## Introduction to RAID technology 

RAID provides two key design goals: 

GLYPH<SM590000> Increased data reliability 

GLYPH<SM590000> Increased input/output (I/O) performance 

When multiple physical disks are set up to use the RAID technology, they are in a RAID array . The IBM Storwize V5000 Gen2 provides the following traditional RAID levels: 

GLYPH<SM590000> RAID 0 

GLYPH<SM590000> RAID 1 

GLYPH<SM590000> RAID 5 

GLYPH<SM590000> RAID 6 

GLYPH<SM590000> RAID 10 

RAID technology can provide better performance for data access, high availability for the data, or a combination. RAID levels define a trade-off between high availability, performance, and cost. 

The RAID concept must be extended to  disk rebuild time  because of increasing physical disk capacity. 

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 becomes more likely. In addition, the spares are idle when they are not being used, which wastes resources. 

Distributed RAID (DRAID) addresses these points and is available for the IBM Storwize V5000 Gen2 in two types: 

GLYPH<SM590000> Distributed RAID 5 (DRAID 5) 

GLYPH<SM590000> Distributed RAID 6 (DRAID 6) 

Distributed RAID reduces the recovery time and the probability of a second failure during rebuild. As with 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. 

Note:  Although Traditional RAID is still supported and is the default choice in the GUI, the suggestion is to use DRAID 6 whenever possible. It depends on the number of disk drives, type, and size. 

## 4.3.3 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 spare, all drives contribute to performance. The spare capacity is rotated across the disk drives so the write rebuild load is distributed across multiple drives and the bottleneck of one drive is removed. 

Figure 4-47 on page 197 shows an example of a distributed RAID 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. 

