There are 2 types of point in time copies: Mirrors
Commonly referred to as split mirrors (this again would be a 100% copy of our data). With a split mirror the primary area of our mirror and the secondary storage area have to be exactly the same usable size. They don’t have to be the same RAID type but as far as our split mirrors are concerned, if we have 100GB usable, that has to be 100GB usable.
Snapshot
This is a virtual representation of a volume at a given point in time. We initially only capture the metadata (the pointers). We will then only copy or replicate changed data. Typically the snapshot size is only going to be about 20% of the volume size.
So snapshots are a virtual representation at a given point in time and a split mirror is a 100% copy at a given point in time.
But in both instances we need a consistent usable copy of data.