Copy-on-write hard-links

Is there a feature in btrfs to manually/explicitly mark hard-links to be copy-on-write? My understanding is that this is what happens when a snapshot is mounted rw and files modified.

Consider this scenario:

I have a base template fs. I make two snapshots of it that are identical. The files in the template and both snapshots are hard-links and have the same inode number.

I change a file in one of the snapshots, and it gets copied on write. I make the same change in the other snapshot, and that, too, gets copied on write. I now have two identical files that are not hard-links any more.

What happens if I remove one of those files and create a hard-link to the file in the other snapshot? Will this implicitly become a copy-on-write file or will the hard-link aspect in the traditional sense be preserved? If I modify the file, will it end up modified in both? Is there a way to explicitly set a COW flag (on a file with hard-links)?

The reason I am asking this is because I am looking into using either VServer or LXC virtualization. VServer has a "hashify" feature that works as I described (copy-on-write hard-linking identical files between multiple guests). But VServer isn't, and is unlikely to ever be in the mainline kernel. LXC is already in the mainline kernel, but relies on the FS to provide this functionality. For future proofing reasons, I would prefer to use LXC+btrfs, but hashify is too valuable a feature to sacrifice for staying with the mainline. Also note that simple block-level dedupe isn't sufficient for the full benefit in this context - hard-linking has the additional benefit that multiple copies of DLLs in multiple guests will not use separate memory when hard-linked (i.e. their inodes are the same). This equates to a very substantial memory saving (poor man's KSM) in addition to the disk space savings when there are many guests.

TIA.

Gordan

I'm afraid you can't do this. hard linking between subvolumes isn't allowed. But, what you can do is use the clone ioctl to make a new inode that references all of the data extents of an existing file, which would be a kind of COW hard link.

Checkout bcp from btrfs-progs or cp --reflink from the latest..well wherever cp comes from.

-chris

Would the inodes on the clone file be the same for purposes of loading a dynamic library? Specifically, say the file I am cloning is a DLL. Normally, if a DLL is hard-linked, if two programs dynamically load it from two different hard-links, it'll still only use one bit of shared memory. Will this also hold true for the cloned files? My understanding is that it will not since it's not the same inode. Is that the case?

Thanks.

Gordan