Help with Understanding on Drive Selection

[Nano]

Hi,

 

My current understanding of Unraid is the following.

 

Parity = Data mirrors from the Data Drives Nightly, like raid but not instantly like and allows for different sizes such as 8TB Parity and 2 4TB Data Disks

 

Data = In use constantly, (if cache not used ) and will backup to parity ( nightly ?? hourly ?? )

 

Which leads to my question.

 

I would want to use amazing NAS Drives for my Data Disk's but i should use the CHEAPEST Desktop HDD for parity.

 

I'm sure my understanding is wrong and want someone to ELI5 if i am indeed incorrect.

[trurl]

1 hour ago, Nano said:

Parity = Data mirrors from the Data Drives Nightly, like raid but not instantly like and allows for different sizes such as 8TB Parity and 2 4TB Data Disks

This is all wrong. And since the rest of what you said is based on this misconception no point in commenting on them.

 

Unraid parity is realtime. Any time a data disk in the array is written, parity is updated immediately.

 

Parity is not a mirror at all, and in fact, has no data. Parity is just that, parity. The concept of parity is used in a lot of different ways in computers and communications. Parity is just a bit that, combined with other corresponding bits, allows a missing bit to be calculated.

 

In the case of Unraid, parity plus all of the other disks in the array allows the data for a missing disk to be calculated.

 

Here is the wiki on parity:

 

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

[Nano]

Following your explanation, would you atleast need a 8TB drive as parity for 2 data disks.

 

Second question, if you had 1 data disk and 1 parity disk and the data disk or parity fails, can you rebuild the arrary by replacing the disk

[trurl]

5 hours ago, Nano said:

Following your explanation, would you atleast need a 8TB drive as parity for 2 data disks.

Parity must be at least as large as the largest single data disk in the array. In your example above with 2x4TB data disks, only a 4TB parity disk would be required.

 

6 hours ago, Nano said:

Second question, if you had 1 data disk and 1 parity disk and the data disk or parity fails, can you rebuild the arrary by replacing the disk

Yes, that is basically the whole point of parity. If you had 20 data disks and one parity disk, and one of those 20 data disks failed, then it could be rebuilt from parity plus all the remaining disks. Did you read the wiki link I gave?

[Nano]

On 3/31/2019 at 11:45 PM, trurl said:

Parity must be at least as large as the largest single data disk in the array. In your example above with 2x4TB data disks, only a 4TB parity disk would be required.

 

Yes, that is basically the whole point of parity. If you had 20 data disks and one parity disk, and one of those 20 data disks failed, then it could be rebuilt from parity plus all the remaining disks. Did you read the wiki link I gave?

I understand that, so if both data disks failed you lose the Array, would the 8TB allow for 2 Drive Failures ? Or is it always one?

[JonathanM]

2 hours ago, Nano said:

I understand that, so if both data disks failed you lose the Array, would the 8TB allow for 2 Drive Failures ? Or is it always one?

You never lose any more disks than have failed. All good disks can still be used, so you never "lose the Array".

 

Each parity drive, 1 or 2, allows for that many failed drives to be emulated. If you have 2 parity drives, you can have 2 data drive failures that are seamlessly emulated. 1 parity drive, 1 data drive emulated.

 

Parity size is irrelevant, only the capacity of the largest data disk is used on the parity disk. Having an 8TB parity disk and 4TB data disks is wasteful until you upgrade one or more of your data drives to 8TB.