it is copy-on-write, but it is not a patch, it's a completely different block.
basically, assume you have a file using blocks 4, 5, and 6. That is in the
current filesystem (aka: consistency checkpoint.)
if you don't change the file, and then create a new snapshot (#1), the blocks
are the same, and the inode in snapshot 1 points to the same data blocks (4,
5, and 6.)
now, say you append to the file. snapshot 1 still points to (4,5,6), and the
current filesystem will point to (4,5,7), where block 7 is copied data from
block 6, plus the appendage.
snapshots are a read-only view of the file system at the time of the snapshot.
for a graphical interpretation, see http://www.netapp.com/technology/level3/300
2.html, page 11.
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