fienode - discover identical CoW copies

Something analogous to an inode for Copy-on-Write (CoW, cp --reflink) files.

Discover when two files on a CoW filesystem share identical physical data.

Why's it called fienode? It's analogous to an inode, but derived from File Extents.

Warning: Proof-of-concept. Alpha quality software, use at your own risk.

See Also:

sharedextents, for comparing physical extents between two files
filefrag -v, for listing physical extents

Installation

From source

go get github.com/pwaller/fienode

Binary Download

See releases.

fienode-linux-amd64 v1.0

Usage: `fienode <filename>...`

For example:

$ fienode foo
foo 8a53b838c97f1f9712e6a77e2bc00dd1922d32e9
$ cp foo bar
$ fienode bar
bar c829d10e6ccc0f90fc41d1492e423e2cabfe2bca
$ cp --reflink foo baz
$ fienode baz
baz 8a53b838c97f1f9712e6a77e2bc00dd1922d32e9

# (note: baz and foo share the same hash)

How it works

The result is the a SHA1 hash of the physical extents of the file.

See "How to verify a file copy is reflink/CoW?" on unix.stackexchange.com for more information.

Caveats

There may be bugs. This will delete all your data and eat your cat. When it does, that is your problem. Keep backups, folks.

I just copied a large file x to y with cp --reflink x y. For about a minute, it gave equal results. Thereafter, BTRFS decided - seemingly at random - to coalesce two extents in one of the files but not the other. So fienode then returned different results, even though the majority of the file was actually shared.