On Fri, Apr 30, 2010 at 08:35:50PM -0500, Jon Nelson wrote: > On Fri, Apr 30, 2010 at 5:46 PM, Keld Simonsen <keld@xxxxxxxxxx> wrote: > > 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. > > These aren't 4K disks (as far as I know), so I'm not sure what you > mean by alignment issues. > Using 255 heads, 63 sectors per track: > > /dev/sda1 starts on sector 63 and ends on sector 7807589 > /dev/sda2 starts on sector 11711385 and ends on sector 482528340 I dont know much about this, and I have not tested it, but try to make LVM and raid on sector numbers divisionable by the raid block size. > /dev/sdb is partitioned the same > /dev/sda2 and /dev/sdb2 form the raid10,f2. > > > Were your dd's done on the raw devices, or via a file system? > > Raw (logical) devices carved out of the volume group. I always advise to do performance tests on the file system, it is closer to the performance that you will actually see in service. I even had some strange results with hdparm yesterday, where a hdparm gave about 60 MB/s , and a "cat file >/dev/null" gave 180 MB/s on the same raid. 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
