What exactly is Parity Consistency Check that Synology NAS runs when a new disk is added and why does it normally take a long time to complete?

What exactly is "Parity Consistency Check" that Synology NAS runs when a new disk is added and why does it normally take a long time to complete?

Why does the process takes a long time? (can take up to a week when adding a new drive, when all I seem to have is three 8Tb drives)

I only have two 8Tb disks, in my Synology NAS using SHR. Would it be better to copy over the data that I have there now elsewhere, then insert a new disk into NAS, and initialize them all as a new SHR drive, and then copy my data back from elsewhere to the new SHR? It seems like it will take faster to do that than to wait for the natural Parity Consistency Check that is running.

This has been running 24 hours and just about 20% now, that seems awfully long time.... which again makes me wonder if it's better to do the "copy over, re-initialize, and copy back" method

With a quick look I didn't find out how Synology's Parity Consistency Check actually works. Anyhoo my best guess is that when the new drive is being built the data is validated against stored checksum values to ensure integrity. Something along those lines.

This is a common woe in Synology forums, Reddit etc. According to this Reddit thread "when you start talking volumes measured in TB, the parity check is measured in days".

According to Synology knowledge center the resync speed can be adjusted in the storage manager. That might speed up the process. User who posted this thread to Synology forums says disabling iSCSI LUNs speeded up the process considerably.