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Amazon Elastic Compute Cloud
User Guide for Linux Instances

Instance Store Swap Volumes

Swap space in Linux can be used when a system requires more memory than it has been physically allocated. When swap space is enabled, Linux systems can swap infrequently used memory pages from physical memory to swap space (either a dedicated partition or a swap file in an existing file system) and free up that space for memory pages that require high-speed access.

Note

Using swap space for memory paging is not as fast or efficient as using RAM. If your workload is regularly paging memory into swap space, you should consider migrating to a larger instance type with more RAM. For more information, see Resizing Your Instance.

The c1.medium and m1.small instance types have a limited amount of physical memory to work with, and they are given a 900 MiB swap volume at launch time to act as virtual memory for Linux AMIs. Although the Linux kernel sees this swap space as a partition on the root device, it is actually a separate instance store volume, regardless of your root device type.

Amazon Linux AMIs automatically enable and use this swap space, but your AMI may require some additional steps to recognize and use this swap space. To see if your instance is using swap space, you can use the swapon -s command.

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[ec2-user ~]$ swapon -s Filename Type Size Used Priority /dev/xvda3 partition 917500 0 -1

The above instance has a 900 MiB swap volume attached and enabled. If you don't see a swap volume listed with this command, you may need to enable swap space for the device. Check your available disks using the lsblk command.

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[ec2-user ~]$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT xvda1 202:1 0 8G 0 disk / xvda3 202:3 0 896M 0 disk

Here, the swap volume xvda3 is available to the instance, but it is not enabled (notice that the MOUNTPOINT field is empty). You can enable the swap volume with the swapon command.

Note

You must prepend /dev/ to the device name listed by lsblk. Your device may be named differently, such as sda3, sde3, or xvde3. Use the device name for your system in the command below.

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[ec2-user ~]$ sudo swapon /dev/xvda3

Now the swap space should show up in lsblk and swapon -s output.

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[ec2-user ~]$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT xvda1 202:1 0 8G 0 disk / xvda3 202:3 0 896M 0 disk [SWAP] [ec2-user ~]$ swapon -s Filename Type Size Used Priority /dev/xvda3 partition 917500 0 -1

You also need to edit your /etc/fstab file so that this swap space is automatically enabled at every system boot.

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[ec2-user ~]$ sudo vim /etc/fstab

Append the following line to your /etc/fstab file (using the swap device name for your system):

/dev/xvda3       none    swap    sw  0       0

To use an instance store volume as swap space

Any instance store volume can be used as swap space. For example, the m3.medium instance type includes a 4 GB SSD instance store volume that is appropriate for swap space. If your instance store volume is much larger (for example, 350 GB), you may consider partitioning the volume with a smaller swap partition of 4-8 GB and the rest for a data volume.

Note

This procedure applies only to instance types that support instance storage. For a list of supported instance types, see Instance Store Volumes.

  1. List the block devices attached to your instance to get the device name for your instance store volume.

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    [ec2-user ~]$ lsblk -p NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT /dev/xvdb 202:16 0 4G 0 disk /media/ephemeral0 /dev/xvda1 202:1 0 8G 0 disk /

    In this example, the instance store volume is /dev/xdvb. Because this is an Amazon Linux instance, the instance store volume is formatted and mounted at /media/ephemeral0; not all Linux operating systems do this automatically.

  2. (Optional) If your instance store volume is mounted (it lists a MOUNTPOINT in the lsblk command output), unmount it with the following command.

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    [ec2-user ~]$ sudo umount /dev/xvdb
  3. Set up a Linux swap area on the device with the mkswap command.

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    [ec2-user ~]$ sudo mkswap /dev/xvdb mkswap: /dev/xvdb: warning: wiping old ext3 signature. Setting up swapspace version 1, size = 4188668 KiB no label, UUID=b4f63d28-67ed-46f0-b5e5-6928319e620b
  4. Enable the new swap space.

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    [ec2-user ~]$ sudo swapon /dev/xvdb
  5. Verify that the new swap space is being used.

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    [ec2-user ~]$ swapon -s Filename Type Size Used Priority /dev/xvdb partition 4188668 0 -1
  6. Edit your /etc/fstab file so that this swap space is automatically enabled at every system boot.

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    [ec2-user ~]$ sudo vim /etc/fstab

    If your /etc/fstab file has an entry for /dev/xvdb (or /dev/sdb) change it to match the line below; if it does not have an entry for this device, append the following line to your /etc/fstab file (using the swap device name for your system):

    /dev/xvdb       none    swap    sw  0       0

    Important

    Instance store volume data is lost when an instance is stopped; this includes the instance store swap space formatting created in Step 3. If you stop and restart an instance that has been configured to use instance store swap space, you must repeat Step 1 through Step 5 on the new instance store volume.