- IO Queue
– I/O request is queued at the Volume level - Pend
– Channel/Path contention - Connect
– Data transfer time - Disconnect
– Seek time (arm movement), rotational delay
– Cache miss
Common explanations/reasons:
IOQ – the channel path or device is physically busy, too many datasets on the logical volume, or too many requests for data. The I/O rate is too high. Buffer Pool tuning may help this problem. You may have to move some datasets to another volume.
Defining all devices as 3390-3 on a RAID type system may result in too many datasets allocated to a device. SMS is a common cause of these allocation problems.
Pend – Channel/Path contention – some devices may need to be moved to another channel or path, or moving datasets to a volume on another channel or path will alleviate this. Indication of heavy I/O loads and/or data transfer rates.
Connect – Data transfer time. The length of the data (I.e. 4K page) divided by the transfer speed
Disconnect – Find the necessary cylinder, wait for the data to come around to the Read head on the access arm (rotational delay, or latency) Occurs because of a cache ‘miss’.