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On May 12, 2009, at 2:51 PM, Jerry Feldman wrote: > Could you elaborate. Right now I think the jury is still out on > these devices. Personally, back in then 1970s I was under the > impression that Say that you are running some form of real-time (or near-real-time), high volume system. The one I have experience is institutional equities trading. A potential bottleneck is writing out the transaction logs. Say that you need a minimum of 15,000 IOPS to keep up with your heaviest volume (I exaggerate by a factor of 10 for the sake of example). What are your options? The ones I had looked at are: * Lots of rotating media striped together. Given disks that can handle bursts of 300 IOPS I would need at least 50 such disks to handle the load. Least expensive up front costs but requires the most power and cooling over a long term, especially if the application grows. * Rotating media with lots of battery backed up cache. This is cheaper on disk but in the case of EMC it actually costs a lot more up front than the disks. Does not scale at all. DMX frames have a fixed, finite, relatively small (IIRC, 128GB on DMX3) cache capacity. * Flash-based SSDs. These can handle the load but are quite expensive, but not as expensive as EMC cache. Flash-based SSDs have the same wear problems as every other form of flash-based media. Much of the expense is in redundant cells to offer MTBF comparable to rotating media. Immune to power loss. * SRAM or DRAM-based SSDs. Fastest media, less expensive than flash and cache although more expensive than rotating media. Longer MTBF than flash; no wear leveling needed. More scaleable than cache. Susceptible to power failure if the batteries fail. For high performance needs, where cache and rotating media are insufficient, there is a reasonable need for either flash or RAM SSDs. Which is appropriate depends on the application. For my example, based on a real project I worked on, I would use flash-based SSDs because I want the power fault tolerance. For an application that requires the performance but not necessarily the fault tolerance I would consider RAM-based SSDs. --Rich P.
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