tmpfs
The tmpfs facility allows the creation of filesystems whose contents reside in virtual memory (RAM).
Since the files on such filesystems typically reside in RAM, file access is extremely fast.
If a tmpfs filesystem is unmounted, or machine is shut down, all its contents are discarded (lost).
A tmpfs filesystem has the following properties:
• The filesystem can employ swap space when physical memory pressure demands it.
• The filesystem consumes only as much physical memory and swap space as is required to store the current contents of the filesystem. Even though, running df -h on a tmps volume will show its maximum size (by default) as 50% of the physical RAM
• During a remount operation (mount -o remount), the filesystem size can be changed (without losing the existing contents of the filesystem).