HP ProLiant DL380 G5 Data Protection Storage Server Administration Guide (434184-001, September 2006)

Arr ays

See Figure 17. With an array controller installed in the system, the capacity of several physical drives

(P1–P3) can be logically combined into one or mo re logical units (L1) called arrays. When this is

done, the read/write heads of all the c onstituent physical drives are active simultaneously, dramatically

reducing the ov

erall time required for data transfer.

NOTE:

Depending on the storage server model, array conﬁguration may not be possible or necessary.

P1 P3P2

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Figure 17 Conﬁguri ng arrays from physical drives

Because the read/write heads are simultaneously active, the same amount of data is written to each

drive during any given time interval. Each unit of data is termed a block. The blocks form a set of data

stripes over all the ha rd drives in an array, as shown in Figure 18.

B11B10 B12

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Figure 18 RAID 0 (data striping) (S1-S4) of data blocks (B1-B12)

For data in the array to be readable, the data block sequence within each stripe must be the same.

This sequencing process is performed by the array controller, which sends the d ata blocks to the drive

write heads in the correct order.

A natural consequence of the striping process is that each hard drive in a given array contains the

same number of data blocks.

NOTE:

If one hard drive has a larger capacity than other hard drives in the same array , the extra capacity is

wastedbecauseitcannotbeusedbythearray.

Fault tolerance

Drive failure, although rare, is potentially catastrophic. For example, using simple striping as shown in

Figure 18, failure of any hard drive leads to failure of all logical drives in the same array, and hence to

data loss.

To protect against data loss from hard drive failure, storage servers should b e con ﬁgured with fault

tolerance. HP recommends adhering to RAID 5 conﬁgurations.

HP ProLiant DL380 G5 Data Protection Storage Server 39