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--- vms:debian [2019/01/17 05:00]
admin [Login using an rsa key]
+++ vms:debian [2019/01/17 05:44]
admin [Swap File]
@@ Line 273: / Line 273: @@
 As stated above, using a swap partition isn't always the best option when using a VM since it might become necessary to resize the swap space when the VM's memory configuration is changed based on its utilization. When a swap partition is used, it becomes necessary to "play" with the partitions sizes, which means changing the partitions scheme and oftentimes even the virtual disk's size, which is tedious...
+Therefore, in the case of VMs, it is often much easier to create the swap space using a system file, you can learn more about this in [[https://www.digitalocean.com/community/tutorials/how-to-configure-virtual-memory-swap-file-on-a-vps|this Digital Ocean's article]]. Here are the steps to create and use a swap file.
+First to make sure swap is not already activated on the system use the ''free'' command:
+<Code:bash>
+> free
+              total        used        free      shared  buff/cache   available
+Mem:        2058304       37680     1759884        2968      260740     1872108
+Swap:             0           0           0
+</Code>
+The ''free : 0'' value on the ''Swap:'' line means swap is indeed **not activated**
+We'll create the partition file under ''/var'' and name it ''swap.img'', then change its permissions to ''600'' so no user will be able to access it:
+<Code:bash>
+> touch /var/swap.img
+> chmod 600 /var/swap.img
+</Code>
+=== Sizing ===
+Deciding on the swap size is really case dependent. In general, it is recommended recommend to set it to 1-2x the available system RAM. So, if you have a 512mb RAM VM, use 512mb-1gb swap. If you have a 1gb RAM VM use 1gb-2gb swap, etc. This is not a hard and fast rule, for example if you have a 4gb RAM VM it may be best to use little (512mb) or no swap at all.\\
+We use the ''dd'' command to stretch our swap file size, filling it with zeroes to the size we need (here 2Gb):
+<Code:bash>
+> dd if=/dev/zero of=/var/swap.img bs=1024k count=2048
++0 records in
++0 records out
+2147483648 bytes (2.1 GB, 2.0 GiB) copied, 1.57258 s, 1.4 GB/s
+</Code>
+Next, we prepare the file to be usable as the swap file:
+<Code:bash>
+> mkswap /var/swap.img
+Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
+no label, UUID=f8d9bdfe-4090-4bc6-8f7a-fc74b64946ad
+</Code>
+Then we turn on swapping:
+<Code:bash>
+> swapon /var/swap.img
+> free
+              total        used        free      shared  buff/cache   available
+Mem:        2058304       38784       67972        2968     1951548     1847628
+Swap:       2097148           0     2097148
+</Code>
+We now see that the swap is active. We could turn off swapping with the ''swapoff /var/swap.img'' command.\\
+Turning the swap on this way will **not activate swapping on the next boot**, so we need to modify ''/etc/fstab'' to have the swap file activated at boot time:
+<Code>
+> nano /etc/fstab
+ADD THOSE LINES
+# swap file
+/var/swap.img    none    swap    sw    0    0
+</Code>
+Finally, one could define the system's //swappiness//, which tells the Linux kernel/VM handler how likely it should be to use VM. It is a percent value, between 0 & 100. A usual recommendation for VMs is 30:
+<Code:bash>
+> sysctl -w vm.swappiness=30
+vm.swappiness = 30
+</Code>
+That's about it !\\
+If you want to know what your current system's swappiness is, use ''sysctl -a | grep swappiness''.
 ===== Shell Customization & Utilities =====