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

From: Carlos Maiolino <cmaiolino@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Marcin Sura <mailing-lists@sura.pl>, xfs@oss.sgi.com
Subject: Re: mkfs.xfs -d su=XXX,sw=YYY and future volume resize
Date: Mon, 18 Jul 2016 12:10:41 +0200	[thread overview]
Message-ID: <20160718101041.GA3788@redhat.com> (raw)
In-Reply-To: <20160715230725.GV1922@dastard>

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@fromorbit.com
> 
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-- 
Carlos

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