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HP OpenVMS Systemsask the wizard |
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The Question is: Hi, We have a number of controller based RAID 5 sets configured via an HSZ50 ocntroller. I have a question which has puzzled me and would be interested in any input you have. All things being equal, from purely a performance point of view, if we had 6 files all being updated at the same time by the same application, would be it be better to to have a single logical VMS disk based on a 6 member RAID 5 set(using one disk from ea ch of six available shelves) or would we benefit by splitting each file to seperate partition on the RAID 5 set and mounting the six VMS logical disks like this. Please assume that writeback and read cache are enabled on the unit. I get the feeling that it will boil down to a question of how the caches are utilized, but lacking this information, I am unable to form an opinion. Regards Edwin The Answer is : When a single application updates multiple files on individual partitions on a controller disk unit, it is effectivly guaranteeing a certain "seek" distance. This is not optimal. This does not involve the configuration: whether the unit was RAID-5, RAID-1, or JBOD, though with RAID at least there are some odds that the files will be distributed across different spindles. In the opinion of the OpenVMS Wizard, the caching algorithms do not play a particular role here. When a single application needs to more or less simultaneously update multiple files, then either those files should be on independent disk spindles, or they should be located as close together as is possible. In the same partition, in other words. You can further request the files be allocated nearby using the allocation attributes available via the FDL "POSITION" and associated subfunctions for "FILE" and LBN proximity. File placement is often the least effective of the available performance "tweaks". Pre-allocation, correct bucketsizes, global buffers, faster I/O subsystems, and having the appropriate numbers of buffers available are all typically more effective controls. But once all those settings are near optimal, then by all means consider the implications of file placement for that last measure of application performance.
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