fallocate failing inside my docker container

Question

I'm trying to run a script inside my Linux Ubuntu 14.10 Docker container which uses the fallocate command like so:

fallocate -l 10M 10meg

However, when I run this script, and even when I run the command when I have ssh'd into my container, I get the following error:

root@~$>> fallocate -l 10M 10meg
fallocate: fallocate failed: Operation not supported

Any ideas why this is failing inside my container?

Answer 1

at the time of writing, fallocate() syscall is not supported by Union FS, the filesystem used by Docker. You can either use truncate as follows (recommended):

truncate -s 10M 10meg

or dd (expensive):

dd if=/dev/zero of=10meg bs=1M count=10