Why Use Raid-Dp; Single-Parity Raid Using Larger Disks - IBM N Series Hardware Manual

10.2 Why use RAID-DP

As mentioned earlier, traditional single-parity RAID offers adequate protection against a
single event. This event can be either a complete disk failure or a bit error during a read. In
either event, data is re-created by using both parity data and data that remains on unaffected
disks in the array or volume.
If the event is a read error, re-creating data happens almost instantaneously and the array or
volume remains in an online mode. However, if a disk fails, the lost data must be re-created.
The array or volume remains in a vulnerable degraded mode until data is reconstructed onto a
replacement disk or global hot spare disk. This degraded mode is where traditional
single-parity RAID fails to meet the demands of modern disk architectures. In single-parity
RAID, the chance of secondary disk failure is increased during rebuild times, increasing the
risk of data loss.
Modern disk architectures have continued to evolve, as have other computer-related
technologies. Disk drives are orders of magnitude larger than they were when RAID was first
introduced. As disk drives have gotten larger, their reliability has not improved, and the bit
error likelihood per drive has increased proportionally with larger media. These three factors
(larger disks, unimproved reliability, and increased bit errors with larger media) have serious
consequences for the ability of single-parity RAID to protect data.
Given that disks are as likely to fail now as when RAID technology was first introduced, RAID
is still vital. Integrating RAID-DP when one disk fails, RAID re-creates data from both parities
and the remaining disks in the array or volume onto a hot spare disk. But because RAID was
introduced, the significant increases in disk size have resulted in much longer reconstruction
times for data lost on the failed disk.
It takes much longer to re-create lost data when a 274 GB disk fails than when a 36 GB disk
fails (Figure 10-2). In addition, reconstruction times are longer because the larger disk drives
in use today tend to be ATA-based. ATA-based drives run more slowly and are less reliable
than smaller, SCSI-based drives.
Figure 10-2 Disk size versus reconstruction time

10.2.1 Single-parity RAID using larger disks

The various options to extend the ability of single-parity RAID to protect data as disks
continue to get larger are not attractive. The first option is to continue to buy and implement
storage using the smallest disk sizes possible so that reconstruction completes quicker.
However, this approach is impractical. Capacity density is critical in space-constrained data
Chapter 10. Data protection with RAID Double Parity
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