On Thu, Apr 26, 2012 at 01:17:43PM +0200, Torbjørn Thorsen spake thusly: > * How will using iSCSI change the partition alignment problem ? > I'm guessing you have found the difference the only way that works; by trying. > It seems unclear why AoE would cause the unaligned writes, so maybe there > isn't > a good answer to this question.
I presume it changes the partition alignment problem but not actually introducing one. I don't know how it is possible that an alignment problem is being introduced by using a VM. It makes no sense to me. But the performance is what it is. And when doing writes I see lots of corresponding reads indicative of an alignment problem. And it isn't necessary a partition alignment problem: I'm pretty sure that at some point I actually exported a raw unpartitioned hard drive into then Xen VM and still had issues. It almost seems like Xen itself is causing it. > * How do you actually see from the output of iostat that you're writes > aren't aligned ? > I usually use "iostat -dxm 1" to get an overview, but I don't fully > understand all the output. I run iostat and watch the "rrqm/s wrqm/s r/s w/s" columns. If I am inside the VM and I do a: dd if=/dev/zero of=testfile bs=4M then in the iostat output I would expect to see a lot of write requests merged (wrqm/s) and writes (w/s) per second. I would expect to see few if any reads. Instead I see a significant number of reads perhaps totalling around 1/4 as many writes. This causes lots of extra seeking which really slows down IO. > In a non-AoE context, I have a dm RAID6, where I see all the disks and > the md device > when using iostat. > I can there see the md device hitting 100% utilization, while the disk > don't go above ~60%. > Is that a symptom of unaligned writes ? Not necessarily. The main indicator of unaligned writes is seeing a lot of reads when only writes should be happening. -- Tracy Reed
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