Tvheadend

There's just one recording location (with sub-directories under it depending on the config option), so it should be easy to determine the amount of free space on that location's device. To get that, use statvfs(), but also be wary about quota'ed disks so use quotactl() for that. There should be a config option to allow the user to suggest free space warning level (either percentage or perhaps GB free) - should the filestore space go below the warning level, there should be some sort of alert (and a log entry). It gets a bit tricky how often you display that warning (probably only when it goes from being OK to being in warning state, so you'd need to know what the previous state was, so you don't keep displaying the alert).