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--- vms:debian [2018/11/25 04:30]
admin [SMART]
+++ vms:debian [2019/04/09 12:24]
admin [Swap File]
@@ Line 269: / Line 269: @@
 > systemctl reload sshd
 </Code>
+===== Swap File =====
+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>
+To make swappiness persistent:
+<Code:bash>
+> nano /etc/systectl.conf
+...
+###################################################################
+# Swap usage
+#
+# vm.swappiness = 0         The kernel will swap only to avoid an out of memory condition
+# vm.swappiness = 1         Minimum amount of swapping without disabling it entirely.
+# vm.swappiness = 10        This value is sometimes recommended to improve performance when sufficient memory exists in a system.
+# vm.swappiness = 60        The default value.
+# vm.swappiness = 100       The kernel will swap aggressively.
+###################################################################
+vm.swappiness = 10
+</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 =====
@@ Line 421: / Line 505: @@
 ==== Time Sync ====
-One important aspect when setting up a new system is time synchronization, to achieve this we'll use ''ntp''. Debian 9 has time synchronization built in and activated by default using the standard ''ntpd'' time server, provided by the ''ntp'' package.
+One important aspect when setting up a new system is time synchronization, to achieve this we'll use ''ntp''. Debian 9 should have time synchronization built in and activated by default using the standard ''ntpd'' time server, provided by the ''ntp'' package.
 First lets check whether the ''ntp'' time server is running:
@@ Line 453: / Line 537: @@
 You can refer to [[https://blog.sleeplessbeastie.eu/2015/04/27/how-to-manage-system-services-on-debian-jessie/|this list of systemctl commands]] to see how to manage services.
+To configure your system's time zone use the ''dpkg-reconfigure tzdata''
 ==== SMART ====