Verify filesystem is aligned to stripes

[Michael Monnerie]

On Freitag, 26. November 2010 Dave Chinner wrote:
> FWIW, for workloads that do random, small IO, XFS works best when you
> turn off aligned allocation and just let it spray the IO at the
> disks. This works best if you are using RAID 0/1/10. All the numbers
> I've been posting are with aligned allocation turned off (i.e. no
> sunit/swidth set).

That's interesting to read.

Why would sunit/swidth be slower then? I'd thought that XFS then would 
know one stripe is 64k and I have 8 disks so it should try to pack 
8*64=512kb in one junk on disk, and that especially for small files it 
would write them like that.

The man page just says inodes, log are stripe aligned, and file tails 
>512k extended to full stripes on append. I thought that even the 
inode/log alignment alone would help a lot.

Now what is the advantage on skipping sunit/swidth altogether?
And what is the difference when it's on RAID10 to RAID6?

I'm always eager to understand performance issues ;-)

-- 
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Michael Monnerie, Ing. BSc

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