I ran a backup today and noticed one file because it took so long to back up. I'm using FC25 on this machine, and used the OS packages of Docker. I seem to have a 100G file on a 12G partition: root@toshi7:/var/lib/docker/devicemapper/devicemapper# ls -lh total 34M -rw------- 1 root root 100G Oct 25 22:02 data -rw------- 1 root root 2.0G Oct 25 22:02 metadata root@toshi7:/var/lib/docker/devicemapper/devicemapper# df -h Filesystem Size Used Avail Use% Mounted on ... /dev/sda8 16G 12G 3.8G 75% / tmpfs 7.8G 776M 7.1G 10% /tmp /dev/mapper/home_crypt 237G 211G 24G 91% /home ... /var/ is part of / , not a separate partition. The file appears to be a plain file, not a link or device. How exactly did my backup process (rsync to an external hard drive) manage to back up 100G of data from a 12G partition? "docker info" contains some very interesting output: I've included that below. The most obvious/naive solution would simply be to exclude /var/lib/docker/devicemapper/devicemapper/ from my backups. Because I'm guessing I don't need a backup of the "Data loop file?" Is that correct? To my surprise, the file and its backup share an md5sum value - but then, I haven't touched docker in the intervening time. But I'd still really like to know either where it's storing 100G of data, or what's going on if it's not actually storing that much. It's very weird, and makes it awfully hard to calculate how big your backup will be when your partitions are evidently behaving like Tardises. Containers: 0 Running: 0 Paused: 0 Stopped: 0 Images: 8 Server Version: 1.12.6 Storage Driver: devicemapper Pool Name: docker-8:8-802519-pool Pool Blocksize: 65.54 kB Base Device Size: 10.74 GB Backing Filesystem: xfs Data file: /dev/loop0 Metadata file: /dev/loop1 Data Space Used: 27.79 MB Data Space Total: 107.4 GB Data Space Available: 4.943 GB Metadata Space Used: 618.5 kB Metadata Space Total: 2.147 GB Metadata Space Available: 2.147 GB Thin Pool Minimum Free Space: 10.74 GB Udev Sync Supported: true Deferred Removal Enabled: false Deferred Deletion Enabled: false Deferred Deleted Device Count: 0 Data loop file: /var/lib/docker/devicemapper/devicemapper/data WARNING: Usage of loopback devices is strongly discouraged for production use. Use `--storage-opt dm.thinpooldev` to specify a custom block storage device. Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata Library Version: 1.02.136 (2016-11-05) Logging Driver: journald Cgroup Driver: systemd Plugins: Volume: local Network: null host bridge overlay Swarm: inactive Runtimes: oci runc Default Runtime: oci Security Options: seccomp Kernel Version: 4.12.11-200.fc25.x86_64 Operating System: Fedora 25 (Twenty Five) OSType: linux Architecture: x86_64 Number of Docker Hooks: 2 CPUs: 8 Total Memory: 15.56 GiB Name: toshi7 ID: TJAV:PKWI:UYK2:UGAQ:GHUP:2HDO:4RBD:U54O:X7DM:SUPY:WQAM:IFTX Docker Root Dir: /var/lib/docker Debug Mode (client): false Debug Mode (server): false Registry: https://index.docker.io/v1/ Insecure Registries: 127.0.0.0/8 Registries: docker.io (secure) -- Giles https://www.gilesorr.com/ gilesorr@gmail.com