Your First Bite of Raspberry Pi

To Swap or Not to Swap

SD cards are getting better every year when it comes to wear leveling and write cycles. The concern is always that excessive use of the SD card will wear it out, and it will become read-only. Because the RPi also suffers from limited and non-expandable RAM, the issue of swap space is highly debated.

Raspbian comes installed by default with a swap file active—not a swap partition, but a swap file. Whether this swap file will wear out your SD card is still up for debate. I tend to be paranoid about such things, so I searched for an alternative. Because I have the 512MB units, I have a little more RAM to work with, so I decided to use zRam to compress some of the system RAM and use it as swap space. How does that work? Basically, zRam allows the system to use RAM as swap space, but in a compressed mode that allows it to appear as more RAM+swap than the system actually contains.

zRam already is compiled into the Raspbian kernel, so if you want to try my method, it just takes a little scripting. I absolutely love the script I found at the eXtremeSHOK.com Web site. That site is off-line at the time of this writing, so I've included the script below. Create a file called /etc/init.d/zram, which contains the following:

#!/bin/bash
### BEGIN INIT INFO
#Provides: zram
#Required-Start:
#Required-Stop:
#Default-Start: 2 3 4 5
#Default-Stop: 0 1 6
#Short-Description: Increased Performance In Linux With zRam 
#(Virtual Swap Compressed in RAM)
#Description: Adapted for Raspbian (Rasberry pi) by eXtremeSHOK.com 
#using https://raw.github.com/gionn/etc/master/init.d/zram
### END INIT INFO

start() {
    mem_total_kb=$(grep MemTotal /proc/meminfo | grep -E --only-matching '[[:digit:]]+')

    modprobe zram

    sleep 1
    #only using 50% of system memory, comment the line 
    #below to use 100% of system memory
    mem_total_kb=$((mem_total_kb/2))

    echo $((mem_total_kb * 1024)) > /sys/block/zram0/disksize


    mkswap /dev/zram0

    swapon -p 100 /dev/zram0
}

stop() {
    swapoff /dev/zram0
    sleep 1
    rmmod zram
}

case "$1" in
    start)
        start
        ;;
    stop)
        stop
        ;;
    restart)
        stop
        sleep 3
        start
        ;;
    *)
        echo "Usage: $0 {start|stop|restart}"
        RETVAL=1
esac

Then you need to make the init script executable and force it to start automatically. The following commands accomplish that:

sudo chmod +x /etc/init.d/zram
sudo update-rc.d zram defaults

Basically, as written, the script will use half the available RAM as compressed swap space. Again, all credit goes to the folks at eXtremeSHOK.com for creating the init script. I've been very happy with the performance, and it enabled me to get rid of the swap file altogether.

What's Next?

Hopefully this intro to the Raspberry Pi will make the device a little less intimidating for folks like me who were hesitant. With its ability to run XBMC, function as a complete desktop system and yet barely sip electricity, the RPi is a great device for countless projects. And at $35, it's absolutely dirt cheap.

We expect to see many, many Raspberry Pi projects around the Linux Journal offices. Hopefully you'll see a few around your place as well. If you have any particularly cool projects or case mods, send a photo to ljeditor@linuxjournal.com. We'd love to see what you're all up to.