Red Hat Training

A Red Hat training course is available for RHEL 8

Chapter 14. 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.

14.1. Overview of swap space

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.

Adding swap space

The following are the different ways to add a swap space:

Removing swap space

The following are the different ways to remove a swap space:

14.3. Creating an LVM2 logical volume for swap

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

14.4. Creating a swap file

You can create a swap file to create a temporary storage space on a solid-state drive or hard disk when the system runs low on memory.

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.

  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

14.5. Extending swap on an LVM2 logical volume

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 sufficient 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

14.6. Reducing swap on an LVM2 logical volume

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

14.7. Removing an LVM2 logical volume for swap

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

14.8. Removing a swap file

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