Linux – Why don’t EC2 ubuntu images have swap

amazon ec2linuxswapUbuntu

I started a couple servers on EC2 and they don't have swap.

Am I doing something wrong or is it that the machines just don't have any?

Best Answer

You are right, the Ubuntu EC2 EBS images don't come with swap space configured (for 11.04 at least). The "regular" instance-type images do have a swap partition, albeit only 896 MB on the one I tested.

If some process blows up and you don't have swap space, your server could come to a crawling halt for a good while before the OOM killer kicks in, whereas with swap, it merely gets slow. For that reason, I always like to have swap space around, even with enough RAM. Here's your options:

  • Create an EBS volume (2-4 times the size of your RAM), attach it to your instance (I like calling it /dev/xvdm for "memory"), sudo mkswap /dev/xvdm, add it to fstab, sudo swapon -a, and you're good to go. I have done this before and it works fine, but it is probably a bit slower than instance store because it goes over the network.

  • Or you might be able to repartition your disk to add a swap partition, though this might require creating a new AMI. I have not been able to do this in a running instance, because I cannot unmount the root file system, and I do not even have access to the disk device (/dev/xvda), only the partition (xvda1).

  • Or you can create a swap file. This is my preferred solution right now.

    sudo dd if=/dev/zero of=/var/swapfile bs=1M count=2048 &&
sudo chmod 600 /var/swapfile &&
sudo mkswap /var/swapfile &&
echo /var/swapfile none swap defaults 0 0 | sudo tee -a /etc/fstab &&
sudo swapon -a

    Done. :) I know a lot of people feel icky about using files instead of partitions, but it certainly works well enough as emergency swap space.

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