Default size for devicemapper data can exceed free space

[amluto]

My /var lives on a 32GB partition. Docker helpfully created a 100GB sparse devicemapper data file, which (unsurprisingly) exceeded my free space rather quickly, causing docker to explode.

Given that there's no way that a sparse file that exceeds the total fs size can work well, docker probably shouldn't do this.

[alexlarsson]

The file will not use more than the actual size anyway, I fail to see how this would be different e.g. with the aufs backend. When you have more data in containers than fit on /var you'll run into problems, independent of the backend.

That said, we have some issues in reclaiming space. One was fixed in 0.7.1, with the removal of -init layers, the other being that devicemapper doesn't seem to be re-sparsing on image removal (reported in another issue somewhere)

[alexlarsson]

other size issue tracked in #3182

[crosbymichael]

Yes, as Alex said, it will not use more than the actual size. The space reclaiming issues have been fixed in the recent versions of docker.