Linux allows you to use part of your RAM as a block device, viewing it as a hard disk partition. The advantage of using a RAM disk is the extremely low latency (even when compared to SSDs). The disadvantage is that all contents will be lost after a reboot. There are two main type of RAM disks:

• ramfs - cannot be limited in size and will continue to grow until you run out of RAM. It's size can not be determined precisely with tools like df. Instead, you have to estimate it by looking at the “cached” entry from free's output.
• tmpfs - newer than ramfs. Can set a size limit. Behaves exactly like a hard disk partition but can't be monitored through conventional means (i.e. iostat). Size can be precisely estimated using df.

Before getting started, let's find out the file system that our root partition uses. Run the following command (T = print file system type, h = human readable):

$ df -Th

We will assume that the file system is ext4. If it's not, just use what you have in what follows:

$ sudo mkdir /mnt/ramdisk
$ sudo mount -t tmpfs -o size=1G ext4 /mnt/ramdisk

If you want the RAM disk to persist after a reboot, you can add the following line to /etc/fstab. Remember that its contents will still be lost.

tmpfs     /mnt/ramdisk     tmpfs     rw,nodev,nosuid,size=1G     0  0

That's it. We just created a 1Gb tmpfs ramdisk with an ext4 file system and mounted it at /mnt/ramdisk. Use df again to check this yourself.

As we mentioned before, you can't get I/O statistics regarding tmpfs since it is not a real partition. One solution to this problem is using pv to monitor the progress of data transfer through a pipe. This is a valid approach only if we consider the disk I/O being the bottleneck.

Next, we will generate 512Mb of random data and place it in /mnt/ramdisk/file first and then in /home/student/file. The transfer is done using dd with 2048-byte blocks.

$ pv /dev/urandom | dd of=/mnt/ramdisk/rand  bs=2048 count=$((512 * 1024 * 1024 / 2048))
$ pv /dev/urandom | dd of=/home/student/rand bs=2048 count=$((512 * 1024 * 1024 / 2048))

Look at the elapsed time and average transfer speed. What conclusion can you draw?