Optimal MDisk group configuration guidelines for storage systems

A managed disk (MDisk) group provides the pool of storage from which virtual disks (VDisks) are created. You must ensure that the entire pool of storage provides the same performance and reliability characteristics.

Notes:

The performance of an MDisk group is generally governed by the slowest MDisk in the group.
The reliability of an MDisk group is generally governed by the weakest MDisk in the group.
If a single MDisk in a group fails, access to the entire group is lost.

Use the following guidelines when you group similar disks:

Group equally performing MDisks in a single group.
Group similar arrays in a single group. For example, configure all 6 + P RAID-5 arrays in one group.
Group MDisks from the same type of storage system in a single group.
Group MDisks that use the same type of underlying physical disk in a single group. For example, group MDisks by fibre-channel or SATA.
Do not use single disks. Single disks do not provide redundancy. Failure of a single disk results in total data loss of the MDisk group to which it is assigned.

Scenario: Similar disks are not grouped together

Under one scenario, you could have two storage systems that are attached behind your SAN Volume Controller. One device is an IBM® TotalStorage® Enterprise Storage Server® (ESS), which contains ten 6 + P RAID-5 arrays and MDisks 0 through 9. The other device is an IBM System Storage® DS5000, which contains a single RAID-1 array, MDisk10, one single JBOD, MDisk11, and a large 15 + P RAID-5 array, MDisk12.

If you assigned MDisks 0 through 9 and MDisk11 into a single MDisk group, and the JBOD MDisk11 fails, you lose access to all of the IBM ESS arrays, even though they are online. The performance is limited to the performance of the JBOD in the IBM DS5000 storage system, therefore slowing down the IBM ESS arrays.

To fix this problem, you can create three groups. The first group must contain the IBM ESS arrays, MDisks 0 through 9, the second group must contain the RAID-1 array, and the third group must contain the large RAID-5 array.

Attention: Do not combine internal SAN Volume Controller solid-state drives (SSDs) and external HDD-based storage systems in the same MDisk group.