In Ubuntu, squashfs is routinely used for snap files, but since these are synthetic filesystems they display as 100% utilization and thus trigger check_disk to issue a false positive DISK CRITICAL error.

Similar false positives have been reported to us for tracefs and tmpfs. The commonly discussed workaround is to exclude these when running the tools (e.g. using the '-X ' parameter for check_all_disks). Since wrappers are typically used for running the underlying tools, it is possible to add a string of -X... parameters.

Unfortunately the number of flags needed is getting extensive, and I wonder if this could be solved more uniformly. For filesystems that are always going to show 100% capacity, a cleaner solution is possible. monitoring-plugins' check_disk.c maintains an internal exclusion list, fs_exclude_list, which already excludes iso9660, and can be modified to add other filesystems to exclude by default.

In other words, check_disk.c would be modified thusly:

np_add_name(&fs_exclude_list, "iso9660");
np_add_name(&fs_exclude_list, "squashfs");
np_add_name(&fs_exclude_list, "tmpfs");
np_add_name(&fs_exclude_list, "tracefs");

This code is added prior to the command line parsing logic, and as such simply sets default behavior. It does not preclude further adding or removing filesystems via the -X and -N parameters. Indeed, if someone were to desire checking tmpfs, they are able to manually add it, via "-N tmpfs".