/dev/full

In Linux, FreeBSD, NetBSD /dev/full, or the always-full device, is a special file that always returns the error code ENOSPC (meaning "No space left on device") on writing, and provides an infinite number of zero bytes to any process that reads from it (similar to /dev/zero). This device is usually used when testing the behaviour of a program when it encounters a "disk full" error.

History
Support for the always-full device in Linux is documented as early as 2007. Native support was added to FreeBSD in the 11.0 release in 2016, which had previously supported it through an optional module called lindev. The full device appeared in NetBSD 8.