How to enable a swap file on a router

In this page I describe how to enable a swap file on a router (Asus RT-AC56U) - we use an Entware installation, so please refer to this page on how to install Entware on a router.
The swap file is used to increase the available physical memory (RAM) with a virtual memory (SWAP) in order to have more resources and to let services have more available memory to be executed. A swap file is created on an external USB drive. We choose two different location where we can store and manage this swap file. One location is inside "/opt" folder (this is the root folder of an Entware installation), the other one is inside the root folder of the mounted USB external drive (the root folder location is stored inside the settings of the router). To create the swap file we use scripts. We give a look to the first script that creates the swap file in "/opt" folder and enable it:

#!/bin/sh

DIR_DL="/opt/swap"

# look for file existence

if [ ! -e "$DIR_DL" ]; then
# create a 256MB swap file ("count" is in Kilobytes)
dd if=/dev/zero of=$DIR_DL bs=1k count=262144

# set up the swap file
mkswap $DIR_DL
fi

# enable swap
swapon $DIR_DL

Inside the above-mentioned script, the file "swap" (the filename of the swap file), together with path "/opt", is put inside variable "DIR_DL". We check if this file exists, if it does not exist we create an empty file with 256 MB size. The "mkswap" instruction is used to make the file a swap file, in this way the operative system knows that this file is to be used as swap file.
The last instruction "swapon" just enables the swap file.
An alternate script to create the swap file is the following one:

#!/bin/sh

SWAP_FOLDER=`nvram get apps_mounted_path`
DIR_DL="$SWAP_FOLDER/swap"

# look for file existence

if [ ! -e "$DIR_DL" ]; then
# create a 256MB swap file ("count" is in Kilobytes)
dd if=/dev/zero of=$DIR_DL bs=1k count=262144

# set up the swap file
mkswap $DIR_DL
fi

# enable swap
swapon $DIR_DL

The above-mentioned script is similar to the first one, but in this case we read the settings of the router using "nvram" instruction in order to achieve the root folder of the mounted USB external drive. This folder name usually starts with "/tmp/mnt" as this folder is the root folder of all mounted USB external drive. The rest of the script is the same as the first one.
After creating the script to create and enable the swap file, we have to execute it whenever we insert the external USB drive, please read the section "How to install Entware on a router" on how to execute a script whenever an external USB drive is mounted.
We need also to create another script to disable the swap file whenever we remove the external USB drive. In this case we just use the instruction "swapoff" to disable the swap file.
The first script file uses the "/opt" folder to disable the swap file:

#!/bin/sh

DIR_DL="/opt/swap"

# disable swap
swapoff $DIR_DL

The second script file reads the router settings in order to know where is the swap file:

#!/bin/sh

SWAP_FOLDER=`nvram get apps_mounted_path`
DIR_DL="$SWAP_FOLDER/swap"

# disable swap
swapoff $DIR_DL

There is an command, inside the router, to know how much swap memory is used and free, this command is named "free", this is a sample output of this command:

free

         total    used     free    shared    buffers
  Mem:  255892   54432   201460         0        168
 Swap:  262140       0   262140
Total:  518032   54432   463600

We can also use the command "cat /proc/swaps" to know all informations about the swap file, this is a sample output of this command:

cat /proc/swaps

Filename    Type  Size    Used    Priority
/opt/swap   file  262140  59256   -1