Is the parity drive's capacity the effective total capacity?

[RebelLion1519]

Hello UnRAID Community!

 

This is my first post, so pardon in advance for noob questions. One of the reasons UnRAID appeals to me is the fact that you can start with a single drive and build up over time. I currently have a 10 TB drive in my desktop, which I'd like to use in the server. The plan would be to build the UnRAID server with that 10 TB drive as the only drive, then add more drives and use the 10 TB as the parity. I understand that any subsequent drives must be smaller than the parity, but does the total capacity of all drives have to be less than the parity? If I added 3x5 TB drives and used the 10 TB as parity, would I have 15 TB of useable space, protected from disk failure? Or would I want to get 2x5 TBs, then later get a 20 TB and make it the parity?

[trurl]

The total capacity of all data drives in the array is the total capacity of the array. Parity contains none of your data and by itself can recover nothing.

 

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

[MAM59]

the really easy answer is: NO 🙂

You can have as many drives as you wish (or your licence allows) but they cannot be bigger each than the parity drive.

So, you may use your 10Tb as Parity and add 10*10Tb for Data, summing up to 100Tb net dataspace.

UNRAID just sums up every single sector of each drive and uses the result for the sector on the parity drive. If a data drive is smaller (aka: does not contain the higher sector numbers) the higher sectors are simply skipped throughout parity generation or check.

The only need is that each sector number of any data drive must have a corresponding sector on the parity drive. Thats why the parity drive has to be the biggest of all.

 

Its a very smart and easy approach, avoiding the bad idea of RAIDs that depend on each drive has to be equal.

 

 

Edited July 12, 2022 by MAM59

[kizer]

My system is the following. 

4TB Parity

4TB x 6 Data

1TB SSD for Cache

2GB USB for the OS

 

So if I ever want to install larger data drives I have to increase the Parity first. So if I went to 10TB Data Drives my first drive would be swapping out the 4TB for the 10TB. You could honestly think of the Parity as wasted/no space until it saves your butt a few times.

 

 

[peterg23]

Spaceinvader One has a good video explaining "How Disk Arrays and Parity Work" here:

 

https://www.youtube.com/watch?v=dX2PvD1qtKw

 

[RebelLion1519]

Thanks guys! I have a much better understanding of how parity works. Okay to ask follow-up questions here? I'm honestly new posting to forums in general. 

 

Can I start my server with just the single 10 TB drive, and then after I've added more drives, re-assign the 10 TB to be parity, or do I have to start with at least one drive in addition to the parity?

[JonathanM]

No data drive can be larger than parity, and you can't make any data slot smaller, only bigger. So if you start with a 10TB data drive and want to keep the data on it, you will need a 10TB drive or larger to assign as parity.

 

If you manually move all the data off the 10TB to other drives so it's empty again, then you could set a new config and assign it as parity.

[trurl]

11 hours ago, RebelLion1519 said:

do I have to start with at least one drive in addition to the parity?

 

On 7/11/2022 at 9:21 PM, trurl said:

Parity contains none of your data

 

You can't start the array without at least one data drive. You can start without parity though. 

 

Since dual parity is an option and that scenario hasn't been mentioned, no data drive can be larger than any parity drive.

[RebelLion1519]

To clarify my question a little better, I understand that you have to have a data drive and don't necessarily need a parity drive. What I was hoping would be I could build the server with the 10 TB as the only drive installed, use it for data, without parity. Then, later, I could add say two 5 TB drives for data and re-assign the 10 TB to be a parity. I think from these responses that would be possible?

[trurl]

If you wanted to keep the data from the 10TB you would have to copy it before assigning it to parity.

 

And since your example had 10TB parity plus 2x5TB data, I don't want you to get the idea that parity must equal the size of all the data disks. Parity just has to be at least as large as the largest single data disk. You could have 10TB parity plus several data disks of various sizes, each data disk could be up to 10TB.