Re: mkfs.xfs -d su=XXX,sw=YYY and future volume resize

[Dave Chinner <david@fromorbit.com>]

On Fri, Jul 15, 2016 at 11:46:47AM +0200, Carlos Maiolino wrote:
> On Thu, Jul 14, 2016 at 10:19:02PM +0200, Marcin Sura wrote:
> >    Hi,
> >    When I create filesystem I can specify su and sw paramteres which match
> >    underlying storage device.
> >    So for example, If I have raid 6 volume made of 6 + 2 disks with stripe
> >    size of 256k I create xfs with:
> >    mkfs.xfs -d su=256k,sw=6 <disk>
> >    But what if I will extend original volume by 2 disks. sw will 8. Will
> >    be there a performance panelty compared to filesystem created from
> >    scratch with correct sw parameter?
> >    BR
> >    Marcin
> 
> Complementing Dave's answer, you can also mount the filesystem with new
> alignment configuration.
> 
> You can use: sunit=value and swidth=value for mounting the FS with the new
> settings. man xfs for more info.

That doesn't change the fact that everything that already existed in
the filesystem would now be misaligned and hence have performance
issues on read and/or overwrite. Only newly written data will be
aligned to the new stripe width.

Also, keep in mind that stripw width cahnges are an alignment change
you generally canot make through the mount options. Static metadata
such as allocation group headers are laid out according to the
physical geometry of the filesystem, and that cannot be changed.
hence the mount options can only change the stripe unit/width when
they don't change the alignment constraints. e.g. doubling of the
stripe width will work because the alignemtn of the stripe units is
still the same. However, changing from 6 disks to 8 won't work
because it is a different physical alignment...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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