Changing Raid Type w/o Reinstalling

bscott@ntisys.com bscott at ntisys.com
Tue Dec 31 08:50:01 CST 2002


On Tue, 31 Dec 2002, at 9:33am, eric at interplas.com wrote:
>> I'd like to take my / and /usr partitions and reassign them as raid 0
>> instead of raid 5.
>
> I think that physically impossible.  Data bits are stripped across your 3+
> drives.  No one drive in you system has the "whole" picture.

  It is not physically impossible.  The controller has the "whole picture"  
of the LV (logical volume).  It can rearrange things while keeping the OS's
view of the LV consistent.  That is, after all, the whole point of a RAID
controller.  Going from RAID5 (striped with parity) to RAID5 (striped, no
redundancy) should be quite possible.  All you would have to do is recombine
the parity information and consolidate the blocks to free up the remaining
space.  It should, in fact, be similar to what happens when a disk fails and
you rebuild it (except you're rebuilding to a different RAID level).  Now
that I think about, I've never tried this particular reconfiguration (from
RAID5 to RAID0), and that does seem like a rather unusual thing to want to
do, so maybe the controller doesn't implement it, but it could.

  What you cannot do is reconfigure an LV to storage that is smaller than
that which can contain the LV.  For example, if you have three disks that
contain a single LV in RAID5, you cannot reconfigure to RAID1 (mirrored
disks), because the LV is larger than a single disk.  I can't remember if
the MegaRAID controllers can truncate an LV; I don't think they can.

-- 
Ben Scott <bscott at ntisys.com>
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