On 05/23/2013 09:37 PM, Chris Mason wrote: > Quoting Bernd Schubert (2013-05-23 15:33:24) >> Btw, any chance to generally use chunksize/chunklen instead of stripe, >> such as the md layer does it? IMHO it is less confusing to use >> n-datadisks * chunksize = stripesize. > > Definitely, it will become much more configurable.
Actually I meant in the code. I'm going to write a patch during the weekend. > >> >>> >>> Using buffered writes makes it much more likely the VM will break up the >>> IOs as they go down. The btrfs writepages code does try to do full >>> stripe IO, and it also caches stripes as the IO goes down. But for >>> buffered IO it is surprisingly hard to get a 100% hit rate on full >>> stripe IO at larger stripe sizes. >> >> I have not found that part yet, somehow it looks like as if writepages >> would submit single pages to another layer. I'm going to look into it >> again during the weekend. I can reserve the hardware that long, but I >> think we first need to fix striped writes in general. > > The VM calls writepages and btrfs tries to suck down all the pages that > belong to the same extent. And we try to allocate the extents on > boundaries. There is definitely some bleeding into rmw when I do it > here, but overall it does well. > > But I was using 8 drives. I'll try with 12. Hmm, I already tried with 10 drives (8+2), doesn't make a difference for RMW. > >> Direct-io works as expected and without any RMW cycles. And that >> provides more than 40% better performance than the Megasas controller or >> buffered MD writes (I didn't compare with direct-io MD, as that is very >> slow). > > You can improve MD performance quite a lot by increasing the size of the > stripe cache. I'm already doing that, without a higher stripe cache the performance is much lower. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
