A storage pool provides the pool of storage from which volumes are created. You must ensure that the MDisks that
make up each tier of the storage pool have the same performance and
reliability characteristics.
Notes:
- The performance of a storage pool is generally
governed by the slowest MDisk in the storage pool.
- The reliability of a storage pool is generally
governed by the weakest MDisk in the storage pool.
- If a single MDisk in a pool fails, access to the entire pool is
lost.
Use the following guidelines when you group similar disks:
- Group MDisks with equal performance in a single tier of a pool.
- Group similar arrays in a single tier. For example, configure
all 6 + P RAID-5 arrays in one tier of a pool.
- Group MDisks from the same type of storage system in a single tier of a pool.
- Group MDisks that use the same type of underlying physical disk
in a single tier of a pool. 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 storage pool to which it is assigned.
Important: When grouping internal RAID MDisks into
pools using similar technology disks, the arrays need to use a similar slow_write_priority setting to prevent creating a single
array that cannot avoid writing to a slow component. Failure to do
this would result in an immediate critical performance bottleneck
if a single drive in the pool became slow in the redundancy mode array. A simple exception to this rule is when using the
IBM Easy Tier function, the flash drive arrays are set to redundancy mode (as the write response time technique
is too coarse for typical flash drive latencies) and the spinning
disk arrays are set to latency mode.
Scenario: Similar disks are not grouped together
Under one scenario, you could have two externalstorage systems that are attached behind your system.
One device is an IBM TotalStorageEnterprise Storage Server (ESS),
which contains ten 6 + P RAID-5 arrays and MDisks 0 through 9. The
other device is an IBM System StorageDS5000, 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 storage pool, 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 DS5000storage system, therefore slowing down the IBM ESS arrays.
To fix this
problem, you can create three pools. The first pool must contain the
IBM ESS arrays, MDisks 0 through 9, the second pool must contain the
RAID 1 array, and the third pool must contain the large RAID 5 array.