Difference Between a tmpfs and ramfs RAM Disk

There are two file system types built into most modern Linux distributions which allow you to create a RAM based storage area which can be mounted and used link a normal folder.

What is a memory based file system (RAM disk)?

A memory based file system is something which creates a storage area directly in a computers RAM as if it were a partition on a disk drive. As RAM is a volatile type of memory which means when the system is restarted or crashes the file system is lost along with all it’s data.

The major benefit to memory based file systems is that they are very fast – 10s of times faster than modern SSDs. Read and write performance is massively increased for all workload types. These types of fast storage areas are ideally suited for applications which need repetitively small data areas for caching or using as temporary space. As the data is lost when the machine reboots the data must not be precious as even scheduling backups cannot guarantee that all the data will be replicated in the even of a system crash.

tmpfs vs. ramfs

The two main RAM based file system types in Linux are tmpfs and ramfs. ramfs is the older file system type and is largely replaced in most scenarios by tmpfs.

ramfs

ramfs creates an in memory file system which uses the same mechanism and storage space as Linux file system cache. Running the command free in Linux will show you the amount of RAM you have on your system, including the amount of file system cache in use. The below is an example of a 31GB of ram in a production server.

free -g

       total used free shared buffers cached

Mem:   31    29   2    0      0       8

-/+ buffers/cache: 20 11

Swap:  13    6    7

Currently 8GB of file system cache is in use on the system. This memory is generally used by Linux to cache recently accessed files so that the next time they are requested then can be fetched from RAM very quickly. ramfs uses this same memory and exactly the same mechanism which causes Linux to cache files with the exception that it is not removed when the memory used exceeds threshold set by the system.

ramfs file systems cannot be limited in size like a disk base file system which is limited by it’s capacity. ramfs will continue using memory storage until the system runs out of RAM and likely crashes or becomes unresponsive. This is a problem if the application writing to the file system cannot be limited in total size. Another issue is you cannot see the size of the file system in df and it can only be estimated by looking at the cached entry in free.

tmpfs vs. ramfs

The two main RAM based file system types in Linux are tmpfs and ramfs. ramfs is the older file system type and is largely replaced in most scenarios by tmpfs.

ramfs

ramfs creates an in memory file system which uses the same mechanism and storage space as Linux file system cache. Running the command free in Linux will show you the amount of RAM you have on your system, including the amount of file system cache in use. The below is an example of a 31GB of ram in a production server.

free -g

       total used free shared buffers cached

Mem:   31    29   2    0      0       8

-/+ buffers/cache: 20 11

Swap:  13    6    7

Currently 8GB of file system cache is in use on the system. This memory is generally used by Linux to cache recently accessed files so that the next time they are requested then can be fetched from RAM very quickly. ramfs uses this same memory and exactly the same mechanism which causes Linux to cache files with the exception that it is not removed when the memory used exceeds threshold set by the system.

ramfs file systems cannot be limited in size like a disk base file system which is limited by it’s capacity. ramfs will continue using memory storage until the system runs out of RAM and likely crashes or becomes unresponsive. This is a problem if the application writing to the file system cannot be limited in total size. Another issue is you cannot see the size of the file system in df and it can only be estimated by looking at the cached entry in free.

See my other post for details on how to create a RAM disk in Linux.