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ep:labs:03:contents:tasks:ex6 [2019/10/13 17:40]
radu.mantu created
ep:labs:03:contents:tasks:ex6 [2020/08/03 16:41] (current)
cristian.marin0805
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-==== 05. [10pFeedback ​====+==== 06. [30pRAM disk ====
  
-  * Please take minute ​to fill in the **[[https://docs.google.com/forms/d/e/1FAIpQLSfsMBl2EFu10jJG2qHEiSsR-qYr3wkzQPfDwjhChKnjRtDT_w/viewform | feedback form]]** for this lab.+Linux allows you to use part of your RAM as 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.  
 +<note tip> 
 +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**. 
 +</​note>​ 
 +=== [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): 
 + 
 +<code bash> 
 +$ df -Th 
 +</code> 
 +The result should look like this: 
 +<​code>​ 
 +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 
 +</​code>​ 
 + 
 +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: 
 + 
 +<code bash> 
 +$ sudo mkdir /mnt/ramdisk 
 +$ sudo mount -t tmpfs -o size=1G ext4 /mnt/​ramdisk 
 +</​code>​ 
 + 
 +<​note>​ 
 +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. 
 + 
 +<​code>​ 
 +tmpfs     /​mnt/​ramdisk ​    ​tmpfs ​    ​rw,​nodev,​nosuid,​size=1G ​    ​0 ​ 0 
 +</​code>​ 
 +</​note>​ 
 + 
 +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. 
 + 
 +=== [15pTask 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. 
 + 
 +<code bash> 
 +$ 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)) 
 +</​code>​ 
 + 
 +Look at the elapsed time and average transfer speedWhat conclusion can you draw?
ep/labs/03/contents/tasks/ex6.1570977636.txt.gz · Last modified: 2019/10/13 17:40 by radu.mantu
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