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List:       evms-devel
Subject:    Re: [Evms-devel] Make RAID-1 software on a non RAID system (I know, probably alrea dy asked)
From:       Kevin Corry <corryk () us ! ibm ! com>
Date:       2003-01-16 18:22:38
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On Thursday 16 January 2003 11:24, Fabio Corazza wrote:
> But I didn't find exactly what I need, searching
> on the ml through sf's web site.
>
> I've configured a fresh newly installed debian
> system, with the following hardware configuration:
>
> - Controller SCSI Adaptec 29160 UW-160
> - Two identical HD IBM 36.7GB UW-160 (same model, same size)
> - The rest doesn't matter.
>
> The system has been installed on /dev/sda, its
> partition table looks like this:
>
  <...snip...>
>
> The other disk contains no-data, I thought It would be
> the mirror of the other. After a little playing with
> Linux's native MD drivers, I switched into EVMS, behind
> the hint of a friend.
>
> The ncurses UI is very nice but I still don't know how to
> organize my RAID-1 arrays. I would like to create an
> identical mirror of /dev/sda into /dev/sdb, including
> all partitions, and making boot from the RAID device.
>
> Is this possible with EVMS? What should I do exactly?

Unfortunately, this is not possible, either with EVMS or native MD. If you 
want to create a mirror, it must be done up-front. A device with an existing 
filesystem cannot be mirrored.

First, MD needs space at the end of the device to store its metadata 
(possibly up to 128k). This would most likely overwrite the filesystem data 
and/or metadata.

And even if there was enough room to shrink the filesystem to add the MD 
metdata, MD makes no guarantees as to which side of the mirror will be used 
to do the initial sync. So you run the risk of sycning the useless data from 
the new disk over the existing filesystem data.

In addition, MD devices cannot be partitioned. In order to break an MD device 
into smaller pieces, you would need to add the MD device to an LVM group, and 
create volumes from the group.

If you have your heart set on using mirroring, I would suggest backing up 
your data and reorganizing your hard drives. If this is something you are 
willing to do, let me know and I can give you some specific tips on how to 
set them up.

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


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