Understanding DRU Spanning


What is DRU Spanning

DRU spanning is the mechanism allowing you to assemble several drives to make 1 DRU. It’s useful when your system is made of drives of very different sizes – best is to assemble them so that the sizes of DRUs is consistent.


In the above diagram, setup 2 uses the DRU spanning feature and is the recommended setup for the following reasons:
– better RAID performance (the lower the DRU count, the better the performance)
– better protection level

Setup 2 has a better protection level than setup 1 as, for instance, DRU2 can lose all of its 3 drives but such event will only count as 1 failure.
Similarly, DRU1 can lose both disks and it will only be counted as 1 failure.
Put another way, each disk failure in setup 2 is only a partial DRU failure whereas a disk failure in setup 1 is a full DRU failure.


Common Questions

Q: What happens if I need to replace a disk in DRU2 (setup 2) with a larger one?

A: You can replace all the disks in a DRU or a few of them, copy the data over to the new disk(s), and use the advanced RAID reconfiguration feature without having to re-create the RAID.

Q: How do you put more than one drives in a single DRU using text configuration

A: This is straight forward if using the WebUI (just add all the applicable drives to the same DRU).
Text configuration: data=DRU1{/Path1;Path2;…;/etc.} (where each path represents a separate disk)



Originally 18 UoR:

9 x 1TB drives
2 x 1.5TB drives
7 x 2TB drives


By allocating  another 1TB drive to parity, you could have:
– 1 PPU with 2TB + 1TB (= 3TB)
– 1 DRU with 2x 1.5TB
– 6 DRUs with 2TB + 1TB
– 1 DRU with 2x 1TB

That will be down from 17 to only 8 DRUs. 

Again, the only thing you would lose here is that one TB drive to parity (2TB vs 3TB for parity).



Be Sociable, Share!

4 Responses to “Understanding DRU Spanning”

  1. Ross November 22, 2012 at 1:41 AM #

    Does DRU spanning erase any data to bring them together?

  2. Harold September 23, 2013 at 3:04 PM #

    In the example above, what is the recovery process if one of the 1.5 TB drives die and I want to replace it with a 3 TB drive? Is it replace, restore, reconfigure?

  3. eric July 31, 2014 at 12:41 PM #

    excellent question harold. did you ever get an answer??

  4. Brahim July 31, 2014 at 1:36 PM #

    Yes, you can restore a failed disk to a disk of a larger size.

Leave a Reply