On Wed, Apr 21, 2010 at 12:02:46PM -0500, Jon Nelson wrote: > I was helping somebody else diagnose some issues, and decided to run > comparitive tests on my own raid (raid10,f2). > > The raid10,f2 (md0) is the only physical device backing a volume > group, which is then carved into a bunch of (primarily) ext4 > filesystems. > The kernel is 2.6.31.12 (openSUSE) on a Quad Processor AMD Phenom 9150e system. > The raid is two Western Digital Caviar Blue drives (WDC WD5000AAKS-00V1A0). > > The problem: really, really bad I/O performance under certain circumstances. > > When using an internal bitmap and *synchronous* I/O, applications like > dd report 700-800 kB/s. > When not using a bitmap at all, and synchronous I/O, dd reports 2.5 > MB/s (but dstat shows 14MB/s?) > Without a bitmap and async I/O (but with fdatasync) I get 65MB/s. > *With* a bitmap and using async. I/O (but with fdatasync) I get more > like 65MB/s. > > The system has 3GB of memory and I'm testing with dd if=/dev/zero > of=somefile bs=4k count=524288. > > I'm trying to understand why the synchronous I/O is so bad, but even > so I was hoping for more. 65MB/s seems *reasonable* given the > raid10,f2 configuration and all of the seeking that such a > configuration involves (when writing). > > The other very strange thing is that the I/O patterns seem very > strange. I'll see 14MB/s very consistently as reported by dstat > (14MB/s for each sda, sdb, and md0) for 10-15 seconds and then I'll > see it drop, sometimes to just 3 or 4 MB/s, for another 10 seconds, > and then the pattern repeats. What's going on here? With absolutely > no other load on the system, I would have expected to see something > much more consistent. Hmm, not much response to this. The only idea I have for now is misalignment between raid and LVM boundaries. Were your dd's done on the raw devices, or via a file system? Best regards keld -- 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
