On Sat, Jul 16, 2016 at 09:07:25AM +1000, Dave Chinner wrote:
> 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...
>
Thanks for the detailed info, it was my bad to have not predicted such details
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
>
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--
Carlos
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