StoreVirtual 4730 - Queue Depth

[MichaeliS]

Hi folks!

We're using a HP StoreVirtual 4730 with 4 nodes seperated to two sites. Now i've got questions about the "Queue Depth" paramter showing in performance monitor. 

First one is, what is a bad value or where is the limit to bad performance viewing this sensor?

Second one, where can i check the "queue length" of my StoreVirtual Adapters?

Thanks in advance!

[Stor_Mort]

Hi MichaeliS, 

Queue depth is a measure of the load being applied to a storage system (or any resource with a queue). It is a count of the number of pending IO requests in the queue being measured.

You can select different queues in the Performance Monitor. A good starting point is Cluster Queue Depth. I like to monitor both the read and write queues rather than the total.

Queue depth can change very quickly as production load changes and is not tightly coupled to latency. However,  persistently high queue depths will tend to accompany high latency.

There is an old rule that an average queue depth which is near the number of spindles in the storage unit is optimal for SATA technology drives (i.e. one transaction pending per spindle.) Double the number of spindles is optimal for SAS technology drives. But that should be taken as a very loose recommendation. Depending on storage system optimizations and traffic characteristics, you may start to see slowdowns at higher or lower levels than that. Queue depth and latency together give a better picture of how the system is performing.

There's nothing wrong with a low queue depth, of course. It just means that the storage system is not being kept as busy as is possible. It could handle more load if the servers demand it.