03. [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.
- 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
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?
Put one screenshot with the tmpfs partition in df output and one screenshot of both pv commands and write your conclusion.
