LSI Series 518 2.3.8Parity, 2.3.9Hot Spares

Table 2.1 describes how disk spanning is used for RAID 10 and RAID 50.

Table 2.1 Spanning for RAID 10 and RAID 50

Level Description

10Conﬁgure RAID 10 by spanning two contiguous RAID 1 logical drives. The RAID 1 logical drives must have the same stripe size.

50Conﬁgure RAID 50 by spanning two contiguous RAID 5 logical drives. The RAID 5 logical drives must have the same stripe size.

Note: Spanning two contiguous RAID 0 logical drives does not produce a new RAID level or add fault tolerance. It does increase the size of the logical volume and improves performance by doubling the number of spindles.

2.3.8Parity

Parity generates a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets. Parity data does not fully duplicate the parent data sets. In RAID, this method is applied to entire drives (dedicated parity) or to stripes across all disk drives in an array (distributed parity).

RAID 5 combines distributed parity with disk striping. If a single disk drive fails, it can be rebuilt from the parity and the data on the remaining drives. Parity provides redundancy for one drive failure without duplicating the contents of entire disk drives, but parity generation can slow the write process.

2.3.9Hot Spares

A hot spare is an extra, unused disk drive that is part of the disk subsystem. It is usually in standby mode, ready for service if a drive fails. Hot spares permit you to replace failed drives without system shutdown or user intervention.

MegaRAID SCSI 320-2 implements automatic and transparent rebuilds using hot spare drives, providing a high degree of fault tolerance and zero downtime. The MegaRAID SCSI 320-2 RAID Management software allows you to specify physical drives as hot spares. When a hot spare is needed, the MegaRAID SCSI 320-2 controller assigns the hot spare that

2-8	Introduction to RAID