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