[lug] Server Partitioning Recommendation

Hugh Brown hugh at math.byu.edu
Fri Jan 19 06:41:41 MST 2007

Daniel Webb wrote:
> On Thu, Jan 18, 2007 at 06:44:03PM -0700, dio2002 at indra.com wrote:
>> I thought the whole purpose of LVM was so that you "can" span multiple
>> physical devices.  Why would I not want to span LVM across physical
>> devices?
> There was a discussion on this subject a while back on this list where I
> played devil's advocate.  Basically, you divide your mean time before failure
> by the number of drives if you span multiple physical devices with a logical
> volume.  I can't remember my statistics class well enough, but it's probably
> not that simple.  I'll pretend though.  ;)
> One of the things I got from that conversation, though, is that anybody who
> spans devices with LVM better be spanning RAID devices or at least understand
> the risk they're taking (it could be OK if you have very solid automated
> backups, for instance).
> I'm curious, though, what happens when you put LVM on RAID5 and then add a
> drive to it?  

I suspect the answer is that it depends.  I know that there's a maximum 
LV size and that the size of the PE is dependent upon a variety of 
factors.  I assume that if you don't exceed any parameters by adding the 
disk, then you'd just have more PEs available to allocate.  I'd want to 
be sure and test this before doing it in production.

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