mobileraid MR8X, Detailed User'S Manual

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SAS-to-SATA II RAID Subsystem
15
Installation and Configuration Manual
1.3 RAID Concepts
he 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 5, 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.