Functions And Limitations Of Raid Schemes - HP 381513-B21 - Smart Array P800 Controller RAID Technology Brief

Functions and limitations of RAID schemes

Before creating large arrays with a high number of disk drives or with high-capacity disk drives, IT
managers should consider the limitations of available RAID schemes in protecting data during a
single- or multiple-drive failure. RAID schemes, called levels, are differentiated by the method each
uses to provide fault tolerance. Note that the RAID level numbers do not correlate with the degree of
fault protection provided. Table 1 illustrates the RAID levels described in this section.
In a RAID 0 implementation, user data is striped 2 across all the drives in the array. For large files,
reading this data in parallel from the separate drives is faster than reading the file from a single drive.
Also, many small files can be read in parallel. However, this RAID scheme offers no fault tolerance;
the entire array will fail if one drive fails.
RAID 1 is a mirroring scheme that stores identical data on two sets of drives. It is used in applications
that require very high availability. RAID 1 has high fault tolerance, but it has low storage efficiency
because it requires twice the number of drives required for RAID 0.
RAID 1+0 is implemented as a striped array of mirrored drives. It is best suited for sites that need high
performance and maximum reliability, but are willing to forgo storage efficiency. RAID 1+0 can
withstand the failure of half the drives as long as no two drives in a mirrored pair fail; however, it
sacrifices storage efficiency.
RAID 5 is implemented as a striped array of three or more drives. Parity information is calculated for
each stripe of data and is placed on a different drive than the related data (see Table 1). The parity
information is spread across all drives in the array and occupies the equivalent capacity of one
physical drive. RAID 5 provides good performance and can withstand the loss of a single drive
without failure of the array. If a second drive fails before the first failed drive can be replaced,
however, the entire array will fail.
RAID 6 (ADG) is an extension of RAID 5 for implementation on arrays of four or more drives. The
data and two sets of parity information are striped across all drives in the array. The additional set of
parity improves the fault tolerance of the array but results in lower write performance. The two sets of
parity information are stored in different locations across the drives in the array and occupy the
equivalent capacity of two physical drives. RAID 6 protects against the simultaneous failure of two
drives in the array.
Striping is the distribution of data over multiple disk drives to improve performance. Data is interleaved in
2
groups of sectors known as "stripes" across the drives.
3