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Chapter 13. Getting started with swap

Swap space provides temporary storage for inactive processes and data when physical memory is full, helping prevent out‑of‑memory errors. It acts as extra memory, allowing the system to keep running. However, swap can slow performance, so optimize physical memory use first.

13.1. Overview of swap space
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Swap space in Linux is used when physical memory (RAM) is exhausted. When more memory is required and RAM is full, inactive memory pages are moved to swap. Swap helps systems with limited RAM but is no substitute for adding more actual memory.

Swap space is located on hard drives, which have a slower access time than physical memory. Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files.

In years past, the recommended amount of swap space increased linearly with the amount of RAM in the system. However, modern systems often include hundreds of gigabytes of RAM. As a consequence, recommended swap space is considered a function of system memory workload, not system memory.

13.3. Creating an LVM2 logical volume for swap
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You can create an LVM2 logical volume for swap. Assuming /dev/VolGroup00/LogVol02 is the swap volume you want to add.

Prerequisites

  • You have enough disk space.

Procedure

  1. Create the LVM2 logical volume of size 4 GB: 

    # lvcreate -n LogVol02 -L 4G VolGroup00
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  2. Format the new swap space: 

    # mkswap /dev/VolGroup00/LogVol02
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  3. Add the following entry to the /etc/fstab file: 

    /dev/VolGroup00/LogVol02 none swap defaults 0 0
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  4. Regenerate mount units so that your system registers the new configuration: 

    # systemctl daemon-reload
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  5. Activate swap on the logical volume: 

    # swapon -v /dev/VolGroup00/LogVol02
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Verification

  • To test if the swap logical volume was successfully created and activated, inspect active swap space by using the following command: 

    # cat /proc/swaps
Filename               Type        Size         Used        Priority
/dev/dm-6         partition     4194300            0              -2
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    # free -h
               total        used        free      shared  buff/cache   available
Mem:            30Gi       1.2Gi        28Gi        12Mi       995Mi        28Gi
Swap:          4.0Gi          0B       4.0Gi
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13.4. Creating a swap file
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Set up a swap file to provide virtual memory that prevents system crashes when physical RAM becomes full. This solution is particularly useful when you need additional swap space without repartitioning disks or modifying existing storage configurations.

Prerequisites

  • You have enough disk space.

Procedure

  1. Determine the size of the new swap file in megabytes and multiply by 1024 to determine the number of blocks. For example, the number of 1024-byte sized blocks required for a 64 MB file is 65536.

  2. Create an empty file: 

    # dd if=/dev/zero of=/swapfile bs=1024 count=65536
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    Replace 65536 with the value equal to the required block size.

  3. Set up the swap file with the command: 

    # mkswap /swapfile
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  4. Change the security of the swap file so it is not world readable. 

    # chmod 0600 /swapfile
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  5. Edit the /etc/fstab file with the following entries to enable the swap file at boot time: 

    /swapfile none swap defaults 0 0
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    The next time the system boots, it activates the new swap file.

  6. Regenerate mount units so that your system registers the new /etc/fstab configuration: 

    # systemctl daemon-reload
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  7. Activate the swap file immediately: 

    # swapon /swapfile
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Verification

  • To test if the new swap file was successfully created and activated, inspect active swap space by using the following command: 

    $ cat /proc/swaps
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    $ free -h
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13.5. Creating a swap volume by using the storage RHEL system role
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You can use the storage RHEL system role to create or modify swap volumes on block devices.

Prerequisites

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: Create a disk device with swap
  hosts: managed-node-01.example.com
  roles:
    - rhel-system-roles.storage
  vars:
    storage_volumes:
      - name: swap_fs
        type: disk
        disks:
          - /dev/sdb
        size: 15 GiB
        fs_type: swap
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    The volume name (swap_fs in the example) is currently arbitrary. The storage role identifies the volume by the disk device listed under the disks: attribute.

    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.storage/README.md file on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
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    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
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13.6. Extending swap on an LVM2 logical volume
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You can extend swap space on an existing LVM2 logical volume. Assuming /dev/VolGroup00/LogVol02 is the volume you want to extend by 2 GB.

Prerequisites

  • You have enough disk space.

Procedure

  1. Disable swapping for the associated logical volume: 

    # swapoff -v /dev/VolGroup00/LogVol02
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  2. Resize the LVM2 logical volume by 2 GB: 

    # lvresize -L +2G /dev/VolGroup00/LogVol02
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  3. Format the new swap space: 

    # mkswap /dev/VolGroup00/LogVol02
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  4. Enable the extended logical volume: 

    # swapon -v /dev/VolGroup00/LogVol02
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Verification

  • To test if the swap logical volume was successfully extended and activated, inspect active swap space: 

    # cat /proc/swaps
Filename               Type        Size         Used        Priority
/dev/dm-6         partition     6291452            0              -2
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    # free -h
               total        used        free      shared  buff/cache   available
Mem:            30Gi       1.2Gi        28Gi        12Mi       994Mi        28Gi
Swap:          6.0Gi          0B       6.0Gi
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13.7. Reducing swap on an LVM2 logical volume
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You can reduce swap on an LVM2 logical volume. Assuming /dev/VolGroup00/LogVol02 is the volume you want to reduce.

Procedure

  1. Disable swapping for the associated logical volume: 

    # swapoff -v /dev/VolGroup00/LogVol02
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  2. Clean the swap signature: 

    # wipefs -a /dev/VolGroup00/LogVol02
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  3. Reduce the LVM2 logical volume by 512 MB: 

    # lvreduce -L -512M /dev/VolGroup00/LogVol02
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  4. Format the new swap space: 

    # mkswap /dev/VolGroup00/LogVol02
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  5. Activate swap on the logical volume: 

    # swapon -v /dev/VolGroup00/LogVol02
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Verification

  • To test if the swap logical volume was successfully reduced, inspect active swap space by using the following command: 

    $ cat /proc/swaps
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    $ free -h
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13.8. Removing an LVM2 logical volume for swap
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You can remove an LVM2 logical volume for swap. Assuming /dev/VolGroup00/LogVol02 is the swap volume you want to remove.

Procedure

  1. Disable swapping for the associated logical volume: 

    # swapoff -v /dev/VolGroup00/LogVol02
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  2. Remove the LVM2 logical volume: 

    # lvremove /dev/VolGroup00/LogVol02
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  3. Remove the following associated entry from the /etc/fstab file: 

    /dev/VolGroup00/LogVol02 none swap defaults 0 0
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  4. Regenerate mount units to register the new configuration: 

    # systemctl daemon-reload
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Verification

  • Test if the logical volume was successfully removed, inspect active swap space by using the following command: 

    $ cat /proc/swaps
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    $ free -h
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13.9. Removing a swap file
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You can remove a swap file.

Procedure

  1. Disable the /swapfile swap file: 

    # swapoff -v /swapfile
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  2. Remove its entry from the /etc/fstab file accordingly.

  3. Regenerate mount units so that your system registers the new configuration: 

    # systemctl daemon-reload
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  4. Remove the actual file: 

    # rm /swapfile
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