btrfs: imaging a volume to an external file

I recently built a system with a btrfs root partition. The most compelling reason for adopting btrfs compared to ext4, in my case, was live copy-on-write snapshots, with near-zero latency. Compared to ext4, when a full system backup entailed taking down the system, mounting from a live distribution, and building a partclone image on removable media, the promise of snapshots is that the snapshot can be captured on the backup media while the system is live.

To my astonishment, no tool exists, however, that will capture the entire snapshot to a single file on external media, such that if the system is restored from that file, all applications will have the same view of the filesystem as before the crash (except that the other snapshots or subvolumes are missing).

The documentation suggests mirroring the directory tree using a tool such as rsync, or using btrfs-send/btrfs-receive to capture the incremental changes on another system. In the first case, I have always found it nearly impossible to recreate all the metadata in a file tree exactly by mirroring a file tree rather than imaging the file system, and have little optimism that a restore would be very smooth. I always find that some metadata, be it permissions, timestamps, hidden files, or so on, are not properly captured. The problem is compounded when the transfer occurs across file systems of different types. The other suggestion assumes another btrfs file system is available, which is not always the case.

Are any suggestions available for saving or restoring a volume-level image, similar to a partclone file, but representing only a select subvolume?

1 Answer

btrfs send and btrfs receive are exactly the tools you need.

I have always found it nearly impossible to recreate all the metadata in a file tree exactly by mirroring a file tree rather than imaging the file system, and have little optimism that a restore would be very smooth. I always find that some metadata, be it permissions, timestamps, hidden files, or so on, are not properly captured.

Have you tried the tools? I have had no such problems with btrfs. btrfs send and btrfs receive do not work with files, they work with entire subvolumes, having access to all Btrfs metadata natively.

The problem is compounded when the transfer occurs across file systems of different types. The other suggestion assumes another Btrfs file system is available, which is not always the case.

You will need another Btrfs filesystem only if you want to replicate the subvolume there. This is often desirable, so you can mount it and get access to its files. And this is unavoidable, if you want to backup your subvolumes incrementally. You need the exact previous snapshot(s) in both places to incrementally send the next one.

With full dump, however, you get a standalone stream, which is exactly an image (but not a browsable backup, unless you btrfs restore it). Work with it like with any other stream:

btrfs send /source/subvolume >/another/filesystem/subvolume-image # just a file
# (or you can gzip it and/or send with nc on the fly, whatever)
# then later
</another/filesystem/subvolume-image btrfs receive /some/btrfs/directory

Where /some/btrfs/directory may belong to the same Btrfs filesystem as /source.