DL-SNAP is a feature designed for directory level file backups.
It is implemented on top of lustre ldiskfs without modification of ext4 disk format, though we set a special original flag to FEATURE of the superblock in order to check whether DL-SNAP is enabled.
The feature makes execution of e2fsck command failed although no modification to ext2 file system structure.
DL-SNAP uses COW(Copy On Write) mechanism to reduce backup time and storage usage. When we create a snapshot, only inodes are created. At this point, new data blocks are not allocated on OST. Data blocks of snapshot are allocated when original file is modified.
Not only root users but also ordinary users can create snapshots. Users can create a snapshot of a directory using lfs command with snapshot option, and restore files by usual process such as cp command.
For more information, please check the presentation at LUG2016.