Questions on RAID alignment

[Emmanuel Florac]

Le Thu, 20 Sep 2012 13:11:38 +0200
Spelic <spelic at shiftmail.org> écrivait:

> 1- Does XFS support alignment to concatenation RAIDs (e.g. linear
> over raid1) similarly to striped ones? I am guessing probably su and
> sw should not be specified and the agcount should be multiple of the
> number of the concatenated devices, is that correct?

No, it's better to set your sw/su accordingly to your RAID setup
(example: 8 drives RAID-10 256k stripe -> su=256k,sw=4) and let
mkfs.xfs determine itself how many ag it wants, to adapt it both to the
striping and filesystem size. 

> 2- Can mkfs parameters be altered after creation, e.g. after growing
> a RAID and then growing the XFS fs, can I also alter su, sw, agcount
> somehow?

agcount will be adjusted by xfs_growfs AFAIK. For the other parameters,
not so much.
 
> 3- I seem to remember there was an xattr that one could attach
> to /home (if /home was under XFS) so that every user (subdir
> of /home) would get a different AG in a different RAID device. I
> cannot find that information anymore on the internet; could you
> refresh my mind?
> 

I really have no idea on this.

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