On a system with memory-intense workload with common peak loads, a large swap memory can be useful to store large memory contents not needed at the moment. While using swap instead of memory will certainly have great impact on performance, sometimes this is preferable over adding more memory to the machine, as disk space is much cheaper. Sometimes there is simply no more memory, maybe a physical machine that is out of free slots, and there isn't any larger memory modules on the market. At other times the slower performance on peak loads may be preferable over the application crashing with out of memory error.
In some cases swap memory needs to be increased, a live example could be expanding the memory in the machine, and so also expanding the swap space to match the double of the new memory size.In this tutorial you will learn:
- How to check swap space.
- How to identify swap volume.
- How to extend swap volume.
- How to add another swap volume.
Software Requirements and Conventions Used
|Category||Requirements, Conventions or Software Version Used|
|System||RHEL 8 / CentOS 8|
|Other||Privileged access to your Linux system as root or via the
# - requires given linux commands to be executed with root privileges either directly as a root user or by use of
How to increase swap size on RHEL 8 step by step instructions
In our lab setup, the swap partition is on a logical volume. This is the most common setup, as LVM is a very flexible way to handle disk space. The LVM tutorial covers the architecture and usage of the Logical Volume Manager, so we will not go into details when using
lvextend to increase the size of the swap volume. We'll increase the initial 1GB swap to 2GB.
We need to find out which volume is our swap partition:
# grep swap /etc/fstab /dev/mapper/rhel_rhel8lab-swap swap swap defaults 0 0
As by default volume names are generated with short hostname, and while creating a logical volume we can explicitly name it, the volume name will vary.
In our example we take note of the volume name with the path, which is
/dev/rhel_rhel8lab/swap. These two names for the partition may be confusing at first glance, but if we list both, we can see they are actually symlinks pointing to the same device:
# ls -al /dev/rhel_rhel8lab/swap lrwxrwxrwx. 1 root root 7 Nov 25 18:35 /dev/rhel_rhel8lab/swap -> ../dm-1 # ls -al /dev/mapper/rhel_rhel8lab-swap lrwxrwxrwx. 1 root root 7 Nov 25 18:35 /dev/mapper/rhel_rhel8lab-swap -> ../dm-1
We'll use these interchangeably in the next steps.
We need to disable swapping on the volume while we work on it. We use the full volume name that we found in the fist step.
# swapoff -v /dev/mapper/rhel_rhel8lab-swap swapoff /dev/mapper/rhel_rhel8lab-swap
Do not disable swap on a production server while it is under heavy load, and is using the swap excessively! In such a scenario add another swap partition instead.
To increase the size of the swap we extend the logical volume holding it. In this example with one more Gigabyte:
# lvextend -L +1G /dev/rhel_rhel8lab/swap Size of logical volume rhel_rhel8lab/swap changed from 1.00 GiB (256 extents) to 2.00 GiB (512 extents). Logical volume rhel_rhel8lab/swap successfully resized.
Note that you need to ensure you have sufficient free space in the volume group holding the logical volume, as you would on any other volume extension that contains filesystems.
We re-create the swap on the extended volume:
# mkswap /dev/mapper/rhel_rhel8lab-swap mkswap: /dev/mapper/rhel_rhel8lab-swap: warning: wiping old swap signature. Setting up swapspace version 1, size = 2 GiB (2147479552 bytes) no label, UUID=defca15e-a5ed-4fe8-bddd-5f11a3c76e80
We turn swapping back on on the extended swap volume:
# swapon -v /dev/mapper/rhel_rhel8lab-swap swapon: /dev/mapper/rhel_rhel8lab-swap: found signature [pagesize=4096, signature=swap] swapon: /dev/mapper/rhel_rhel8lab-swap: pagesize=4096, swapsize=2147483648, devsize=2147483648 swapon /dev/mapper/rhel_rhel8lab-swap
It is a good practice to always verify that the modification of the system is successful. In this case
free -m(memory information with values in Megabytes) should show that we have 2GB of swap:
# free -m total used free shared buff/cache available Mem: 1989 1201 225 17 562 617 Swap: 2047 0 2047
How to Add new swap partition on RHEL 8 step by step instructions
When disabling the swap to increase it's size is not appropriate, we can increase overall swap by adding another volume that is a swap partition as well. When we are finished we simply give it to the system, which will start using it as needed.
We create a new logical volume with
LVMas we would if we'd like a new volume that stores a filesystem:
# lvcreate -L 1G -n swap2 rhel_rhel8lab Logical volume "swap2" created.
On the same lab machine this will result with a full path name of the new partition as
/dev/mapper/rhel_rhel8lab-swap2, the same as
Create the swap with
# mkswap /dev/rhel_rhel8lab/swap2 Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes) no label, UUID=a319fb8d-18b8-42b7-b6bf-cafb27aaec2b
Turn on swap on the new volume:
# swapon /dev/rhel_rhel8lab/swap2
And verify the results:
# free -m total used free shared buff/cache available Mem: 1989 1198 153 14 637 623 Swap: 2047 0 2047
For the operating system to be able to use the new swap partition after reboot, we need to add it to
# grep swap /etc/fstab /dev/mapper/rhel_rhel8lab-swap swap swap defaults 0 0 /dev/mapper/rhel_rhel8lab-swap2 swap swap defaults 0 0
The difference between the two methods can be outlined by checking the contents of
/proc/swaps, where we can see that by adding another volume instead of extending the one we already have will result in two devices used for swapping:
# cat /proc/swaps Filename Type Size Used Priority /dev/dm-1 partition 1048572 0 -2 /dev/dm-2 partition 1048572 0 -3