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Verify filesystem is aligned to stripes

To: xfs@xxxxxxxxxxx
Subject: Verify filesystem is aligned to stripes
From: Spelic <spelic@xxxxxxxxxxxxx>
Date: Wed, 24 Nov 2010 19:39:56 +0100
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Hi there,
I thought there was a way to empirically check that the filesystem is correctly aligned to RAID stripes, but my attempts fail. I don't mean by looking at sunit and swidth from xfs_info, because that would not detect if there is some LVM offset problem.

I am particularly interested for parity RAIDs in MD.

I was thinking at "iostat -x 1": if writes are aligned I shouldn't see any reads from the drives in a parity RAID...

unfortunately this does not work:
- a dd streaming write test has almost no reads even when I mount with "noalign", with sufficiently large stripe_cache_size such as 1024. If it is smaller, always reads, even if xfs is aligned. - a kernel untar will show lots of reads at any stripe_cache_size even if I'm pretty sure I aligned the stripes correctly on my 1024k x 15 data disks and the .tar.bz2 file was in cache. I tried with both xfs stripes autodetection in 2.6.37-rc2 and by specifying su and sw values by hand, which turned out to be the same; I was without LVM so I'm pretty sure alignment was correct. Why are there still lots of reads in this case?

so I'm pretty clueless. Anyone has good suggestions?
Thank you

PS, OT: do you confirm it's not a good idea to have agsize multiple of stripe size like the mkfs warns you against? Today I offsetted it by +1 stripe unit (chunk) so that every AG begins on a different drive but performances didn't improve noticeably. Wouldn't that cause more unfilled stripes when writing?

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