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List:       evms-devel
Subject:    Re: [Evms-devel] Extending a RAID-5 using EVMS?
From:       Kevin Corry <corryk () us ! ibm ! com>
Date:       2003-01-16 13:43:23
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> > 2. Combine two raid5 arrays with drive linking, thereby extending the
> > volume.  (I think this is possible, someone say if it's not)
>
> This should work (if not in EVMS, it should be possible with md and LVM)
>   but it 'wastes' a physical disk as the second array also needs an
> extra disk. Also, IIRC, RAID-5 needs a minimum of 3 disks, so instead of
> adding a single disk (like a 180GB or so) at a time, I'll have to add
> three...
>
> Apparently, there is a tool to do something like this on md, but
> according to the online reports, it's like buying a lottery ticket - if
> you win, you get to keep your data. If you lose, you should have taken a
> backup beforehand.

There is a tool called raidreconf. Jakob Ostergaard emailed us about it back 
in May. You can read through his description at:
http://marc.theaimsgroup.com/?t=102059081900002&r=1&w=2
It's an off-line tool, I would certainly recommend a full backup ahead of 
time.

> I would accept if this functionality meant that the parity information
> was ditched during the re-build of the array. It would be a risk, but I
> suspect many would be willing to take that chance (assuming that
> retaining and re-building the parity info is the major problem with this
> feature). Add the disk, ditch the parity info, extend the array and
> re-build parity across all disks.

We haven't gone too much into the details of implementing RAID-5 expand. Our 
hope is that the parity would not be ditched completely, but rather updated 
as each stripe is expanded. That way if the expand process died, it could be 
restarted at the spot it was interrupted.

-- 
Kevin Corry
corryk@us.ibm.com
http://evms.sourceforge.net/


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