Raid-Dp Overview; Protection Levels With Raid-Dp; Larger Versus Smaller Raid Groups - IBM N Series Hardware Manual

RAID-DP significantly increases the fault tolerance from failed disk drives over traditional
RAID. Based on the standard mean time to data loss (MTTDL) formula, RAID-DP is about
10,000 times more reliable than single-parity RAID on the same underlying disk drives. With
this level of reliability, RAID-DP offers significantly better data protection than RAID 1
mirroring, but at RAID 4 pricing. RAID-DP offers businesses the most compelling TCO
storage option without putting their data at increased risk.

10.3 RAID-DP overview

RAID-DP is available at no additional fee or special hardware requirements. By default, IBM
System Storage N series storage systems are shipped with the RAID-DP configuration.
However, IBM System Storage N series Gateways are not. The initial configuration has three
drives configured as shown in Figure 10-5.
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Figure 10-5 RAID-DP Initial factory setup

10.3.1 Protection levels with RAID-DP

At the lowest level, RAID-DP offers protection against two failed disks within the same RAID
group. It also offers protection from a single disk failure followed by a bad block or bit error
before reconstruction has completed. A higher level of protection is available by using
RAID-DP in conjunction with SyncMirror. In this configuration, the protection level is up to five
concurrent disk failures. That is, you are protected against four concurrent disk failures
followed by a bad block or bit error before reconstruction is completed.

10.3.2 Larger versus smaller RAID groups

Configuring an optimum RAID group size for a volume requires balancing factors. Decide
which factor (speed of recovery, assurance against data loss, or maximizing data storage
space) is most important for the volume that you are configuring.
Advantages of large RAID groups
Large RAID group configurations offer the following advantages:
More data drives available
A volume configured into a few large RAID groups requires fewer drives reserved for parity
than that same volume configured into many small RAID groups.
Better system performance
Read/write operations are usually faster over large RAID groups than over smaller RAID
groups.
ONTAP
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Chapter 10. Data protection with RAID Double Parity
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