Extending the Omega’s RAM with a Swap File on USB Storage

While the Omega comes with a decent 64MB of RAM, you might find yourself in a situation where this is simply not enough! This tutorial will walk you through how you can use USB Storage and a Swap File to extend the amount of memory available to your Omega.

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What is a Swap File?

A Swap File is a special file on a hard drive or flash storage that is used by the operating system to store information not currently being used by the device’s RAM. This allows the system and programs to use more memory than just by using the physically available RAM memory.

Swap Files, therefore, implement virtual memory. This is also known as paging and is used in every modern desktop and mobile operating system to manage and optimize the use of memory.

For more information, check out the Wikipedia article on Paging.

The Setup Procedure

Let’s get to setting up our very own Swap File and extending the available memory on the Omega!

Step 1: Install Required Packages

First, we will install the swap-utils package that will allow us to create and use Swap Files:

opkg update
opkg install swap-utils

Step 2: USB Storage

Plug in a USB drive and make sure it’s mounted. For best results, you will need to setup automatic mounting. See the USB Storage tutorial for details on the procedures.

For the purposes of this tutorial, let’s assume the USB device was mounted to /mnt/sda1.

Step 3: Create Swap File on the USB storage

Now we need to create the file on the USB drive that will be used as the Swap file. We will be using the dd utility to help us create the file. The dd utility is meant to convert and copy files, it is very powerful so it can potentially be very destructive if used incorrectly, stay on your toes when using dd.

The specific dd command we will be running will look like:

dd if=/dev/zero of=/mnt/sda1/swap.page bs=1M count=256

This command will create a file on the USB drive that is 256MB with all of its bytes initialized to 0x00. This is the file that will be used as the Swap File, so we will effectively be extending our RAM by 256MB.

Let’s go through each part of the command:

• The if argument specifies the source of the data, in this case /dev/zero is a special system file that outputs as many 0x00 bytes as requested.
• The of argument tells dd where to create the new file. In our scenario, the USB drive was mounted at /mnt/sda1 and we want to make a swap.page file on the top level of the drive.
• The bs and count arguments will specify the size of the file that will be created:
  • The bs argument actually stands for Block Size, with a Block being the number of bytes that are read, written, or converted at one time. We are setting our Block size to 1 Million Bytes or 1 Megabyte (MB)
  • The count argument defines how many blocks are to be created. So we are creating 256 blocks that are 1MB each, therefore the file will be 256MB in total

Step 4: Setup the Swap File

Now that the Swap File has been created, we need to tell Linux to setup this file as a swap area:

mkswap /mnt/sda1/swap.page

Running the free command will show that nothing has changed yet:

root@Omega-1302:~# free
             total         used         free       shared      buffers
Mem:         61152        42528        18624           96        11648
-/+ buffers:              30880        30272
Swap:            0            0            0

Step 5: Activate the Swap File

We’re finally ready to activate the Swap File and actually expand our available memory:

swapon /mnt/sda1/swap.page

Now, when free is run again, we will see that the Swap row is populated:

root@Omega-1302:~# free
             total         used         free       shared      buffers
Mem:         61152        42644        18508           96        11648
-/+ buffers:              30996        30156
Swap:       262140            0       262140

The units of the numbers displayed by the free command are kilobytes. The total Swap size is 262140 kilobytes which is equivalent to 256MB, so we can confirm that the Swap File is active and being used as memory.

Note: When the Omega is rebooted, the USB Swap Page will no longer be used. Step 5 will have to be repeated after every boot unless some automatic method for activating the Swap Page is created…

Going Further

So it’s a little problematic that the Swap File needs to be activated manually after every boot, lets figure out how to automate this tedious little activity.

Automatically Activating the Swap File

The /etc/rc.local will be run automatically after every boot, so this is perfect for our purposes.

Add the following to your /etc/rc.local file:

# activate the swap file on an external USB drive
SWAP_FILE="/mnt/sda1/swap.page"
if [ -e "$SWAP_FILE" ]; then
        swapon /mnt/sda1/swap.page
fi

Make sure this code is placed above the exit 0 line that already exists in the file!

This will first check that the Swap File exists, and will then activate the Swap File. Note that this depends on the USB drive being automatically mounted during boot, see the USB Storage tutorial for details on how to accomplish automatic mounting.

Try adding this code, reboot your Omega (with the USB drive still plugged in), and running free to confirm the Swap File is indeed being used.

Summary

Using a Swap File allows us to use other types of storage (USB, flash) to extend the amount of RAM available in our system. All modern desktop and mobile operating systems implement swap files in one way or another since storage is generally much cheaper than memory. This isn’t exactly equivalent to adding more RAM since memory is much, much faster than storage, but for situations where memory usage becomes an issue, this method is incredibly useful.

Happy hacking!