Rick, While the arrays have all this parallelism built in so that sequential IO is pre-fetched concurrently from many disks into the cache (on some RAID designs), the effectiveness is somewhat discounted when this massively parallel data stream from disk to cache is transferred one SSCH at a time on one channel at a time from cache to host.
While PAV provides a degree of parallelism at the volume and dataset level, it does not provide any parallelism at requesting program level. Using wide striping with striped datasets (RAID-0) means that the buffers can be processing reads and writes to many volumes in parallel across many channels. In simple terms, a four way striped dataset can be processed up to four times faster than an un-striped dataset. The other advantage of striping, especially wide striping, is prevention or mitigation of hot spots. Even with PAV we have all seen how the reporting phase of batch processing can bog down when the new master file is suddenly hammered by 25 reporting programs. There are a few ways to relieve this such as IO avoidance with DLF, or creating clone datasets with FCV2 or similar. Another way is to stripe the dataset so that the IO is spread across many volumes, paths, and usually more disk drives. This is the whole logic behind LDEV Striping (HDS parlance). Array Groups with 4 disk drives, RAID-5 and 10, do not handle hot spots as well as Array Groups with 8 disk drives. LDEV striping allows volumes to be striped over 16 or 32 disk, which in turn reduce skew and soften the hot spots that create sibling pend. The same logic applies to striped datasets. RAID-0 datasets for random activity need no explanation - it is just better than sliced bread. Ron > At the risk of sounding obtuse, I have to ask the question: why is > striping even an issue today? > > Given the architecture of modern "DASD-like" storage systems and the > advent of Dynamic PAV, the hardware and operating system facilities > SEEM > to address all of the performance considerations that might seem to > mitigate in favor of striping. > ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
