How to create a read-only snapshot in btrfs?

mount btrfs backup

8,734

Solution 1

btrfs subvolume snapshot -r /path/to/mounted/subvolume /path/to/snapshot

(Note that this did not work on older versions of Ubuntu.)

Solution 2

Are you using Ubuntu?

Ubuntu ships with an old version of btrfs-tools (aka "btrfs-progs"), and thus the userland tools do not support read-only snapshots.

See this bug (and it's parent bug):

https://bugs.launchpad.net/ubuntu/+source/btrfs-tools/+bug/924621

Solution 3

According to Btrfs wiki, read-only snapshots are available since Linux 2.6.38. So make sure you have a recent kernel. Apart from that, you should be just able to do it using ro mount option (mount -o [other-options],ro ...).

8,734

imz -- Ivan Zakharyaschev

My name is Ivan Zakharyaschev, Иван Захарьящев, imz.

Updated on September 18, 2022

Comments

imz -- Ivan Zakharyaschev almost 2 years

As far as I understand, the snapshots made by btrfs subvolume snapshot are writable by default and are visible at a certain path under the mountpoint of your btrfs volume.

How to setup the system so that there is a gurantee during the normal operation of the system that nothing is written to the snapshot? But without making it invisible to the users (i.e., it should remain accessible for reading by any users with respect to their permissions).

(For now, I've just chmod og-rwx /.snapshots where my snapshots are mapped to. But that doesn't fulfill the second of my wishes, and also the guarantee is not strong enough: a process with root's rights could spoil something there.)

Does one need to do some tricks with mount to achieve this goal?
- Admin almost 13 years
  
  In LVM, there is AFAIU a dedicated lvchange --permission r for this purpose.
- Admin almost 13 years
  
  Comparing the default situation in btrfs (i.e., writable snapshots) to LVM: Can LVM snapshots be writable?.
- Admin about 12 years
  
  What distro are you using? Ubuntu ships with an old version of btrfs-tools, and thus does not provide read-only snapshots.
Mark Norgren over 12 years

Doesn't work for me either on Ubuntu 11.10.
Adrian Ratnapala almost 8 years

@AleksandrDubinsky Can we at least try to generalise this away from Ubuntu. I expect the version of btrfs, or at least its user-space tools is more relevant. We know at least that v0.19 of the command line tool didn't work. Can we get a version number for that does work?