How much space NAS snapshots consume?

Solution 1:

Most NAS snapshots implement copy-on-write, so the snapshot itself initially takes up no space (or next-to-none, there is some overhead). But any block that is changed whilst the file it's part of is snapshotted must be copied. If an entire file is deleted whilst it's still in a snapshot, all those blocks must still be kept.

So in your case above, as long as that file is still in a snapshot, you only have 9GB left. Worse still, if the file is not deleted but completely altered, so that every block is changed, you will only have 8GB left, as 1GB is used up by the file in its current incarnation, and 1GB is used by the original file. If the file is completely changed every day, and every daily snapshot is kept, you'll run out of space completely before the end of day ten.