Lost available space after expanding volume

Anstilion · August 13, 2014

Hi, I recently purchased a Synology 1813+, and I'm really impressed so far. But the thing eating me now, is that after expanding a SHR-2 volume consisting of 4x2TB Samsung SpinPoints and 2x3TB WD Reds with another 2x3TB WD Red's (needed the last two for backup as I was coming over from another NAS) my used space went up from 3,784 TB used (which is what i measure when adding all the data on the shares together) to 6,37 TB used space on the volume. What on earth is using 2,586 TB that i can't see??
And if I have to take the volume down to recreate it, are there any performance advantages of going with RAID 6 instead of SHR-2? Any insight and help into this is greatly appreciated

SuperSoph_WD · August 13, 2014

Hey, Viking!

Sounds like a great NAS configuration you have going on. I did some research for you and came across the RAID calculator from Synology's website:
https://www.synology.com/en-us/support/RAID_calculator

I put your configuration details and compared SHR-2 to RAID 6:
Before you expanded your volume, the comparison between both configurations was identical, both SHR-2 and RAID 6 indicating that you had:
4x2TB + 2x3TB

8 TB - available space
4 TB - used for protection
2 TB - unused space
After expanding, however, SHR-2 definitely has the advantage due to the fact that you don't have any unused space:

4x2TB + 4x3TB
SHR-2
14 TB - available space
6 TB - used for protection
If you decide to rebuild the volume and configure to RAID 6, you will have:
12 TB - available space
4 TB - used for protection
4 TB - unused space

I did some snapshots for you to see the difference for yourself. From what you can see thanks to the RAID calculator, is that in RAID 6 array, you will have 4 TB of unused space. This means that the space can't be utilized and is thus wasted. Plus another 4 TB used for HDD protection. While with the SHR-2 configuration, you just have 6TB used for HDD protection with 14 TB available space.

I think what you are experiencing now is not exactly lost available space, but just space used for protection.

I hope this helps.

SuperSoph_WD