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ep:labs:02:contents:tasks:ex2 [2019/10/09 06:50] radu.mantu |
ep:labs:02:contents:tasks:ex2 [2025/02/11 23:22] (current) cezar.craciunoiu [02. [10p] Feedback] |
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| - | ==== 02. [20p] Swap space ==== | + | ==== [10p] Feedback ==== |
| - | + | ||
| - | <note warning> | + | |
| - | Before starting this task, call the assistant to show him your progress. If you manage to freeze your PC, it might prove tricky to do so afterwards. | + | |
| - | </note> | + | |
| - | + | ||
| - | === [10p] Task A - Swap File === | + | |
| - | + | ||
| - | First, let us check what swap devices we have enabled. Check the //NAME// and //SIZE// columns of the following command: | + | |
| - | <code> | + | |
| - | $ swapon --show | + | |
| - | </code> | + | |
| - | + | ||
| - | If you ever installed a Linux distro, you may remember creating a separate //swap partition//. This, however, is only one method of creating swap space. The other is by adding a //swap file//. Run the following commands: | + | |
| - | <code> | + | |
| - | $ sudo swapoff -a | + | |
| - | $ sudo dd if=/dev/zero of=/swapfile bs=1024 count=$((4 * 1024 * 1024)) | + | |
| - | $ sudo chmod 600 /swapfile | + | |
| - | $ sudo mkswap /swapfile | + | |
| - | $ sudo swapon /swapfile | + | |
| - | + | ||
| - | $ swapon --show | + | |
| - | </code> | + | |
| - | + | ||
| - | Just to clarify what we did: | + | |
| - | * disabled all swap devices | + | |
| - | * created a 4Gb zero-initialized file | + | |
| - | * set the permission to the file so only //root// can edit it | + | |
| - | * created a swap area from the file using **mkswap** (works on devices too) | + | |
| - | * activated the swap area | + | |
| - | + | ||
| - | The new swap area is temporary and will not survive a reboot. To make it permanent, we need to register it in [[https://en.wikipedia.org/wiki/Fstab|/etc/fstab]] by adding a line such as this: | + | |
| - | + | ||
| - | <code> | + | |
| - | /swapfile swap swap defaults 0 0 | + | |
| - | </code> | + | |
| - | + | ||
| - | Now that we created a //swap file//, what are the advantages / disadvantages when compared to a //swap partition//? | + | |
| - | * advantages: | + | |
| - | * easier to manage | + | |
| - | * similar performance | + | |
| - | * disadvantage: | + | |
| - | * can be affected by disk fragmentation (not the case for a partition) | + | |
| - | + | ||
| - | === [10p] Task B - Does it work? === | + | |
| - | + | ||
| - | In one terminal run **vmstat** and look at the //swpd// and //free// columns. | + | |
| - | <code> | + | |
| - | $ vmstat -w 1 | + | |
| - | </code> | + | |
| - | + | ||
| - | In another terminal, open a python shell and allocate a bit more memory than the available RAM. Identify the moment when the newly created swap space is being used. | + | |
| - | + | ||
| - | One thing you might notice is that the value in **vmstat**'s "free" column is lower than before. This does not mean that you have less available RAM after creating the swap file. Remember using the **dd** command to create a 4GB file? A big chunk of RAM was used to buffer the data that was written to disk. If "free" drops to unacceptable levels, the kernel will make sure to reclaim some of this buffer/cache memory. To get a clear view of how much available memory you actually have, try running the following command: | + | |
| - | + | ||
| - | <code bash> | + | |
| - | $ free -h | + | |
| - | </code> | + | |
| - | + | ||
| - | Observe that once you close the python shell and the memory is freed, //swpd// still displays a non-zero value. Why? There simply isn't a reason to clear the data from the swap area. If you really want to clean up the used swap space, try the following: | + | |
| - | <code> | + | |
| - | $ vmstat | + | |
| - | $ sudo swapoff -a && swapon -a | + | |
| - | $ vmstat | + | |
| - | </code> | + | |
| - | + | ||
| - | <solution -hidden> | + | |
| - | Output here: | + | |
| - | + | ||
| - | {{ :ep:labs:ep2017_l2_ex05.png?550 |}} | + | |
| - | </solution> | + | |
| + | Please take a minute to fill in the **[[https://forms.gle/NpSRnoEh9NLYowFr5 | feedback form]]** for this lab. | ||
