On Feb 10, 2013, at 2:09 PM, Ellis H. Wilson III wrote: > On 02/10/13 04:41, Vincent Diepeveen wrote: >> SSD's are not about bandwidth, they're about latency. > > This is a bit aggressive of a vantage point -- let's tone it back: > "SSD's aren't always the cheapest way to achieve bandwidth, but > they are > critical for latency-sensitive applications that are too large for > main > memory." >
SSD's are never the cheapest way to achieve bandwidth and never will be. > In any event, your original statement used to be wholly correct. > It has > changed to a certain degree to "SSDs are about IOPs," which isn't > quite > the same thing. However, more pointedly, with modern HDDs barely > approaching 200MB/s and SSD solutions approaching 2-4GB/s, this is an > increasingly limited viewpoint. We have to start considering their > use > for bandwidth. Find me an application that needs big bandwidth and doesn't need massive storage. So any SSD solution that's *not* used for latency sensitive workloads, it needs thousands of dollars worth of SSD's. In such case plain old harddrive technology that's at buy in price right now $35 for a 2 TB disk (if you buy in a lot, that's the actual buy in price for big shops and you nor i get them for that price of course), or $17.5 a terabyte, that's unbeatable in performance for storage and bandwidth. We speak about a sustained 200MB/s for dirt cheap RAID harddrives here. Put 16 of them in a raid partition and you can get more than you can deliver over the network from the file server and more than your motherboard can effectively handle a second. We speak about a buy in price of total peanuts for 16 harddrives here, and the same storage in SSD is worth a total fortune. So using SSD's is just for latency. Anyone not using them for that i would never hire. > >> With a raid array of cheapo disks we can also get 3GB/s bandwidth, >> more than most 2 socket nodes effectively can handle. > > 3GB/s divided by 200MB/s gives me something like 15 drives, unless my > math is wrong, which will be something like $2-$3K, and that's really > only possible in RAID0, so you're only going to get the capacity of > one > drive. If all I'm looking for is bandwidth I'd rather spend that > 3k on > an expensive SSD (or RAID a bunch of cheaper SSDs) and get it for far > less power, wire complexity, space consumption, and risk of failure. > Moreover, it'll have better latency. This gap will continue to widen, > so while we can talk about 15 disks reasonably right now, in a year > we'll be talking more like 25-30 and then it just becomes absurd. > Just > buy the SSD(s) at that point. > >> Only theoretically a higher bandwidth will be possible (benchmarks >> huh). >> >> However getting 20 bytes from a SSD is in the few dozens of >> microseconds, versus several milliseconds for the cheapskate disks. >> >> That factor of 50-100 difference roughly in latency difference is the >> reason SSD's exist. >> >> Any bandwidth test of a SSD is total nonsense. > > (I wish you'd put [In my personal opinion] in front of all of your > sentences. It would make them less nails on a chalkboard.) > > So what happened to "perfectly parallel"? Seems to me like a > perfectly > parallel device would be well tuned to deliver good bandwidth. > > Best, > > ellis > _______________________________________________ > Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin > Computing > To change your subscription (digest mode or unsubscribe) visit > http://www.beowulf.org/mailman/listinfo/beowulf _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
