After preclearing the array and parity, all discs will be zeroed, so why is it necessary to build the parity?

When you add a disk it is not necessary to build parity.

 

I mean a brand new array/configuration.

 

If unraid can see the preclear signature, why is there still a need to calculate the parity. All discs will be zero, therefore the parity will also be zero.

But the parity has got to be MADE zero...

 

Also the disks are not complete zero since there is a filesystem with data on there.. even if there are no files.

But in the case were one sets up a new array, and all data disks as well as the parity-to-be have been pre-cleared, the exact contents are already known - zeros! No file systems on the data disks yet, formatting is later in the process.

If a clear parity and a set of clear drives are all added at the same time in a new configuration, then it should be valid to format the disks and just have parity update like normally when formatting instead of doing a complete parity sync. It has been many years since I had that scenario, so I don't know if unRAID does a parity sync anyway or not.

 

Seems like a special case that is probably not really checked for, but maybe you can manually prevent the full sync by checking the parity is valid box before starting.

If all disks are cleared, including parity(s), you can check the "parity is already valid" when first starting the array.