Chapter 10. Data protection with RAID Double Parity
151
Figure 10-4 RAID-DP
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
approximately 10,000 times more reliable than single-parity RAID on the same underlying
disk drives. With this level of reliability, RAID-DP offers 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 included with the RAID-DP configuration.
However, IBM System Storage N series Gateways are not. The initial configuration has three
drives that are configured, as shown in Figure 10-5.
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 completes. A higher level of protection is available by using RAID-DP
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 that is configured into a few large RAID groups requires fewer drives that are
reserved for parity than that same volume that is configured into many small RAID groups.
RAID-DP:
Tolerates up to 2
Failed drives
RAID-DP uses
Two parity
Drives.
ONTAP
PARITY
PARITY
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