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Re: Using xfs_growfs on SSD raid-10

To: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Subject: Re: Using xfs_growfs on SSD raid-10
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 11 Jan 2013 09:12:01 +1100
Cc: Alexey Zilber <alexeyzilber@xxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <50EF32B9.1040608@xxxxxxxxxxxxxxxxx>
References: <CAGdvdE3VnYKg8OXFZ-0eALuhK=Qdt-Apj0uwrB8Yfs=4Uun3UA@xxxxxxxxxxxxxx> <50EE33BC.8010403@xxxxxxxxxxxxxxxxx> <CAGdvdE3eeQY1xX0Zdskr461D6ag+JC4tWEozhK32108G3y_=9A@xxxxxxxxxxxxxx> <50EE5649.60608@xxxxxxxxxxxxxxxxx> <CAGdvdE1OWbjutd2etq79_n=EPOyKMbpseXA9PPQxLVLRy2yToQ@xxxxxxxxxxxxxx> <50EF32B9.1040608@xxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Thu, Jan 10, 2013 at 03:29:29PM -0600, Stan Hoeppner wrote:
> On 1/10/2013 1:19 AM, Alexey Zilber wrote:
> > Hi Stan,
> > 
> >    Thanks for the details btw, really appreciate it.  Responses inline
> > below:
> > 
> >  >>
> >>> Only the sw=3 is no longer valid, correct?  There's no way to add sw=5?
> >>
> >> You don't have a 5 spindle array.  You have a 3 spindle array and a 2
> >> spindle array.  Block striping occurs WITHIN an array, not ACROSS arrays.
> >>
> > 
> > That's correct, but I was going with the description of the sw option as
> > "number of data disks" which is constantly increasing as you're adding
> 
> That is correct.  But "data disks" is in the context of a single striped
> array.  Remember, what you're doing here is aligning XFS write out to
> the stripe size of the array.
> 
> > disks.  I realize that block striping occurs independently within each
> > array, but I do not know how that translates into parity with the way xfs
> > works with the logical disks.  
> 
> The takeaway here is this:  If you concatenate two or more striped
> arrays together with a single XFS atop, you must use identical arrays,
> or at least arrays with the same overall stripe size.  Otherwise you
> cannot achieve or maintain alignment with XFS.

Hi Stan - excellent information dump, but there's one important
aspect of growing filesystems like this that you missed - the _end_ of
each volume must also be stripe aligned so that when you concatenate
the new one it starts at an aligned address. If the end of the
existing volume is not aligned, then it's very hard to align the new
volume correctly....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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