# Drive Space Utilization

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Suppose I made an unraid with a bunch of 4tb drives. Then later on I add a couple of bigger drives, say 6TB. They'd have to be made into parity drives; I understand the biggest-drive rule there. But I wonder about the utilization of space on these mixed size drives. So let me phrase the question most succinctly:

"Do I gain, in this scenario, 8 or 12 TB available storage?"

Thanks!

3 minutes ago, dnyberg said:

Suppose I made an unraid with a bunch of 4tb drives. Then later on I add a couple of bigger drives, say 6TB. They'd have to be made into parity drives; I understand the biggest-drive rule there. But I wonder about the utilization of space on these mixed size drives. So let me phrase the question most succinctly:

"Do I gain, in this scenario, 8 or 12 TB available storage?"

Thanks!

You gain no space if those drive(s) have to be used as parity.

6 minutes ago, dnyberg said:

Do I gain, in this scenario, 8 or 12 TB available storage?"

An easier way to consider it is that you simply add up the sizes of all the non-parity drives and that is the usable space (regardless of whether they are mixed sizes).

Ok, let me clarify: assume the 4TB initial build will have 2 of the 4TB drives as parity. So we add two 6TB, make them the parity drives, free up two of the 4TB's for general storage pool. Depending on how the 6TB drives are used, I have gained either 8 or 12 TB of useful space. Which is it, please?"

15 minutes ago, dnyberg said:

Ok, let me clarify: assume the 4TB initial build will have 2 of the 4TB drives as parity. So we add two 6TB, make them the parity drives, free up two of the 4TB's for general storage pool. Depending on how the 6TB drives are used, I have gained either 8 or 12 TB of useful space. Which is it, please?"

Ah, ok, that makes it much more clear.  You are taking those 4TB drives that use to be the parity drives and adding them for storage.

In that case you would end up with 8TB of space available.

If you only replace your initial 2 4TB parity drives by 2 6TB drives, you gain NO useful space.

If you only replace your initial 2 4TB parity drives by 2 6TB drives, then add those 2 4TB drives back as Data drives, you gain 2x4TB of useful space.

41 minutes ago, itimpi said:

you simply add up the sizes of all the non-parity drives and that is the usable space

Okay, I think I understand the system now.

Just to confirm, now I have a bunch of 4TBs, and two 6TBs as parity drives.

Now I add a 3rd 6 TB, this one set as a data drive. My useful storage has now increased by 6TB?

21 minutes ago, dnyberg said:

Ok, let me clarify: assume the 4TB initial build will have 2 of the 4TB drives as parity. So we add two 6TB, make them the parity drives, free up two of the 4TB's for general storage pool. Depending on how the 6TB drives are used, I have gained either 8 or 12 TB of useful space. Which is it, please?"

Assuming you keep all drives you gain 8TB as that is the additional space in the 2x4TB data drives you have added to the array.  You ignore the size of the parity drives in any such calculation, the fact that they are bigger than your largest data drive is irrelevant - it just means that you can now add 6TB data drives without having to upgrade the parity drives.

Just now, dnyberg said:

Now I add a 3rd 6 TB, this one set as a data drive. My useful storage has now increased by 6TB?

Yes

Thanks, I understand now.

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