Re: Data Offset

[NeilBrown <neilb@suse.de>]

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On Thu, 10 May 2012 19:42:10 +0200 Piergiorgio Sartor
<piergiorgio.sartor@nexgo.de> wrote:

> Hi,
> 
> after the RAID-5 problem, I just realized that
> other RAIDs I have, including a multi RAID-6,
> have different data offset for each component.
> 
> This seems to be quite of a problem, in case
> "Create" is used to recover an array.
> 
> Obviouly, if a 4 disks RAID-5 has 2 disks with
> one offset and 2 with another, it will not be
> possible to re-create it (saving the data).

I certainly won't be easy.  Though if someone did find themselves in that
situation it might motivate me to enhance mdadm in some way to make it easily
fixable.

> 
> Is there any way to fix/prevent such issue?
> Shouldn't "mdadm" make sure all offset are
> the same? Or try, at least...

I'm not sure.  Maybe...

With linux-3.5 and mdadm-3.3 (both unreleased) you will probably be able to
  mdadm --grow --data-offset=5M
and that will happen.  At least for RAID10.  Other levels might follow later.

Should mdadm keep them always the same?  The reason that it doesn't is that I
thought that you could change the data offset by removing each device and
adding it back as a spare with a new data_offset.  Maybe that isn't such a
good idea.

I suspect that I got a chorus of people all saying "please keep data_offset
consistent" - and particular if I received a patch which did that - then I
would probably change mdadm accordingly.

NeilBrown


> 
> What I noticed is that, adding a disk later,
> might cause different offsets.
> 
> Any idea?
> 
> Thanks,
> 
> bye,
> 


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