RHEL 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.
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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.
Adding swap space
The following are the different ways to add a swap space:
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For example, you may upgrade the amount of RAM in your system from 1 GB to 2 GB, but there is only 2 GB of swap space. It might be advantageous to increase the amount of swap space to 4 GB if you perform memory-intense operations or run applications that require a large amount of memory.
Removing swap space
The following are the different ways to remove a swap space:
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For example, you have downgraded the amount of RAM in your system from 1 GB to 512 MB, but there is 2 GB of swap space still assigned. It might be advantageous to reduce the amount of swap space to 1 GB, since the larger 2 GB could be wasting disk space.
16.2. Recommended system swap space
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The recommended size of a swap partition depends on the amount of RAM in your system and whether you want sufficient memory for your system to hibernate. The recommended 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 recommendations 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.
| Table 16.1. Recommended swap space | ||
|---|---|---|
| Amount of RAM in the system | Recommended swap space | Recommended swap space if allowing for hibernation |
| ⩽ 2 GB | 2 times the amount of RAM | 3 times the amount of RAM |
| > 2 GB – 8 GB | Equal to the amount of RAM | 2 times the amount of RAM |
| > 8 GB – 64 GB | At least 4 GB | 1.5 times the amount of RAM |
| > 64 GB | At least 4 GB | Hibernation not recommended |
For border values such as 2 GB, 8 GB, or 64 GB of system RAM, choose swap size based on your needs or preference. If your system resources allow for it, increasing the swap space can lead to better performance.
Note that distributing swap space over multiple storage devices also 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. This can be problematic if running applications rely on the additional swap space and might run into low-memory situations. Preferably, perform swap resizing from rescue mode, see Debug boot options. When prompted to mount the file system, select Skip.
16.3. 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 2 GB:
# lvcreate VolGroup00 -n LogVol02 -L 2G
Format the new swap space:
# mkswap /dev/VolGroup00/LogVol02
Add the following entry to the /etc/fstab file:
/dev/VolGroup00/LogVol02 none swap defaults 0 0
Regenerate mount units so that your system registers the new configuration:
# systemctl daemon-reload
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
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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
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.
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.
Set up the swap file with the command:
# mkswap /swapfile
Change the security of the swap file so it is not world readable.
# chmod 0600 /swapfile
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.
Regenerate mount units so that your system registers the new /etc/fstab configuration:
# systemctl daemon-reload
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
16.5. 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 sufficient disk space.
Procedure
Disable swapping for the associated logical volume:
# swapoff -v /dev/VolGroup00/LogVol01
Resize the LVM2 logical volume by 2 GB:
# lvresize /dev/VolGroup00/LogVol01 -L +2G
Format the new swap space:
# mkswap /dev/VolGroup00/LogVol01
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
16.6. 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
Disable swapping for the associated logical volume:
# swapoff -v /dev/VolGroup00/LogVol01
Clean the swap signature:
# wipefs -a /dev/VolGroup00/LogVol01
Reduce the LVM2 logical volume by 512 MB:
# lvreduce /dev/VolGroup00/LogVol01 -L -512M
Format the new swap space:
# mkswap /dev/VolGroup00/LogVol01
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
16.7. 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
Remove the LVM2 logical volume:
# lvremove /dev/VolGroup00/LogVol02
Remove the following associated entry from the /etc/fstab file:
/dev/VolGroup00/LogVol02 none swap defaults 0 0
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
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You can remove a swap file.
Procedure
Disable the /swapfile swap file:
# swapoff -v /swapfile
Remove its entry from the /etc/fstab file accordingly.
Regenerate mount units so that your system registers the new configuration:
# systemctl daemon-reload
Remove the actual file:
# rm /swapfile