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Old 04-23-2007, 02:57 PM   #1
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EVMS converting raid types

I'd like to use EVMS to create a RAID using 500GB drives and expand it over time. It seems that as long as I stick with the same RAID type I'll be fine in doing that--EVMS supports expanding and shrinking of even RAID volumes. Is it possible to convert between RAID types? If possible, I'd like to start with 2 500GB in either a RAID 0 or 1, but then convert it to RAID 5 when adding the 3rd disk sometime in the future. Is it possible to do this non-destructively, or need I wait until sometime in the future when I get additional drives?
Old 04-24-2007, 04:42 PM   #2
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I think I found my answer.

He starts with a RAID 1, then converts to a 2 disk RAID 5 and expands to 3 disks. The raid conversion is done directly with mdadm after converting to Compatibility Volumes in EVMS. This works because RAID 5 on 2 disks is a mirror with RAID 5 metadata. After that, EVMS is used to convert from the Compatibility Volume again and expand the array. Someone replied to this article stating it worked with his real drives and real data.

Definitely not the outcome I expected, but this certainly makes me happy.

This took me some thinking to get my head around why a RAID5 of 2 disks would really be a mirror. Here's why:
First, familiarize yourself with RAID 5.
Now, if we have 2 disks, one disk will be data for a block and parity for the next, and vice versa. Since we only have 1 data block, parity pN will be data dN XOR 0 = dN=pN. It is a mirror!
Disk 1....Disk 2

The only caveat is that this requires madm knows how to calculate the parity when only 1 data block exists, which apparently it already does, so we're ok. Definitely worth experimenting with LOOP devices just to be sure madm hasn't changed.

Last edited by bobpaul; 04-24-2007 at 05:16 PM.



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