On Sun May 31 2009, Michael Tokarev wrote: > Maurice Hilarius wrote: > > A friend writes: > > > > On a recent machine set up with Raid5. > > On a AMD Phenom II X4 810, and 4GB ram. > > 4 Seagate 7200.12 SATA 1TB drives, > > > > I'm getting some rather impressive numbers for sequential read > > (300MB/s+) and write (170MB/s+) but the random read is proving to be > > absolutely atrocious. > > iostat says its going at about 0.5MB/s, > > The key thing about random i/o is the block size. With, say, 512bytes > blocks and single thread you will see less than 0.5Mb/sec. With 64kbytes > blocksize it will be much better. > > To diagnose: first try the same test on bare disk without raid layer. > Next try to vary block size and number of concurrent threads doing I/O. > There's no tweaks needed really. I happen to be the friend Maurice was talking about. I let the raid layer keep its default chunk size of 64K. The smaller size (below like 2MB) tests in iozone are very very slow. I recently tried disabling readahead, Acoustic Management, and played with the io scheduler and all any of it has done is make the sequential access slower and has barely touched the smaller sized random access test results. Even with the 64K iozone test random read/write is only in the 7 and 11MB/s range. It just seems too low to me. > /mjt > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Thomas Fjellstrom tfjellstrom@xxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
