I currently have 6 - 3TB drives in my unRAID server and I'd like to start buying 6TB drives as the price has come down. I know I will need a 6TB parity drive to cover the new 6TB array drives, so would it be possible to use two of my old 3TB together to create a single 6TB parity drive for unRAID?

Only if you have a hardware RAID card.  But even there, I wouldn't recommend it as you would effectively be doubling the chances that the parity "drive" would die (as there's two members)

I'm not sure the idea is really that useful either.

Let's look at a couple examples:

 

starting point, 3x 3TB drives, (1 parity, 2 data, 6TB total).

you have effectively 6TB of data storage.

add 1 6TB drive (parity). retask 3TB parity as data.

data increase: +3TB (9TB)

add 2nd 6TB drive (data).

data increase: +6TB (15TB)

add 3rd 6TB drive (data)

data increase: +6TB (21TB)

 

now, starting from the same 3x 3TB...

add: 1st 6TB (data), retask 2x 3TB as parity (RAID0 6TB)

data increase: 0TB (9TB)

add 2nd 6TB drive (data)

data increase: +6TB (15TB)

add 3rd 6TB drive (data)

data increase: +6TB (21TB)

 

you've come to the same place, but now you've added complexity and increased failure potential.

 

about the only hardware RAID i've considered is potentially RAID1 for the parity drive, but even that is problematic.

Edited November 9, 20196 yr by sota

The other problem with trying to RAID together 2x3TB drives to get 6TB is if the result is even 1 byte less than a genuine 6TB drive then you will not be able to use it as parity in conjunction with 6TB data drives.

This is all really good info, thank you.

My interest in doing something like this is to increase the IO speed for the parity drive RN my parity drive slows the entire array down to 30mbps I was thinking use super cheep 2tb 7200rpm drives in a raid 10 basicaly giving my parity set 4x read and 2x write speed

 

1 hour ago, Linearburn said:

My interest in doing something like this is to increase the IO speed for the parity drive RN my parity drive slows the entire array down to 30mbps I was thinking use super cheep 2tb 7200rpm drives in a raid 10 basicaly giving my parity set 4x read and 2x write speed

 

I doubt you would get much improvement in speed, certainly nothing like what you seem to expect. And others in this thread have already explained why this is a bad idea. 

1 hour ago, Linearburn said:

slows the entire array down to 30mbps

I assume this is the write speed you are talking about. See here:

 

 

1 hour ago, Linearburn said:

giving my parity set 4x read and 2x write speed

Parity is not even involved in reading data so you can see your expectations have no basis. 

In addition to that Turbo Write topic I linked, here is a wiki link to a good overview for a better idea of how Unraid (IS NOT RAID) actually works:

 

https://wiki.unraid.net/UnRAID_6/Overview