Pool configuration guidelines

A storage pool provides the pool of storage from which volumes are created. For optimal performance and reliability, you must ensure that the MDisks that make up each tier of the storage pool have the same characteristics.

Notes:
  1. Generally, the slowest MDisk in the storage pool governs the performance of that storage pool.
  2. Generally, the weakest MDisk in the storage pool governs the reliability of that storage pool.
  3. 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 storage systems types 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.
  • When possible, include similar sized MDisks in a storage pool tier to make it easier to balance the MDisks in that storage pool tier.
Important: When you group internal RAID MDisks into pools by using similar technology disks, the arrays must use a similar slow_write_priority setting to prevent creating a single array that cannot avoid writing to a slow component. Failure to use a similar setting results 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 you use 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.