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Chapter 15. Getting started with swap

Use the swap space to provide temporary storage for inactive processes and data, and prevent out-of-memory errors when physical memory is full. The swap space acts as an extension to the physical memory and allows the system to continue running smoothly even when physical memory is exhausted. Note that using swap space can slow down system performance, so optimizing the use of physical memory, before relying on swap space, can be more favorable.

15.1. Overview of swap space
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Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM.

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.

15.3. 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 2 GB: 

    # lvcreate VolGroup00 -n LogVol02 -L 2G

  2. Format the new swap space: 

    # mkswap /dev/VolGroup00/LogVol02

  3. Add the following entry to the /etc/fstab file: 

    /dev/VolGroup00/LogVol02 none swap defaults 0 0

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

    # systemctl daemon-reload

  5. Activate swap on the logical volume: 

    # swapon -v /dev/VolGroup00/LogVol02

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
               total        used        free      shared  buff/cache   available
Mem:            30Gi       1.2Gi        28Gi        12Mi       994Mi        28Gi
Swap:           22Gi          0B        22Gi
     
    # free -h
               total        used        free      shared  buff/cache   available
Mem:            30Gi       1.2Gi        28Gi        12Mi       995Mi        28Gi
Swap:           17Gi          0B        17Gi

Use a standard swap partition for swap space. You can use standard partitioning tools like fdisk or parted to create a dedicated partition.

Prerequisites

  • You have enough unallocated disk space on a drive.
  • You have the name of the disk device, for example, /dev/sda).

Procedure

  1. Start the partitioning tool for your disk, for example, /dev/sda: 

    $ fdisk /dev/sda

  2. Create a new partition of the desired size and set its type to Linux swap:

    Note

    For fdisk, use the 't' command to change the partition type, entering the hex code 82 for Linux swap.

  3. Write the changes and exit the utility.

  4. Format the new swap partition (assuming the new partition is /dev/sda4): 

    # mkswap /dev/sda4

  5. Add an entry to the /etc/fstab file to enable the swap partition at boot time: 

    # /dev/sda4 none swap defaults 0 0

  6. Regenerate mount units to register the new configuration: 

    # systemctl daemon-reload

  7. Activate the swap partition immediately: 

    # swapon -v /dev/sda4

Verification

  • To confirm the swap partition was created and activated, inspect the active swap space: 

    # cat /proc/swaps
# free -h

15.5. 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 block size of a 64 MB swap file is 65536.

  2. Create an empty file: 

    # dd if=/dev/zero of=/swapfile bs=1024 count=65536

    Replace 65536 with the value equal to the required block size.

    Note

    On modern filesystems such as ext4 or XFS, fallocate is preferable instead of dd. This method is faster and avoids unnecessary disk I/O.

  3. Set up the swap file with the command: 

    # mkswap /swapfile

  4. Change the security of the swap file so it is not world readable. 

    # chmod 0600 /swapfile

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

    /swapfile none swap defaults 0 0

    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

  7. Activate the swap file immediately: 

    # swapon /swapfile

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
     
    $ free -h

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

    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.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml

    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

15.7. 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/LogVol01 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/LogVol01

  2. Resize the LVM2 logical volume by 2 GB: 

    # lvresize /dev/VolGroup00/LogVol01 -L +2G

  3. Format the new swap space: 

    # mkswap /dev/VolGroup00/LogVol01

  4. Enable the extended logical volume: 

    # swapon -v /dev/VolGroup00/LogVol01

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-1          partition    16322556           0              -2
/dev/dm-4          partition     7340028           0              -3
     
    # free -h
               total        used        free      shared  buff/cache   available
Mem:            30Gi       1.2Gi        28Gi        12Mi       994Mi        28Gi
Swap:           22Gi          0B        22Gi

15.8. Reducing swap on an LVM2 logical volume
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You can reduce swap on an LVM2 logical volume. Assuming /dev/VolGroup00/LogVol01 is the volume you want to reduce.

Procedure

  1. Disable swapping for the associated logical volume: 

    # swapoff -v /dev/VolGroup00/LogVol01

  2. Clean the swap signature: 

    # wipefs -a /dev/VolGroup00/LogVol01

  3. Reduce the LVM2 logical volume by 512 MB: 

    # lvreduce /dev/VolGroup00/LogVol01 -L -512M

  4. Format the new swap space: 

    # mkswap /dev/VolGroup00/LogVol01

  5. Activate swap on the logical volume: 

    # swapon -v /dev/VolGroup00/LogVol01

Verification

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

    $ cat /proc/swaps
     
    $ free -h

15.9. 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

  2. Remove the LVM2 logical volume: 

    # lvremove /dev/VolGroup00/LogVol02

  3. Remove the following associated entry from the /etc/fstab file: 

    /dev/VolGroup00/LogVol02 none swap defaults 0 0

  4. Regenerate mount units to register the new configuration: 

    # systemctl daemon-reload

Verification

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

    $ cat /proc/swaps
     
    $ free -h

15.10. Removing a swap file
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You can remove a swap file.

Procedure

  1. Disable the /swapfile swap file: 

    # swapoff -v /swapfile
  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

  4. Remove the actual file: 

    # rm /swapfile

15.11. Adjusting Virtual Memory (VM) Tunables
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Virtual Memory (VM) tunables are kernel parameters that control how the system manages memory, including how aggressively it uses swap space. Adjusting these settings can significantly impact system performance based on the workload.

The vm.swappiness parameter controls the kernel’s tendency to move processes out of physical memory and into swap space. The value is a percentage, ranging from 0 to 100:

  • Higher Value (e.g., 60 or 100): The kernel will swap processes to disk more aggressively. This can free up physical memory for other uses (like the file system cache), but it may increase I/O operations and latency if swapped processes are frequently needed.
  • Lower Value (e.g., 0 or 10): The kernel will try to avoid swapping and keep processes in physical memory for as long as possible. This is generally preferred for desktop systems or servers with fast I/O and large amounts of RAM.

Following other relevant VM tunables can influence swap behavior:

  • vm.dirty_ratio: The percentage of system memory that can be filled with "dirty" pages (memory pages that have been modified but not yet written to disk) before the kernel begins actively writing them out.
  • vm.dirty_background_ratio: The percentage of system memory that can be filled with dirty pages before a background kernel process begins writing them out. This is a less aggressive action than vm.dirty_ratio.

Procedure

  1. View the current value: 

    $ cat /proc/sys/vm/swappiness

    The default value is 60.

  2. Set the value temporarily, for example, to 10: 

    # sysctl vm.swappiness=10

  3. Set the value permanently to persist across reboots by editing the /etc/sysctl.d/99-sysctl.conf or a similar configuration file: 

    vm.swappiness = 10

  4. Apply the changes: 

    # sysctl -p
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