There can be latency spikes in response from a RAID and such a spikes can in effect slow down all of the RAIDs as one I/O completion waits for all of the striped pieces to complete.

These latency spikes impact on throughput may be to stall all the I/O or to delay a few I/Os while others continue. It depends on how the I/O is striped across the devices. If the volumes are constructed as stripes to span all devices, and the I/Os are sized to be full stripes, the I/Os will stall, since every I/O has to touch every device. If the I/Os can be completed by touching a subset of the devices, then those that do not touch a high latency device can continue at full speed, while the stalled I/Os can complete and catch up later.

In large storage configurations, it is possible to lay out the volumes to maximize the opportunity for the I/Os to proceed in parallel, masking most of the effect of a few instances of high latency.

There are at least three classes of events that cause high latency I/O operations, as follows:

The first two events affect a single logical unit number (LUN). The third event affects all the LUNs on a controller. The first and third events appear to happen at random. The second event is repeatable.