Overwriting copy functionality in filesystem

[Bharath Vedartham]

On Sat, Mar 23, 2019 at 03:01:56PM -0400, Valdis Klētnieks wrote:
> On Sat, 23 Mar 2019 22:29:45 +0530, Bharath Vedartham said:
> 
> > I was wondering how we can overwrite the copy functionality while
> > writing our own filesystem in linux. 
> > VFS does not offer any sort of API for copy. I think it calls create and
> > write when we execute the copy the file/dir.
> 
> Which you can verify using strace.  Which you should already be familiar with
> if you have the experience needed to write a usable filesystem.
>
Yes I did that. I have observed that it is a mixture of
create,read,write system calls.
> > I am interested in overwriting the way copy happens in my
> > filesystem(which I am writing for fun :p). 
> 
> And what, exactly, do you want copy to do differently on the API level,
> and on the file system level?
> 
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. 

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.