Monday, October 8, 2012

Difference between RAMfs and TMPfs

Ramfs vs Tmpfs

Primarily both ramfs and tmpfs does the same thing with few minor differences.
  • Ramfs will grow dynamically.  So, you need control the process that writes the data to make sure ramfs doesn’t go above the available RAM size in the system. Let us say you have 2GB of RAM on your system and created a 1 GB ramfs and mounted as /tmp/ram. When the total size of the /tmp/ram crosses 1GB, you can still write data to it.  System will not stop you from writing data more than 1GB. However, when it goes above total RAM size of 2GB, the system may hang, as there is no place in the RAM to keep the data.
  • Tmpfs will not grow dynamically. It would not allow you to write more than the size you’ve specified while mounting the tmpfs. So, you don’t need to worry about controlling the process that writes the data to make sure tmpfs doesn’t go above the specified limit. It may give errors similar to “No space left on device”.
  • Tmpfs uses swap.
  • Ramfs does not use swap.

4. Disadvantages of Ramfs and Tmpfs

Since both ramfs and tmpfs is writing to the system RAM, it would get deleted once the system gets rebooted, or crashed. So, you should write a process to pick up the data from ramfs/tmpfs to disk in periodic intervals. You can also write a process to write down the data from ramfs/tmpfs to disk while the system is shutting down. But, this will not help you in the time of system crash.
Table: Comparison of ramfs and tmpfs
Fill maximum space and continue writingWill display errorWill continue writing
Fixed SizeYesNo
Uses SwapYesNo
Volatile StorageYesYes

If you want your process to write faster, opting for tmpfs is a better choice with precautions about the system crash.

NFR tips

1) Memory Hogging Tip
      a) Create a tmpfs
      b) Write data to it This will consume RAM

       mount -t tmpfs tmpfs -o size=512m dd if=/dev/zero of=/test bs=1024 count=524288 

2) CPU hogging
    # This script will fill 1GB zeros to log.txt, 1KB at a time. 
    # This is done to create a load on the CPU 
     while true
     dd if=/dev/zero of=log.txt bs=1024 count=10485700 

Getting the top comand output to a file

top -n 1 -b > file