Arrays; Fault Tolerance - HP StoreEasy 1000 Storage Administrator's Manual

Arrays

See Figure 31 (page
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 overall time required for data transfer.
NOTE: Depending on the storage system model, array configuration may not be possible or
necessary.
Figure 31 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 32 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 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 wasted because it cannot be used by the array.

Fault tolerance

Drive failure, although rare, is potentially catastrophic. For example, using simple striping as shown
in Figure 32 (page
array, and hence to data loss.
To protect against data loss from hard drive failure, storage systems should be configured with
fault tolerance. HP recommends adhering to RAID 5 configurations.
The table below summarizes the important features of the different kinds of RAID supported by the
Smart Array controllers. The decision chart in the following table can help determine which option
is best for different situations.
54 Storage management overview
54). With an array controller installed in the system, the capacity of several
54), failure of any hard drive leads to failure of all logical drives in the same
Figure 32 (page 54).