Ideally you would use a snapshot-aware NAS - such as a NetApp filer. I remember having a presentation from and discussion with them about 6 years ago, and I was extremely impressed with their concept.
From their white paper:
Files are composed of blocks, which are linked together to form files. Each block pertaining to a file contains bits of information that are part of the file. File systems keep track of the information stored in blocks.
WAFLŪ is write-optimized and always writes new disk blocks to available locations on disk. Pre-existing data blocks are never overwritten. This is true whether a new file is being created or an existing file is being updated. This process minimizes disk drive seeks, which improves performance. Thus preexisting blocks still exist and can be accessed in the form of Snapshot copies until they are deleted. The amount of disk space reserved for such copies is configurable on a per-volume basis.
Snapshots are not copies of data, rather Snapshots record the state of the blocks in the file system at a given point in time and provide read-only access to that image of the file system.
Interestingly, they use RAID 4 in their filers - the dedicated parity disk avoids having to overwrite data blocks with parity data, thus maximising the space available for data and shadow copies.
SD