> Fortunately writes on the file system level can be merged across file system barriers, resulting in few barriers that have to be passed to the block device level.
But what about eliminating repeated writes to the same place? Take this contrived example:
repeat 1000 times:
write first byte of file A
write first byte of file B
A COW file system may well be able to merge the writes, but it would require a lot of intelligence for it to see that most of the writes could actually be skipped. And a traditional file system would be even worse off.