On Sun, Mar 24, 2019 at 10:06:36AM -0400, Valdis Klētnieks wrote: > 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. > That is a very interesting use case. Any examples of filesystems like that in the kernel? I was thinking of a use case where we are copying a huge file (say 100 GB), if we do copy-on-write we can speed up /bin/cp for such files i feel. Any comments on this? > > 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? > I have been looking at overlayfs. But overlayfs uses a lower read-only layer and an upper layer where changes are reflected. I am more interested in filesystems without any layering which support copy-on-write. Are there other filesystems in the kernel which support copy-on-write? > Hint: How do file systems that support point-in-time snapshots work? _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
