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As the high capacity of disk devices (DASDs, disk units, arms and drives) for iSeries & AS/400 systems get available, less arms are required to satisfy capacity needs. This will lead to configure some disk arms for meeting the workload that is placed on them. Lack of the disk arms will bottleneck processor’s performance. In order, to avoid such bottleneck, minimum number of the disk arms is required for the optimum performance on every processor level. The number is independent of quantity of the drives required to meet desired storage ability.
Why it is very important to have sufficient disk arms? Lack of the disk arms will bottleneck the performance for many reasons:
The physical disk drive will just perform some number of the disk accesses every second before performance begins to get impacted. Whereas today’s technology allows the disk to carry out in excess of hundred operations every second, performance is been impacted far before the number is reached. Impact begins to be felt when the number of the disk accesses reaches around 40percent of maximum. This is explained by queuing the theory.
In case, more of accesses are requested this can be done in the given period (often the fraction of second), some requests should wait in the queue while previous requests are been processed. This happens because disk requests happen in the random time intervals as well as take differing amounts of time. Greater the number of the disk requests every second or longer they take being processed, more queuing happens. For these requests that should spend time in the queue, total response time that equals not just the time for a drive to service access request, however the time spent in the queue.
In order, to maintain the optimum efficiency (some disk requests wanting to wait), drive must probably not have to handle about 30requests or second (that depends on a disk drive, protection approach, controller, as well as other variables). When the disk reaches forty percent utilization, the performance is starting to fall (requests are waiting in a queue).
The performance problems will happen in case, the server is sized very strictly on the capacity with some disk arms that are specified. As an example: For the simplicity, we can ignore the RAID in discussion. Let us assume the customer requirements 175GB of capacity as well as needs to perform 600disk accesses every second. In this instance, these accesses should happen at a disk level, and after accounting for iSeries self caching abilities.