> It took me a while to find out how the block layer can ensure that a
> snapshot is consistent on the filesystem level. The answer is Linux VFS
> method super_operations.freeze_fs().
> https://www.kernel.org/doc/html/latest/filesystems/vfs.html
> Without it a snapshot on block level would be similar to a filesystem
> which was not properly unmounted before power-off.
But as I mentioned, higher-layers (the filesystem layer, and the
applications running on top of that) *should* try and make sure that
a hard failure (kernel crash, power failure, ... these and up taking
a snapshot of your block device) can never result in an
inconsistent state.
That's the core of the ext3 improvement over ext2, for example.
> So there might still filesystems in the Linux kernel which do not support
> LVM snapshots in a safe way.
Of course, just like there are still many applications which write files
non-atomically.
Stefan
