Is btrfs suitable as backup filesystem

backupbtrfsexternal-hddfilesystems

Right now I have a pretty traditional backup filesystem structure on top of ext4. Every time a backup is made, a new folder backup-DATE is created to which files are rsync'ed (with hardlinks made using rsync's --link-dest option).

Since I have read about bitrot, I would like to have a checksum for all files, transparently. Apparently ext4 cannot do that, but btrfs does offer support for data checksums (and even a built-in RAID1 mode). For a start, I would like to use btrfs as a "dumb" filesystem which supports data checksums without using its advanced features such as RAID, subvolume snapshots, send/receive, etc.

However, their wiki do not really inspire confidence in the filesystem for backups purposes:

"While many people use it reliably, there are still problems being
found. You should keep and test backups of your data, and be prepared
to use them." – Getting Started

"Is btrfs stable? Long answer: [..] Whatever you do, we recommend
keeping good, tested, off-system (and off-site) backups." – FAQ.

My use case is to have an offline backup. For that reason the disk will see very little use (as in hours) and will be frequently plugged/unplugged (eSATA or USB 3.0). Having a reliable filesystem is a must. It must not be worse than ext4 wrt. power failures, unclean shutdowns, etc.

Is it actually recommended to use btrfs as filesystem for backup purposes? Are there other properties of btrfs which may make it less (or more) suitable?

Best Answer

I will just provide a short answer because I think this is being overthought.

If you read the main kernel wiki about the btrfs (sub-)commands, you will find that there are two commands for:

making a "backup": btrfs-send
and restore: btrfs-restore

Just in case, this means that it is not (designed to be) a backup, but to be an snapshot filesystem, with the idea of rolling back if needed, not as a backup but as "flexible".

Therefore — no, do not use it as backup — use it as a versioned filesystem where you can test things and go back. Don't rely on it.

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BTRFS – How to Modify a BTRFS Filesystem UUID

There is no way to do that for the moment. Actually, the volume UUID is used in each node of the chunk tree. You'll have to change them in there also assuming that the headers of the chunks/device are not hashed. BTRFS was really not design to allow this kind of backup.

This is really sad, but the easiest way to handle that is to use another computer.

If I may, I'd like to suggest you to stop backuping your data this way.

If your partitions are important as a whole, backup with dd/clonezilla. When you need to restore your backups, restore the whole partition at once. Don't do this kind of hybrid backups: you specifically saved your partitions at the bloc level. So you have to restore it at the bloc level. Otherwise, you are using a spoon to cut the meat. As you certainly noticed, this solution is usually not used because it offers no versatility.
If your data are important, backup with rsync or a similar tool on another disk : your data will always be accessible, you backup exactly what you want, you are backuping at the file level, etc.

Note that BTRFS has some (now limited) backuping features. BTRFS is moving fast, I guess more backuping features will come out in the future.

Oh, you've been warned already ;) Automated Clonezilla backup and GPG encryption

BTW, encryption is easier to apply on files using either LUKS for partition-based encryption or EnFS or EncryptFS for file-based encryption.

How to check (simulate), how much space will be freed after I remove a btrfs subvolume

As demonstrated here, this is actually fairly simple to do.

First, enable btrfs quotas:

# btrfs quota enable /btrfs_subvolume

And then run:

# btrfs qgroup show /btrfs_subvolume
OR
# btrfs qgroup show -f /btrfs_subvolume

Which in Btrfs v3.18.2 shows you this:

qgroupid        rfer       excl
--------        ----       ----
0/260        1.09GiB    1.09GiB

The 0/260 is the subvolume ID, and the excl is the exclusive data in the subvolume. If you delete the subvolume, that's how much space you'll free up.

Edit: According to this link, this appears to be the official recommended way to do this.

Best Answer

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BTRFS – How to Modify a BTRFS Filesystem UUID

How to check (simulate), how much space will be freed after I remove a btrfs subvolume

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