180 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
Introduction to RAID technology
RAID provides two key design goals:
Increased data reliability
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 multiple, traditional RAID levels:
RAID 0
RAID 1
RAID 5
RAID 6
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, as well. In addition, the spares, when they are not being
used, are idle, wasting resources.
Distributed RAID (DRAID) addresses those points and it is available for the IBM Storwize
V5000 Gen2 in two types:
Distributed RAID 5 (DRAID 5)
Distributed RAID 6 (DRAID 6)
Distributed RAID reduces the recovery time and the probability of a second failure during
rebuild. Just 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.
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 4-50 on page 181 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.
Note: Although Traditional RAID is still supported and is the default choice in the GUI, the
suggestion is to use distributed RAID 6 whenever possible.
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