There are 2 types of point in time copies:
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.
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
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.