Chapter 13. Getting started with swap

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.

13.2. Recommended system swap space
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The appropriate size of a swap partition ultimately depends on the system memory workload, not system memory alone. However, the following table provides the recommended minimum guidelines for swap partition size based on the amount of RAM in your system, which are typically set automatically during installation.

The swap partition size is set automatically during installation. To allow for hibernation, however, you need to edit the swap space in the custom partitioning stage.

The following guidelines are especially important on systems with low memory, such as 1 GB or less. Failure to allocate sufficient swap space on these systems can cause issues, such as instability or even render the installed system unbootable.

Expand

Table 13.1. Recommended swap space
Amount of RAM in the system	Recommended swap space	Recommended swap space if allowing for hibernation
Less than 2 GiB	2 times the amount of RAM	3 times the amount of RAM
2 GiB – 8 GiB	Equal to the amount of RAM	2 times the amount of RAM
8 GiB – 64 GiB	At least 4 GiB	1.5 times the amount of RAM
More than 64 GiB	Workload dependent (at least 4GiB)	Hibernation not recommended

Distributing swap space over multiple storage devices improves swap space performance, particularly on systems with fast drives, controllers, and interfaces.

Important

File systems and LVM2 volumes assigned as swap space should not be in use when being modified. Any attempts to modify swap fail if a system process or the kernel is using swap space. Use the free and cat /proc/swaps commands to verify how much and where swap is in use.

Resizing swap space requires temporarily removing it from the system, which is done by disabling the swap volume of the file by using the swapoff command. This can be problematic if running applications rely on the additional swap space and might run into low-memory situations.

If you encounter difficulties disabling swap due to high memory pressure or locked pages, alternatively, perform swap resizing from rescue mode. See Debug boot options. When prompted to mount the file system, select Skip.

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

Create the LVM2 logical volume of size 4 GB:
```
lvcreate -n LogVol02 -L 4G VolGroup00
```
```
# lvcreate -n LogVol02 -L 4G VolGroup00
```
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Format the new swap space:
```
mkswap /dev/VolGroup00/LogVol02
```
```
# mkswap /dev/VolGroup00/LogVol02
```
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Add the following entry to the /etc/fstab file:
```
/dev/VolGroup00/LogVol02 none swap defaults 0 0
```
```
/dev/VolGroup00/LogVol02 none swap defaults 0 0
```
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Regenerate mount units so that your system registers the new configuration:
```
systemctl daemon-reload
```
```
# systemctl daemon-reload
```
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Activate swap on the logical volume:
```
swapon -v /dev/VolGroup00/LogVol02
```
```
# 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

# 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

# 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 and managing a standard swap partition
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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

Start the partitioning tool for your disk, for example, /dev/sda:
```
fdisk /dev/sda
```
```
$ fdisk /dev/sda
```
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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.
Write the changes and exit the utility.
Format the new swap partition (assuming the new partition is /dev/sda4):
```
mkswap /dev/sda4
```
```
# mkswap /dev/sda4
```
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Add an entry to the /etc/fstab file to enable the swap partition at boot time:
```
/dev/sda4 none swap defaults 0 0
```
```
# /dev/sda4 none swap defaults 0 0
```
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Regenerate mount units to register the new configuration:
```
systemctl daemon-reload
```
```
# systemctl daemon-reload
```
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Activate the swap partition immediately:
```
swapon -v /dev/sda4
```
```
# swapon -v /dev/sda4
```
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Verification

To confirm the swap partition was created and activated, inspect the active swap space:
```
cat /proc/swaps
free -h
```
```
# cat /proc/swaps
# free -h
```
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13.5. 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

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.
Create an empty file:
```
dd if=/dev/zero of=/swapfile bs=1024 count=65536
```
```
# 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.
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.
Set up the swap file with the command:
```
mkswap /swapfile
```
```
# mkswap /swapfile
```
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Change the security of the swap file so it is not world readable.
```
chmod 0600 /swapfile
```
```
# chmod 0600 /swapfile
```
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Edit the /etc/fstab file with the following entries to enable the swap file at boot time:
```
/swapfile none swap defaults 0 0
```
```
/swapfile none swap defaults 0 0
```
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The next time the system boots, it activates the new swap file.
Regenerate mount units so that your system registers the new /etc/fstab configuration:
```
systemctl daemon-reload
```
```
# systemctl daemon-reload
```
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Activate the swap file immediately:
```
swapon /swapfile
```
```
# 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
```
```
$ cat /proc/swaps
```
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```
free -h
```
```
$ free -h
```
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13.6. 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

You have prepared the control node and the managed nodes.
You are logged in to the control node as a user who can run playbooks on the managed nodes.
The account you use to connect to the managed nodes has sudo permissions for these nodes.

Procedure

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

---
- 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.

Validate the playbook syntax:
```
ansible-playbook --syntax-check ~/playbook.yml
```
```
$ 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.
Run the playbook:
```
ansible-playbook ~/playbook.yml
```
```
$ ansible-playbook ~/playbook.yml
```
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13.7. 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/LogVol02 is the volume you want to extend by 2 GB.

Prerequisites

You have enough disk space.

