Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 181)

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Theory of Operation

RAID

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.

The Mean Time Between Failure (MTBF) of the array is equal to the

MTBF of an individual drive, divided by the number of drives in the array.

Because of this, the MTBF of a non-redundant array (RAID 0) is too low

for mission-critical systems. However, disk arrays can be made fault-

tolerant by redundantly storing information in various ways.

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 array.

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.

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Acer SmartRAID V Bedienungsanleitung Seite 181