LVM + RAID follow up

[john.abreau at zuken.com (John Abreau)]

On Wed, 2006-11-08 at 13:17 -0500, Derek Atkins wrote:

> 
> Ok, you've confused me again.  Doing it this way, to swap out
> 4 drives you need to have space for 8 drives!  Or are you saying
> that you swap out one drive at a time?  The way I would think about
> it:
[snip]
> Now, I can see how in the next swap-out, when I add a p4 (built into
> an md3), if p4 is greater than p2+p3 then I can see how I could move
> everything from md1 and md2 onto md3, and then combine p2 and p3 and
> build a new array from the newly combined p2+p3.  But this only works
> if I keep getting geometrically-bigger hard drives (well, it needs to
> be Fibonachily-increasing sizes).
> 
> Am I missing something?

It sounds like you're trying to mix together several different layers. 
You mention combining partitions, for instance, but that's at the 
layer below RAID, whereas LVM is the layer above RAID. And the ext3 
layer sits above LVM. 

I've only done this with RAID-1, which is simpler than trying to juggle
drives in a RAID-5 metadevice. I only had to swap 2 drives at a time. 
The HOWTOs I read on it described an alternate way to manage it that 
involved partitioning large drives into several partitions and building
RAID volumes from the partitions. The scheme used multiple md's, and 
combined them into one volume at the LVM layer. 

With this scheme, you can remove md's when removing drives, so you 
don't have md's proliferating infinitely, but you'd still have more 
than one md.  At the LVM layer, you can combine the multiple md's 
into a single virtual volume. 


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John Abreau / Executive Director, Boston Linux & Unix
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