Overwriting copy functionality in filesystem

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

On Sun, 24 Mar 2019 18:48:08 +0530, Bharath Vedartham said:

> I was interested in implementing copy-on-write for my filesystem(for fun
> :P). When I do a "cp" operation, I do not want to create a seperate
> inode for the new file. I only want to create a inode when I make a
> change to the file.

Actually, /bin/cp isn't where copy-on-write gets you benefits. Where it really
shines is when you have a versioning filesystem that keeps track of the last
N versions of a file with minimum overhead. So if you have a 100 megabyte
file, open it, write 5 blocks of data, and close it, you now can read back
either the original or new versions of the file, and you're only using 100M plus
5 blocks plus a tiny bit of metadata.

> There is no vfs api for cp. I would need to make creat syscall aware of the
> fact that it is being executed by "cp". My immediate idea was to check
> if a file with the same data exists in the filesystem but that would be
> way too much overhead.

Have you looked at other filesystems that already support copy-on-write?

Hint:  How do file systems that support point-in-time snapshots work?