[GLLUG] Link two RAIDs in one LVM?

John Winters john at sinodun.org.uk
Tue Apr 28 13:33:55 UTC 2020

On 28/04/2020 12:18, Dr. Axel Stammler via GLLUG wrote:
> Hi,
> I have a 4 TB RAID system (two identical hard disks combined in a 
> RAID-1, created using mdadm). Now, after a few years, this has reached 
> 90% capacity, and I am thinking about first adding another similar 8 TB 
> RAID system and then combining them into one 12 GB RAID 1+0 filesystem. 
> I should be grateful for any tips, especially about buying two 8 TB 
> harddisks.
> Which hardware parameters should I look at?

Make sure your new drives are suitable for a RAID array - not SMR.

Have you checked the status of your current two drives?  After a few 
years use they may be reaching the end of their reliable life.

An alternative approach might be to buy two new larger drives and expand 
your RAID array onto them.

Add, say, one 8 TB drive to your array as an additional mirror and wait 
for it to synchronize.  Add the second one (assuming you have enough 
space and power for 4 drives).  Again wait for synchronisation to 
complete.  Detach the two old drives and then expand the array to fill 
the two new drives.

Hey presto, 8GB of space on two brand new drives with all your data 
intact, plus two archive copies to keep in a cupboard.

Unless you have a pressing need for 12TB of space, I wouldn't be 
inclined to set it up with a mix of old and new drives.  You say it's 
taken you a few years to fill up 4GB.  Do you anticipate the rate of 
data acquisition increasing significantly?

If you really need 12TB, then I'd buy 4 new 6TB drives and make a 
completely new RAID array.


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