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#11
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"idunno" wrote in message om... "Bill Todd" wrote in message ... For small writes, a classic RAID-3 implementation may be slightly faster because its spindles are synchronized (and the mirrored spindles in RAID-10 typically aren't, so the write there will take take the longer of the two disk access times). But if the RAID-10 controller has stable write-back cache such that it can queue up write requests and execute them in optimal order, any such advantage may be reduced or eliminated (as it will be for large writes in any event, where the access times become a smaller percentage of the overall overhead). But that's for purely serial small writes. For multiple small writes requested in parallel, RAID-10 may well be able to process at least some of them in parallel, whereas RAID-3 will serialize them at the parity disk (though should at least be able to queue-optimize their execution). RAID-10, of course, provides potentially significantly better *read* performance for a given usable capacity. It also provides somewhat better availability, since with RAID-3 (or -5) the loss of any two disks results in data loss whereas with RAID-10 data is lost only if the two disks happen to be partners. - bill Thank you. That helps. Are these performance differences maintained when you enable spindle sync on a RAID 10 array with a large write-back cache? Hmmm. Most people with read-dominated workloads would likely consider unsynched spindles a *feature* of RAID-10, since they'd get the faster of the two potential read options (at least if the array were suave) at only small expense in write performance. But if the array allows the option of synching the spindles for RAID-10 operation, and if you're performing only writes (so that the heads can be presumed to be in the same position on both mirror partners before each write request - at least I hope you could presume that), then I can't think of any situation off the top of my head in which the RAID-10 array wouldn't equal the speed of the RAID-3 array, and for parallel write operations it might well be superior (with the large write-back cache, even somewhat larger writes might be able to achieve some parallelism, though you're still going to have a hard limit of half the aggregate streaming bandwidth of the disks - the same value as the limit for a RAID-3 array of equal usable capacity, assuming that its parity generator can keep up). - bill |
#12
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Compelling...
thanks again |
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