The decisions regarding whether dynamic or static PAV should be used depends on the system’s specific IO profile. Typical decision criteria are:
For dynamic PAV
Acceleration of long-running batch processes, switch between day and night operation
Option for dealing with unexpected, lengthy peak loads with little overhead
Reserve "for all contingencies”
For static PAV
Consistent performance for high-performance requirements
In the case of an IO bottleneck ahead of specific, known volumes
For performance-critical data of the most important productive applications
From the performance viewpoint, basically nothing militates against a slight "overconfiguration". The behavior of alias devices "as a reserve" causes no measurable overhead.
The actual performance enhancement depends on, among other things, the cache hit rate and the performance of the RAID groups (see section "RAID levels and their performance"). Significantly higher throughput is achieved above all when the RAID level used can also utilize the parallelism of PAV. This is possible above all with a RAID level with “data striping”, in other words RAID 1/0, RAID 5, and RAID 6. This only applies with restrictions for RAID 1.
Primarily RAID 1/0 is recommended for high-performance application of PAV. If great store is set by the efficient use of the disk capacity and the random write load is not too high, RAID 5 or RAID 6 is also well suited to profit from PAV.
Number of alias devices
What number of alias devices makes sense for a volume depends on the load and the RAID group. One to three alias devices are customary, with the following recommendations applying here:
In an initial estimate, the factor measured without PAV between SW time and HW time can be applied for the number of devices including aliases.
In the case of RAID 1, one alias device is generally sufficient; no further improvements are to be expected here with more devices.
In the case of high-performance RAID groups with data striping (RAID 1/0, RAID 5, RAID 6), on the other hand, with a corresponding load for both throughput-oriented and transaction-oriented scenarios, up to three alias devices can make sense. To provide initial guidance here, please also compare the measured values in section "Performance when PAV is used".
More alias devices generally result in no enhancement as the volume’s natural limits are reached.
Important:
To determine the suitable configuration, a quantitative before-and-after comparison should always take place since the same conditions do not apply for all load situations.