On Sat, 12 Sep 2009, Doug Hughes wrote: > Dave Close wrote: >> Doug Hughes wrote: >> >> >> >> Flash reads are reasonably fast, I agree. Writes are not in my >> experience. And one probably writes to swap at least as often as >> one reads from it. >> > You should check out the Intel X-25E. There are 2 more commonly used > flash technologies these days. There is the multi-level cell which has > higher density but is slower on write and the single-level cell (SLC) > which is faster on write (well under a millisecond) and lower on > density. the MLC (e.g. X-25M) is typically used in cameras and consumer > electronics. The SLC are more typically used on enterprise flash SATA or > SAS interface drives.
MLS is generally slower than SLC, but there is a _lot_ more to it than just that. the Intel X25M (MLC drives) are significantly faster than just about all the SLC drives that had been sold prior to it's release. most flash drives on the market are significantly slower to write to than a normal hard drive. the Intel X25M is about the same speed as a 15Krpm scsi/sas drive and the X25E is significantly faster. Intel recently released a new generation of drives that is noticably faster (at about half the price) than their old ones,and in the time since they released the X25 line of drives many competitors have significantly improved their drives. overall the particular controller chip in the drive makes more of a difference than SLC vs MLC (although I agree that with similar controllers the SLC will be faster) > There is also a number of newer devices that combine the advantages of 3 > different technologies. The NAND flash, the NOR flash, and RAM all in > one chip. the RAM is used as a buffer to accumulate writes and to > satisfy reads from cache, > > NAND: small cell size, low cost per bit. High density. Poor random > access. Fast bulk writes and reads by block programming. Fast writes, > slow reads. fast erase. > > NOR: fast reads (100MB/s), slow writes, lower density, good random > access. slow erase > > adding standard DRAM onto this with a super capacity for flushing the > ram to nonvolatile flash gives you the best of all worlds. Its still > relatively new and relatively expensive. I know of only one vendor using > this in mainstream, and that's Sun with their Readzilla and Logzilla > accelerators for FishWorks. (Other vendors may be doing similar, but I'm > not aware of them) although unless the drive includes battery backup for the DRAM you will loose a lot of data if the drive looses power. also DRAM requires a running processor to keep it going, so it's very expensive from a power point of view. for small amounts SRAM is better as it can maintain it's contents with no processor and much less current. > Most off the shelf flash drives are a combination of NOR and NAND chips > to balance density with speed. I don't believe that this is true. the flash drives that I have seen use several of one particular chip for their data storage. they may have a bit of somethign else in their control logic (possibly using it for the metadata for the mapping/wear leveling layer), but the users data all gets stored on one part number of chip > As I mentioned earlier, using Flash for the ZFS intent log GREATLY > speeds up zfs with NFS over spinning disk media, even when striped. You > do pay some penalty in serial write bandwidth however. You become > bottlenecked on the aggregate write throughput of the flash devices. On > the other hand, if you're just pumping out TB of sequential data, > there's no reason to even consider flash in the mix. Spinning media > kicks butt in this arena (even tape does very well) are you comparing this to having the ZFS intent log on the same drives as your other data? or are you comparing it to having your ZFS intent log on a dedicated high-speed rotating drive? > The Fusion IO card is the fastest thing out there at the moment. It's > part flash, part DRAM, like the logzilla but comes in a PCI-E form > factor that you have to plug into a machine, and it's also very spendy, > but if you really want to speed up your DB by orders of magnitude, you > could shove some of these into a machine with a bunch of PCIE2 slots and > mirror them together and throw indexes or small tables on here. the Fusion IO card is fast, but it's also extrememly expensive. > So, I guess to summarize, before saying that flash writes aren't fast, > have a test drive of the X-25E. Flash got a deservedly bad wrap in the > past for write speed, but things have come a long way in the last couple > of years. if you really need fast writes you need battery backed ram. if you don't need very much (up to a couple of gigs with the writes being converted from many small writes to a few large writes by the hardware) you can use a raid card/chassis with it. if you need a lot you can look at something like ACard's ANS-9010 Serial ATA RAM disk ( http://techreport.com/articles.x/16255 ), it's as much faster than a X25E as an X25E is from a normal hard drive. > (note: I still wouldn't use flash for swap space, yet, but once TB flash > is out, wear leveling and 1000000 cycle flash will make even that point > moot) it all depends on how much you use the swap space. if you are using it a lot you could have problems (but you will also be very slow), if you don't use swap much you can get away with it. David Lang > > refs: > http://www.google.com/url?sa=t&source=web&ct=res&cd=2&url=http%3A%2F%2Fwww.toshiba.com%2Ftaec%2Fcomponents%2FGeneric%2FMemory_Resources%2FNANDvsNOR.pdf&ei=MuerStjmCtHglAfYquW7Bg&rct=j&q=NOR+flash+vs+NAND+flash&usg=AFQjCNG04XmXaryk4ybCb5PsP3BwJOxKxw > http://www.google.com/url?sa=t&source=web&ct=res&cd=3&url=http%3A%2F%2Fforums.techarena.in%2Fweb-news-trends%2F1090657.htm&ei=ZumrSpLtCsbllAfU2dTHBg&rct=j&q=sun+flash+1000000+write+cycles&usg=AFQjCNHxxyYuzcAQKtZNhb6fI7vhEj-9Ug > > pay particular attention to these two from SC08 conference > http://www.pdsi-scidac.org/events/PDSW08/resources/papers/simsa_PDSW.pdf > http://www.pdsi-scidac.org/events/PDSW08/resources/slides/simsa_PDSW.pdf > _______________________________________________ > Discuss mailing list > Discuss@lopsa.org > http://lopsa.org/cgi-bin/mailman/listinfo/discuss > This list provided by the League of Professional System Administrators > http://lopsa.org/ > _______________________________________________ Discuss mailing list Discuss@lopsa.org http://lopsa.org/cgi-bin/mailman/listinfo/discuss This list provided by the League of Professional System Administrators http://lopsa.org/
