[GLLUG] Advice on expanding an existing RAID 1 installation

Iain M Conochie iain at shihad.org
Mon May 26 17:51:24 UTC 2014

On 24/05/14 11:12, John Winters wrote:
> Dear GLLUGers,

Hi John
> I seek a spot of advice on enhancing an existing RAID 1 installation 
> (used for backing up all sorts of systems).
> The current system has 2 x 1TB drives in a RAID 1 configuration. These 
> are running out of space.
> I intend to get 3 x 3TB drives and would like to transfer everything 
> to those.
> The Linux RAID Wiki has a page on extending an existing RAID array, 
> but says I should start by removing one of my existing drives.  
> Presumably this isn't necessary if I have a spare physical 
> slot/controller slot/power lead?  Rather than their proposed approach 
> of remove=>add=>remove=>add, is there any reason I can't do 
> add=>remove=>add=>remove?

Why would you want to extend it? You could create a new array that would 
use the entire space; I am pretty sure any RAID implementation will use 
the smallest device size for the size of the array, so adding a 3TB 
device into a 1TB array will not utilize 3TB. You could remove 1 x 1TB 
drive and replace with 2 x 3 TB drives, create a new array, and the copy 
over the data.
> Second question - I'm undecided about whether to put the 3rd drive 
> into the system to start with (thus three drives up to date all the 
> time) or keep it on a shelf for when it's needed.  Does anyone have 
> strong views on which is better?
> The machine in question is only switched on for a couple of hours each 
> day.  It switches itself on, backs up a selection of other systems, 
> then switches itself off again.

You will _probably_ need to ensure that the amount of time the machine 
is up for will be at least (if not greater) than the amount of time 
taken to rebuild the array. If this is the case, then as has been said 
elsewhere this will be pretty resilient.


> TIA,
> John
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