HP ProLiant DL100 G2 Data Protection Storage Server administration guide (June 2006)

Arr ays

See Figure 13. With an array controller installed in the system, the capacity of several physical drives

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

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

reducing the ov

erall time required for data transfer.

NOTE:

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

P1 P3P2

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Figure 13 Conﬁguring 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 hard drives in an array, as shown in Figure 14.

B11B10 B12

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Figure 14 R AID 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 data 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 14, 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 fro m ha rd drive failure, storage servers should be conﬁgured with fault

tolerance. HP recommends adhering to RAID 5 conﬁgurations.

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