06. [30p] RAM disk

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 types of RAM disks:

  • ramfs - cannot be limited in size and will continue to grow until you run out of RAM. Its 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.

[15p] Task A - Create RAM Disk

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

The result should look like this:

Filesystem     Type      Size  Used Avail Use% Mounted on
udev           devtmpfs  1.1G     0  1.1G   0% /dev
tmpfs          tmpfs     214M  3.8M  210M   2% /run
/dev/sda1      ext4      218G  4.1G  202G   2% / <- root partition
tmpfs          tmpfs     1.1G  252K  1.1G   1% /dev/shm
tmpfs          tmpfs     5.0M  4.0K  5.0M   1% /run/lock
tmpfs          tmpfs     1.1G     0  1.1G   0% /sys/fs/cgroup
/dev/sda2      ext4      923M   73M  787M   9% /boot
/dev/sda4      ext4      266G   62M  253G   1% /home

From the results, we will assume in the following commands that the file system is ext4. If it's not your case, just replace with what you have:

$ 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.

[15p] Task B - Pipe View & RAM Disk

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?

ep/labs/03/contents/tasks/ex6.txt · Last modified: 2020/08/03 16:41 by cristian.marin0805
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