Re: raid5/lvm setup questions

Shane wrote:

Hello all,

I'm building a new server which will use a number of disks
and am not sure of the best way to go about the setup. 
There will be 4 320gb SATA drives installed at first.  I'm
just wondering how to set the system up for upgradability. 
I'll be using raid5 but not sure whether to use lvm over
the raid array.

By upgradability, I'd like to do several things.  Adding
another drive of the same size to the array.  I understand
reshape can be used here to expand the underlying block
device.

Yes, it can.

  If the block device is the pv of an lvm array,

would that also automatically expand in which I would
create additional lvs in the new space.  If this isn't
automatic, are there ways to do it manually?

Not automatic AFAIK - but doable.

What about replacing all four drives with larger units. 
Say going from 300gbx4 to 500gbx4.  Can one replace them
one at a time, going through fail/rebuild as appropriate
and then expand the array into the unused space

Yes.

 or would

one have to reinstall at that point.

No


None of the requirements above drive you to layering lvm over the top.

That's not to say don't do it - but you certainly don't *need* to do it.

Pros:
* allows snapshots (for consistent backups)
* allows various lvm block movements etc...
* Can later grow vg to use discrete additional block devices without raid5 grow
limitations (eg same-ish size disks etc)

Cons:
* extra complexity -> risk of bugs/admin errors...
* performance impact

As an example of the cons: I've just set up lvm2 over my raid5 and whilst
testing snapshots, the first thing that happened was a kernel BUG and an oops...

David