On Sat, 12 Sep 2009, Doug Hughes wrote: > [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.
I had misunderstood you. I thought you were talking about flash drives, not storage arrays (storage arrays do frequently use DRAM, they have a full processor on board and large battery packs) >>> 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 don't doubt your performance number (not much anyway, the X25E is limited to the speed of sata, so a theoretical limit of 300MB/sec, not 3000MB/sec ;-), but the huge win (just like it is with databases) is when you move your intent log off of drives that are seeking all over the place to a drive where you don't have to seek much, if at all, letting your writes _be_ sequential in the first place > 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) interesting David Lang _______________________________________________ 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/
