Swap file isn't re-created after a Linux VM restarts

Article
05/23/2024

Applies to: ✔️ Linux VMs

Original KB number: 4577868

Note

CentOS referenced in this article is a Linux distribution and will reach End Of Life (EOL). Consider your use and plan accordingly. For more information, see CentOS End Of Life guidance.

This article provides a resolution to an issue in which the swap file can't be re-created after a restart of a Linux virtual machine.

Symptoms

On Linux virtual machines (VMs) that are provisioned by cloud-init and that have the Microsoft Azure Linux Agent (waagent) installed, you may discover that the swap file isn't re-created after a restart.

Cause

This problem occurs because of a misconfiguration that causes both the Azure Linux Agent and cloud-init to try to configure the swap file. When cloud-init is responsible for provisioning, the swap file must be configured by cloud-init to enable only one agent (either cloud-init or waagent) for provisioning. This issue can be intermittent because of the timing of when the waagent daemons start.

Resolution

To resolve this problem, follow these steps:

Disable resource disk formatting and swap configuration within waagent configuration /etc/waagent.conf because this task is now handled by Cloud-Init. Set the parameters as follows:

# Format if unformatted. If 'n', resource disk will not be mounted.
ResourceDisk.Format=n

# Create and use swapfile on resource disk.
ResourceDisk.EnableSwap=n

#Mount point for the resource disk
ResourceDisk.MountPoint=/mnt

#Size of the swapfile.
ResourceDisk.SwapSizeMB=0

Restart the Azure Linux Agent. See How to update the Azure Linux Agent on a VM for information about the restart commands for different Linux distributions.
Make sure that the VM is configured to create a swap file by using cloud-init:
1. Create a script /var/lib/cloud/scripts/per-boot/create_swapfile.sh with the following commands:
```
#!/bin/sh
if [ ! -f '/mnt/swapfile' ]; then
fallocate --length 2GiB /mnt/swapfile
chmod 600 /mnt/swapfile
mkswap /mnt/swapfile
swapon /mnt/swapfile
swapon -a 
else
swapon /mnt/swapfile; fi
```
  In some cases, the fallocate command won't create a swap file properly. If a swap file isn't created properly, you can use the alternate script below:
```
dd if=/dev/zero of=/mnt/swapfile bs=1M count=2048
```
2. Make the file executable by using the # chmod +x create_swapfile.sh command.
3. Stop and Start the VM or Redeploy it from the portal, and check for swap enablement. Here is an example of how to enable the swap capability:
```
root@ub1804-ephemeral:/var/lib/cloud/scripts/per-boot# free -m 
total used free shared buff/cache available 
Mem: 7953 296 7384 0 272 7412 
Swap: 2047 0 2047
```

To isolate the issue, compare the logs from /var/log/waagent.log and /var/log/cloud-init.log for the reboot timeframe.

Recommended fix

To avoid this situation completely, deploy the VM by using the swap configuration custom data during provisioning.

Use cloud-init to configure a swap partition on a Linux VM

This article shows you how to use cloud-init to configure the swap partition on various Linux distributions. The swap partition was traditionally configured by the Linux Agent (WALA) based on which distributions required one. This document will outline the process for building the swap partition on demand during provisioning time using cloud-init. For more information about how cloud-init works natively in Azure and the supported Linux distros, see cloud-init overview

Create swap partition for Ubuntu based images

By default on Azure, Ubuntu gallery images do not create swap partitions. To enable swap partition configuration during VM provisioning time using cloud-init - please see the AzureSwapPartitions document on the Ubuntu wiki.

Create swap partition for Red Hat and CentOS based images

Create a file in your current shell named cloud_init_swappart.txt and paste the following configuration. For this example, create the file in the Cloud Shell not on your local machine. You can use any editor you wish. Enter sensible-editor cloud_init_swappart.txt to create the file and see a list of available editors. Choose #1 to use the nano editor. Make sure that the whole cloud-init file is copied correctly, especially the first line.

#cloud-config
disk_setup:
  ephemeral0:
    table_type: gpt
    layout: [66, [33,82]]
    overwrite: true
fs_setup:
  - device: ephemeral0.1
    filesystem: ext4
  - device: ephemeral0.2
    filesystem: swap
mounts:
  - ["ephemeral0.1", "/mnt"]
  - ["ephemeral0.2", "none", "swap", "sw,nofail,x-systemd.requires=cloud-init.service,x-systemd.device-timeout=2", "0", "0"]

The mount is created with the nofail option to ensure that the boot will continue even if the mount isn't completed successfully.

Before deploying this image, you need to create a resource group with the az group create command. An Azure resource group is a logical container into which Azure resources are deployed and managed. The following example creates a resource group named myResourceGroup in the eastus location.

az group create --name myResourceGroup --location eastus

Now, create a VM with az vm create and specify the cloud-init file with --custom-data cloud_init_swappart.txt as follows:

az vm create \
  --resource-group myResourceGroup \
  --name centos74 \
  --image OpenLogic:CentOS:7-CI:latest \
  --custom-data cloud_init_swappart.txt \
  --generate-ssh-keys

Verify swap partition was created

SSH to the public IP address of your VM shown in the output from the preceding command. Enter your own publicIpAddress as follows:

ssh <publicIpAddress>

Once you have SSH'ed into the vm, check if the swap partition was created

swapon -s

The output from this command should look like this:

Filename                Type        Size    Used    Priority
/dev/sdb2  partition   2494440 0   -1

Note

If you have an existing Azure image that has a swap partition configured and you want to change the swap partition configuration for new images, you should remove the existing swap partition. For more information, see Customize Images to provision by cloud-init.

Next steps

For additional cloud-init examples of configuration changes, see the following:

Contact us for help

If you have questions or need help, create a support request, or ask Azure community support. You can also submit product feedback to Azure feedback community.

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