docker disk space grows faster than container's - docker

Question

Docker containers that are modifying files, adding, and deleting extensively (leveldb) are growing disk usage faster that the container itself reports and eventually use up all the disk.
Here's one snapshot of df, and a a second. You'll note that disk space has increased considerably (300Mbytes) from the host's perspective, but the container's self-reported usage of disk space has only increased by 17Mbytes. As this continues the host runs out of disk.
Ubuntu stock 14.04, Docker version 1.10.2, build c3959b1.
Is there some sort of trim-like issue going on here?
root#9e7a93cbcb02:~# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/docker-202:1-136171-d4[...] 9.8G 667M 8.6G 8% /
tmpfs 1.9G 0 1.9G 0% /dev
tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
/dev/disk/by-uuid/0a76513a-37fc-43df-9833-34f8f9598ada 7.8G 2.9G 4.5G 39% /etc/hosts
shm 64M 0 64M 0% /dev/shm
And later on:
root#9e7a93cbcb02:~# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/docker-202:1-136171-d4[...] 9.8G 684M 8.6G 8% /
tmpfs 1.9G 0 1.9G 0% /dev
tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
/dev/disk/by-uuid/0a76513a-37fc-43df-9833-34f8f9598ada 7.8G 3.2G 4.2G 43% /etc/hosts
shm 64M 0 64M 0% /dev/shm

Answer 1

This is happening because of a kernel bug fix that has not been propagated to many mainstream OS distros. It's actually quite bad for newbie Docker users who naively fire up docker on the default Amazon AMI as I did.
Stick with CoreOS Stable, you won't have this issue. I have zero affiliation with CoreOS and frankly am greatly annoyed to have to deal with Yet Another Distro. In the CoreOS distro or other correctly working linux kernel the disk space of container and host track each other up and down correctly as the container frees or uses space. I'll note that OSX or other virtual box distros use CoreOS and thus work correctly.
Here's a long writeup on a very similar issue, but the root cause is a trim/discard issue in devicemapper. You need a fairly recent version of the Linux kernel to handle this properly. I'd go so far as to say that Docker is unfit for purpose unless you have the correct Linux kernel. See that article for a discussion on which version of your distro to use.
Note that above article only deals with management of docker containers and images, but AFAICT it also affects attempts by the container itself to free up disk space during normal addition/removal of files or blocks.
Be careful of what distro your cloud provider is using for cloud container management.