Is a [[virtual filesystem]] that is stored in memory; it uses both RAM and swap space.
> Kernel option: CONFIG_TMPFS

`mount -t tmpfs source target`

A tmpfs can only grow to 50% of RAM avail. unless overridden with size=nbytes mount option. 
Only consumes RAM for actual usage.
Data is ephemeral/volatile.

Mount stacking: mount over existing /tmp, to improve speed of apps using alot of tmp files, then unmount to delete data.

Kernel uses tmpfs for [[system v shared memory]] and [[shared anonymous memory mappings]].
`/dev/shm` or `/run/shm`: [[glibc]] of [[POSIX]] shared memory and [[POSIX]] semaphores.