Multiple small data drives, one larger parity drive

[tommitytom]

I'm planning an unraid install using some components I have knocking about.  I have 4x 2TB drives, and a single 8TB drive.  I notice in the documentation it states "Each parity disk provides redundancy against one data drive failing".  Does this presume that all drives are the same size?  For instance, using the 8TB drive as a parity drive, and the 4x 2TB drives as data drives, wouldn't the single 8TB parity allow for more than a single data drive failure (presumably, up to 3, or even 4 failures)?

[PeteAsking]

No the parity disk must be the largest but it can still only allow for one disk to fail (or itself). The reason for this is that it calculates the parity based on each disks sectors in the same place - so if you had a 100mb parity and a 10mb disk, a 30mb disk and 2 80mb disks the. The first 4 disks are used up to 10 mb, then the remaining 3 disks used for the 11-30mb calculation and the last 2 disks are used from the 31-80 calculation. The final 20mb is zeroed on the parity as there is nothing used in that space. 

[trurl]

Parity has none of your data. Parity PLUS ALL remaining disks are required to calculate the data for a missing disk.

 

Doesn't matter at all how large any of the disks are except none can be larger than parity. So single parity only allows one disk to be rebuilt. Dual parity allows 2.

 

Here is the wiki on parity:

 

https://wiki.unraid.net/UnRAID_6/Overview#Parity-Protected_Array

[tommitytom]

I see, so in my case I would essentially be losing 6tb of my party drive!  I'm guessing that's the best possible option here?  My plan was to eventually fill the machine up with 4-5 8TB drives.  Is it possible to run multiple arrays?  One for my 2TB drives, and one for my slowly expanding collection of 8TB drives?

[itimpi]

1 hour ago, tommitytom said:

I see, so in my case I would essentially be losing 6tb of my party drive!  I'm guessing that's the best possible option here?  My plan was to eventually fill the machine up with 4-5 8TB drives.  Is it possible to run multiple arrays?  One for my 2TB drives, and one for my slowly expanding collection of 8TB drives?

Multiple arrays are not possible although I believe it is a roadmap item, but with no ETA.

 

Having the 8TB as parity is over-kill at the moment, but it does mean you are in a position to add 8TB data drives going forward.