Procedure

Disable swapping for the associated logical volume:
```
swapoff -v /dev/VolGroup00/LogVol02
```
```
# swapoff -v /dev/VolGroup00/LogVol02
```
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Resize the LVM2 logical volume by 2 GB:
```
lvresize -L +2G /dev/VolGroup00/LogVol02
```
```
# lvresize -L +2G /dev/VolGroup00/LogVol02
```
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Format the new swap space:
```
mkswap /dev/VolGroup00/LogVol02
```
```
# mkswap /dev/VolGroup00/LogVol02
```
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Enable the extended logical volume:
```
swapon -v /dev/VolGroup00/LogVol02
```
```
# 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

# 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

# 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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13.8. 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

Disable swapping for the associated logical volume:
```
swapoff -v /dev/VolGroup00/LogVol02
```
```
# swapoff -v /dev/VolGroup00/LogVol02
```
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Clean the swap signature:
```
wipefs -a /dev/VolGroup00/LogVol02
```
```
# wipefs -a /dev/VolGroup00/LogVol02
```
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Reduce the LVM2 logical volume by 512 MB:
```
lvreduce -L -512M /dev/VolGroup00/LogVol02
```
```
# lvreduce -L -512M /dev/VolGroup00/LogVol02
```
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Format the new swap space:
```
mkswap /dev/VolGroup00/LogVol02
```
```
# mkswap /dev/VolGroup00/LogVol02
```
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Activate swap on the logical volume:
```
swapon -v /dev/VolGroup00/LogVol02
```
```
# 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
```
```
$ cat /proc/swaps
```
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```
free -h
```
```
$ free -h
```
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13.9. 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

Disable swapping for the associated logical volume:
```
swapoff -v /dev/VolGroup00/LogVol02
```
```
# swapoff -v /dev/VolGroup00/LogVol02
```
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Remove the LVM2 logical volume:
```
lvremove /dev/VolGroup00/LogVol02
```
```
# lvremove /dev/VolGroup00/LogVol02
```
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Remove the following associated entry from the /etc/fstab file:
```
/dev/VolGroup00/LogVol02 none swap defaults 0 0
```
```
/dev/VolGroup00/LogVol02 none swap defaults 0 0
```
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Regenerate mount units to register the new configuration:
```
systemctl daemon-reload
```
```
# 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
```
```
$ cat /proc/swaps
```
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```
free -h
```
```
$ free -h
```
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13.10. Removing a swap file
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You can remove a swap file.

Procedure

Disable the /swapfile swap file:
```
swapoff -v /swapfile
```
```
# swapoff -v /swapfile
```
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Remove its entry from the /etc/fstab file accordingly.
Regenerate mount units so that your system registers the new configuration:
```
systemctl daemon-reload
```
```
# systemctl daemon-reload
```
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Remove the actual file:
```
rm /swapfile
```
```
# rm /swapfile
```
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13.11. 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

View the current value:
```
cat /proc/sys/vm/swappiness
```
```
$ cat /proc/sys/vm/swappiness
```
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The default value is 60.
Set the value temporarily, for example, to 10:
```
sysctl vm.swappiness=10
```
```
# sysctl vm.swappiness=10
```
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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
```
```
vm.swappiness = 10
```
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Apply the changes:
```
sysctl -p
```
```
# sysctl -p
```
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13.1. Overview of swap space
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13.2. Recommended system swap space
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13.3. Creating an LVM2 logical volume for swap
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13.4. Creating and managing a standard swap partition
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13.5. Creating a swap file
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13.6. Creating a swap volume by using the storage RHEL system role
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13.7. Extending swap on an LVM2 logical volume
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13.8. Reducing swap on an LVM2 logical volume
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13.9. Removing an LVM2 logical volume for swap
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13.10. Removing a swap file
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13.11. Adjusting Virtual Memory (VM) Tunables
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Chapter 13. Getting started with swap

13.1. Overview of swap spaceCopy linkLink copied to clipboard!

13.2. Recommended system swap spaceCopy linkLink copied to clipboard!

13.3. Creating an LVM2 logical volume for swapCopy linkLink copied to clipboard!

13.4. Creating and managing a standard swap partitionCopy linkLink copied to clipboard!

13.5. Creating a swap fileCopy linkLink copied to clipboard!

13.6. Creating a swap volume by using the storage RHEL system roleCopy linkLink copied to clipboard!

13.7. Extending swap on an LVM2 logical volumeCopy linkLink copied to clipboard!

13.8. Reducing swap on an LVM2 logical volumeCopy linkLink copied to clipboard!

13.9. Removing an LVM2 logical volume for swapCopy linkLink copied to clipboard!

13.10. Removing a swap fileCopy linkLink copied to clipboard!

13.11. Adjusting Virtual Memory (VM) TunablesCopy linkLink copied to clipboard!

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13.1. Overview of swap space
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13.2. Recommended system swap space
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13.3. Creating an LVM2 logical volume for swap
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13.4. Creating and managing a standard swap partition
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13.5. Creating a swap file
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13.6. Creating a swap volume by using the storage RHEL system role
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13.7. Extending swap on an LVM2 logical volume
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13.8. Reducing swap on an LVM2 logical volume
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13.9. Removing an LVM2 logical volume for swap
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13.10. Removing a swap file
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13.11. Adjusting Virtual Memory (VM) Tunables
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