Expanding Capacity - Value of reserved space?

Hi everyone,

I have a Drobo v2 with x3 500GB drives + x1 1TB drive. My system is Mac OS (PPC) v10.5.8

Currently Drobo Dashboard (v1.6.8) shows the following storage capacity.
1.1TB Used Space + 239.6GB Free space

And how it’s being used is (according to Dashboard, which is slightly different than what the online calculator says):
Available for Data: 1.33TB
Reserved for Expansion: 465.75 GB
Used for Protection: 496.38 GB
Overhead: 2.44 GB

I want to change out a 500GB drive (the LED briefly turned red today, and cleared itself within a few minutes, back to Green without any action on my part). I am thinking it is time to replace the drive, besides I wanted to add capacity anyway.

When I check the Drobo calculator, and I replace the 500GB drive with a 1TB drive, it says it would map into the following:
Available for Data: 1.8TB
Reserved for Expansion: 0 GB
Used for Protection: 931.0 GB
Overhead: 3.8 GB

If however, I were to put a 1.5TB drive in, it would be used as follows:
Available for Data: 1.8TB
Reserved for Expansion: 464.2 GB
Used for Protection: 932.5 GB
Overhead: 3.8 GB

Basically, using a 1.5TB drive instead of a 1TB drive does not provide any additional “available” data storage immediately, but does offer additional room for expansion.

Projecting into the future, I find that if were to change out a second 500GB drive with a 1TB drive, then it does not matter whether the 1st drive changeout was 1TB or 1.5TB. I still do not get any more available space. Only by changing out the two drives with 1.5TB drives do I see an Available Space increase. Since I would not see the benefit for quite some time, is there any real value to putting a 1.5TB drive in now? or just go with the 1TB upgrade now?

Are there any other inherent benefits of having the extra Reserved for Expansion space? Not sure why I would be spending the extra $$ for a 1.5TB drive now. By the time I could use the 'reserved for expansion" space, drive prices will drop some more.



Allan, I think your logic is correct. My understanding is that you need to subtract the largest drive from the total amount of storage to calculate how much available space you have. If you add a drive larger than your largest drive, Drobo will “reserve for expansion” the difference. You can swap out two drives and replace them with larger drives and then benefit from additional available space, but just adding one that is larger than your largest current drive will not net you any extra space.