Questions on RAID alignment

[Dave Chinner]

On Thu, Sep 20, 2012 at 01:11:38PM +0200, Spelic wrote:
> Hello list
> a few questions
> 
> 1- Does XFS support alignment to concatenation RAIDs (e.g. linear
> over raid1) similarly to striped ones?

No. It's being considered and I know how to do it, but it's lower
priority than other stuff I'm working on at the moment. 

> 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?

If you are concatenating RAID0/5/6, then you shoul duse su/sw as per
the alignment of the volumes being concatenated.

> 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?

You can alter su/sw as a mount option if the filesystem was made
with them initially.

> 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?

Nothing of the sort. if you are using the inode64 allocator, then
each new directory is placed i a different AG. That's as good as it
gets right now.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com