Special Filesystems

Linux widely employs the use of special filesystems for certain tasks. These are particularly useful for accessing various kernel data structures and tuning kernel behavior, or for implementing particular functions. 

Note that some of these special filesystems have mount points, such as proc at /proc or sys at /sys and others do not. Examples of special filesystems that have no mount point include sockfs or pipefs; this means user applications don't interact with them, but the kernel uses them, taking advantage of VFS layers and code. These special filesystems are really not true filesystems; they are kernel facilities or subsystems that find the filesystem structural abstraction to be a useful way to recognize data and functionality.

 

Table: Special Filesystems

FILESYSTEM	MOUNT POINT	PURPOSE
rootfs	None	During kernel load, provides an empty root directory
hugetlbfs	Anywhere	Provides extended page access (2 or 4 MB on X86)
bdev	None	Used for block devices
proc	/proc	Pseudofilesystem access to many kernel structures and subsystems
sockfs	None	Used by BSD Sockets
tmpfs	Anywhere	RAM disk with swapping, re-sizing
shm	None	Used by System V IPC Shared Memory
pipefs	None	Used for pipes
binfmt_misc	Anywhere	Used by various executable formats
devpts	/dev/pts	Used by Unix98 pseudo-terminals
usbfs	/proc/bus/usb	Used by USB sub-system for dynamical devices
sysfs	/sys	Used as a device tree
debugfs	/sys/kernel/debug	Used for simple debugging file access