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HP OpenVMS Systemsask the wizard |
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The Question is: I'm looking to add storage capacity to my CI cluster by purchasing an sw800 cabinet with dual redundant controllers and dual power supplies per shelf, and configure some RAID 3/5 arrays. When purchasing disks, would you get better performance by purchasing 4.3GB drives or half as many 9.1 GB drives to get the same RAID volume capacity? 4.3 GB drives are 7200 RPM where 9.1GB drives are 10,000 RPM You can get 1-9.1 gb drive for less $$ than 2-4.3GB drives. Would you get better performance with the increased number of spindles with the slower drives? Using lower capacity drives reduces your expansion capabilities by using twice as many shelf bays. What about the same scenareo with 9.1 vs 18 GB drives? How to choose... Please, Oh Wizard, give me some clues.... The Answer is : There is no answer to this question without substantially more site characterization information. The OpenVMS performance management manual may be of interest. In isolation, more spindles will provide better I/O throughput, and fewer spindles will provide more storage expansion. That said, disk performance is seldom viewed in isolation. You must take into account application requirements, CPU I/O completion capacity, the available host memory for caching, host controller bandwidth (the CI, in this case), SCSI bandwidth, storage controller bandwidth, available storage controller interconnect (CI) bandwidth, and various other factors. Disk-specific factors include the transfer rate and particularly the spiral read read, as well as the access time -- characteristics of the application I/O activity dictate if access time (many small I/Os) or spiral read rate (fewer larger I/Os) will be of central interest. Additional considerations include expansion capabilities, as you stated, and the ability of the controllers to perform RAID functions such as striping and shadowing (mirroring). Also of interest will be your level of interest in managing the low-level behaviour of the applications and of the disk I/O subsystem. (RAID controllers can often be configured to transparently divide I/O operations across multiple disk spindles, thus increasing I/O throughput.) In this particular configuration, you will particularly want to consider the bandwidth of the CI adapters and the activity on the CI buses. Unless your present storage configuration is severely I/O bound and you have similar storage technologies with those of your planned upgrade, you will likely see improvements from any storage upgrade. (Barring an existing I/O performance limit -- CI bandwidth, for instance -- or an existing non-I/O performance limit -- eg: CPU performance -- that is not specifically related to the disk or RAID controller performance.) Thus the use of the larger disks may be more cost-effective.
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