On 2010-11-10 18:18, Sebastian Kayser wrote: > * Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote: >> On 2010-11-10 09:22, Sebastian Kayser wrote: >>> Just to make sure my understanding is correct: >>> - direct=1 should mitigate (disable?) OS caching effects >>> - sync=1, iodepth=1 should make sure that an I/O has really made it to >>> disk before the next on is issued, i.e. should de-facto disable >>> I/O coalescing or device caching >>> >>> Are these sane/valid assumptions? >> >> Yes, your assumptions are valid. I think the issue here is that you give >> randwrite, but don't also specify that you want overwrites. So what >> happens is that the file will be truncated and then randomly written, >> allowing the file system to place randomly chosen file block together. >> If you remove the test file and re-run with overwrite=1 added, then see >> if those results are more in line with what you expect. > > Thanks for the additional aspect! So the possible pitfall is that w/o > overwrite=1 the test file could be sparse and the file system then > re-arrange random file addresses to fall onto consecutive device blocks? > > (If so, wouldn't that be evil / deliberate data fragmentation?) Depends on whether you care about the write performance or the read-back performance :-) > Tested it again with overwrite=1, same IOPS figures. Test file size as > measured with du -ks is the same in both cases btw. (which combined with > the limited runtime=120 doesn't seem to indicate sparseness in case of > overwrite=0, right?). Was the file pre-created? I'm pretty sure that fio will not lay it out first with a write workload, even for random writes (I could be wrong though, and if the file is indeed non-sparse and the test stopped before all IO was written, then it would have been sparse for this case). > What I will do now is to export the whole 2TB of the disk (instead of > just 10GB) and increase size= to see whether that makes any difference > (hopefully). Other than that, further ideas? Lets see what that yields. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
