[Solved] Dual parity and adding a new drive

[doron]

Hi all,

 

This may be a stupid question (then again maybe not):

 

When adding a new drive to an existing array, common wisdom has it you should preclear it, and once this is done (the drive is all zeros), add it to the array, starting the array with "Parity is valid" to save the rebuild. I presume the underlying assumption is that if the drive is indeed all zero, then the parity is valid after its addition by definition of the math.

This makes sense with single parity drive (since the parity operation is a XOR, and adding a zero to a XOR function does not alter the result)

However with dual parity, I've been asking myself whether this assumption (that the parity is valid) still holds (since parity2 function is not a simple XOR).

 

More concisely, - when adding a new drive that is precleared to an array with dual parity, is it still safe to assume that parity is valid?

 

Thanks in advance for any help re this. 

Edited September 30, 2019 by doron

[trurl]

Parity2 still valid if you add a clear disk. Even though the math is different, the fact the disk is all zeros means the result is unchanged.

 

Also, when you add a clear disk to a new slot, Unraid will recognize it as clear because it will have the clear "signature". There is no need to go through New Config to add the disk, and so no need to tell it parity is already valid. It already knows.

[doron]

Thank you@trurl! Great info. Good to know about the auto recognition, too - I've been going old school with these things.

[itimpi]

8 hours ago, doron said:

When adding a new drive to an existing array, common wisdom has it you should preclear it, and once this is done (the drive is all zeros), add it to the array, starting the array with "Parity is valid" to save the rebuild. I presume the underlying assumption is that if the drive is indeed all zero, then the parity is valid after its addition by definition of the math.

It has never been necessary to make this assumption at the user level as once you have done the pre- clear then Unraid will simply accept any new drive as ready to go without affecting parity as it can tell from the pre-clear signature being present that parity remains valid.

[bonienl]

If a new disk is not precleared, Unraid will start automatically clearing it once the disk is added to the array and the array is started.

After the clearing is finished, Unraid will ask to format the new disk. After format is completed the new disk is ready to go.

 

Edited September 29, 2019 by bonienl

[doron]

Thanks @itimpi and @bonienl. I guess I'm quite behind on the User Interface part of things... Cool (and useful!) stuff.

[trurl]

1 hour ago, doron said:

I guess I'm quite behind on the User Interface part of things

It has worked like this as long as I have been using Unraid (4.7)

[doron]

15 minutes ago, trurl said:

It has worked like this as long as I have been using Unraid (4.7)

... and I've been ignorant of that ever since 5.0 😄