Chapter 6. Storage pools 215
Introduction to RAID technology
RAID technology can provide better performance for data access, high availability for the 
data, or a combination of both. RAID levels define a trade-off between high availability, 
performance, and cost.
When multiple physical disks are set up to use the RAID technology, they are in a RAID array. 
The IBM Storwize V7000 provides multiple, traditional RAID levels:
/SM590000RAID 0
/SM590000RAID 1
/SM590000RAID 5
/SM590000RAID 6 
/SM590000RAID 10
In a traditional RAID approach, whether it is RAID10, RAID5, or RAID6, data is spread among 
drives in an array. However, the spare space is constituted by spare drives, which are global 
and sit outside of the array. When one of the drives within the array fails the system responds 
as follows:
/SM590000For RAID10, all data is read from the mirrored copy.
/SM590000For RAID5 or RAID6, data is calculated from remaining data stripes and parity.
Then, the data is written to a spare drive.
In a disk failure, traditional RAID writes the data to a single spare drive. With increasing 
capacity, the rebuild time is also increased. And the probability of a second failure during the 
rebuild process also becomes more likely. In addition, the spares, when they are not being 
used, are idle, which wastes resources.
Distributed RAID (DRAID) addresses those points.
Distributed RAID
In distributed RAID, all drives are active, which improves performance. Spare capacity is used 
instead of the idle spare drives from traditional RAID. Because no drives are idling, all drives 
contribute to performance. The spare capacity is spread across the disk drives, so the write 
rebuild load is distributed across multiple drives and the bottleneck of one drive is removed.
Distributed RAID reduces the recovery time and the probability of a second failure during 
rebuild. Like traditional RAID, a distributed RAID 5 array can lose one physical drive and 
survive. If another drive fails in the same array before the bad drive is recovered, the MDisk 
and the storage pool go offline as they are supposed to. So, distributed RAID does not 
change the general RAID behavior. 
It is available for the IBM Storwize V7000 in two types: 
/SM590000Distributed RAID 5 (DRAID 5)
/SM590000Distributed RAID 6 (DRAID 6)
Figure 6-34 on page 216 shows an example of a Distributed RAID6 with 10 disks. The 
physical disk drives are divided into multiple packs. The reserved spare capacity (which is 
marked in yellow) is equivalent to two spare drives, but the capacity is distributed across all of 
the physical disk drives. The data is distributed across a single row.