I already know that up to two hard drives can be used as verification disks, and I know that space of parity disk must be larger than any disk in the array. I am confused whether the total capacity of two verification disks is greater than that of other individual hard drives, or whether each verification disk needs to have a larger capacity than other hard drives?

Thank you

22 minutes ago, xemaco said:

each verification disk needs to have a larger capacity than other hard drives?

This, but it's not necessary to be larger, same or larger than existing data disks.

Since the wording might be confusing, I'll say it a different way.

Parity drive(s) must be equal to or larger than any other single drive in the array.  If your parity drives are 22gb and 20gb, then the largest any single drive in the array can be is 20gb.

6 hours ago, xemaco said:

the total capacity of two verification disks

Not relevant at all. Each parity disk is independent, and works with 2 different types of math. Parity 1 is a simple calculation, parity 2 is much more complex. They aren't interchangeable and they don't "team up".