Re: how do I know a subvolume is a snapshot?

["Bernhard Kühnel" <bkue@allchangeplease.de>]

Am 17.07.2019 um 01:24 schrieb Ulli Horlacher:
> How do I know that /mnt/tmp/ss is a snapshot?
> I cannot see a snapshot identifier.

From the btrfs-subvolume man page:

>       A snapshot is a subvolume like any other, with given initial
content. By default, snapshots are created
>       read-write. File modifications in a snapshot do not affect the
files in the original subvolume.


I believe the usual practice is to create snapshots with the -r flag and
follow some naming convention (e.g. place them in a common .snapshots
folder named by date), but you're free to switch between read-only and
read-write mode for a snapshot at any time using the btrfs property command.

That allows for some intereresting feats: e.g. there's no guarantee that
a (now) read-only snapshot actually reflects the source's state at
creation time (if someone modified it and re-applied the ro flag). On
the other hand, reverting to a snapshotted state may be as easy as
making the snapshot rw and changing the mount options to use it's
subvolid - no need to copy any files back and forth.


If there is a specific reason why you want to discern snapshots from
other subvolumes (verification/auditing/forensics?), maybe you can get
help here by elaborating further on that.


Bernhard Kühnel