[email protected] wrote: > On Sat, 12 Sep 2009, Doug Hughes wrote: > >> [email protected] wrote: >>>> >>>> >>>> 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. >> oops. Typo on my part. super 'capacitor', which acts as a battery in >> that it flushes the DRAM to nonvolatile flash storage > > writing to flash takes a considerable amount of power, do they really > put capacitors that large on the boards? (I sure haven't seen anything > that looked like a capacitor that large in any of the disassembled > pictures of flash drives that I see in the reviews) > yes: http://blogs.sun.com/ahl/entry/fishworks_launch But, remember, this is NOT a normal flash drive. >> zfs normally stripes the intent log across the data disks unless you >> tell it otherwise (e.g. 46 disks on a Sun x45000 or x4540) >> You can also put the ZIL on a dedicated high speed rotating drive. >> The x25E outperforms both except in the case where you want high >> sequential throughput. > > well given that the X25E is faster than the rotating storage I would > expect it to be a win there as well. > > but the point I was trying to make was that I suspect most of the > benifit is in the step from a striped intent log to an intent log on > dedicated media, with the move from there to an X25 helping, but not > as much. > I am trying to correct this misconception. I assure you it's much bigger than that for 7200 RPM SATA to X-25E. The latency is lower and throughput is higher. But, don't believe me. Check out the references I posted earlier. Even the consumer X25-M sata SSD is .1msec vs 4.2msec on a 7200RPM SATA enterprise drive. for the X25-E it is .085msec for for 15KRPM SCSI it is 3.9msec. (The fusion IO is .05 msec)
The raw sequential throughput of the X-25E is close to 3000MB/sec on a 64KB record whereas your 9200 RPM drive will be around 100 and the 15KRPM drive will be aroudn 227. (it doubles on smaller block sizes) I strongly encourage everybody interested in this topic to look at the CMU paper on this. Included here again: http://www.pdsi-scidac.org/events/PDSW08/resources/papers/simsa_PDSW.pdf Or just review the PPT slides if you don't want to read the whole paper: http://www.pdsi-scidac.org/events/PDSW08/resources/slides/simsa_PDSW.pdf >> I have evaluated a battery backed ram that was much slower than flash >> for ZIL purposes. I will spare the vendor some embarassment, but I >> was very surprised with the results and still don't fully understand >> why. > > no question that you can find lemons in any technology, many raid > cards have traditionally just tried to be faster than the drives that > they ran (enough to show an improvement), but the good ones compete > with each other on performance, so with any of them I would not expect > that sort of problem. > The weird thing was it was showing 400MB/sec on read/write throughput over SC quite easily using iozone, but for ZIL offloading it was slower than having the ZIL striped across the 7200 RPM sata drives with the data by a factor of 2-4x So, it wasn't just a case of the DRAM device being generically slow. (the vendor didn't necessarily understand it either) _______________________________________________ Discuss mailing list [email protected] http://lopsa.org/cgi-bin/mailman/listinfo/discuss This list provided by the League of Professional System Administrators http://lopsa.org/
