Manage and monitor swap space on Linux

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Most of us don’t think about swap space often unless we run into a problem on our systems that suggests we don’t have enough. Even so, seeing and measuring the adequacy of swap space on a system isn’t too difficult, and knowing what’s normal for your system can help you spot errors. So let’s look at some commands that can help you examine your swap space. But first, let’s cover some basics.

What is swap space and how is it used

Swap space is hard disk space that acts like an extension of the main memory. It is used when the system’s physical memory (RAM) is full and the system needs more memory resources. It’s called “swap” because the system moves some inactive pages in memory to swap so it can hold more data in RAM. In other words, it provides a way to free up RAM on a busy system.

Programs and data use RAM because that is the only way the system can process them. In fact, when a system boots, programs like the kernel and systemd into RAM to get started.

The swap memory can be configured as a separate hard disk partition or set up as a file. Most Linux installations these days create a partition during the installation, and this is optimal. However, you can set up a swap file and use it for your swap area.

Insufficient swap space can cause a problem called “thrashing”, in which programs and data are moved between RAM and the swap area so frequently that the system runs very slowly.

RAM and swap are collectively referred to as “virtual memory”.

How much change do you need?

The recommendation for swap space used to be to double the amount of memory, but that was back when systems didn’t have as much memory as they do today. These recommendations for Ubuntu should probably work well for other distributions as well:

RAM         Swap        Swap (with hibernation)
256MB       256MB       512MB
512MB       512MB       1GB
1GB         1GB         2GB
2GB         1GB         3GB
3GB         2GB         5GB
4GB         2GB         6GB
6GB         2GB         8GB
8GB         3GB         11GB
12GB        3GB         15GB
16GB        4GB         20GB
24GB        5GB         29GB
32GB        6GB         38GB
64GB        8GB         72GB
128GB       11GB        139GB

The distinction between swap and hibernation swap is important. A system in hibernation immediately saves your system status to the hard drive and shuts down. When you wake it up (for example, by lifting the “lid” of a laptop), any programs you were running return to the state they were in when the system went to sleep. Therefore, more swap space is recommended. Not all systems hibernate.

Run this command to see if your system can be put to sleep:

$ which pm-hibernate
/usr/sbin/pm-hibernate

When you get the answer shown above, your system is ready to go to sleep. You can test it out by running this command:

$ sudo pm-hibernate

How can you view the amount of swap space on your Linux system?

You can use the … swapon –show Command to display the swap area on your system.

$ swapon --show
NAME       TYPE      SIZE USED PRIO
/dev/zram0 partition 5.8G 3.3M  100

Another useful command is the for free Command that shows both swap space and memory usage. With -m, the results are displayed in MB instead of KB.

$ free
               total        used        free      shared  buff/cache   available
Mem:         6064768      740736      538288        8060     4785744     5014712
Swap:        6064124        3328     6060796
$ free -m
               total        used        free      shared  buff/cache   available
Mem:            5922         723         525           7        4673        4897
Swap:           5921           3        5918

the sar Command can report on swap space usage.

$ sar -S 1 3
Linux 5.13.9-200.fc34.x86_64 (dragonfly)        09/10/2021      _x86_64_       (2 CPU)

02:09:55 PM kbswpfree kbswpused  %swpused  kbswpcad   %swpcad
02:09:56 PM   6060796      3328      0.05         0      0.00
02:09:57 PM   6060796      3328      0.05         0      0.00
02:09:58 PM   6060796      3328      0.05         0      0.00
Average:      6060796      3328      0.05         0      0.00

Notice in the above output of for free Command that swap space is used modestly, although there is a lot of free space available.

You can also display a swap partition with a command like this:

$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0    7:0    0  32.3M  1 loop /var/lib/snapd/snap/snapd/12704
loop1    7:1    0  55.4M  1 loop /var/lib/snapd/snap/core18/2128
loop2    7:2    0  65.4M  1 loop /var/lib/snapd/snap/powershell/173
loop3    7:3    0  32.3M  1 loop /var/lib/snapd/snap/snapd/12883
sda      8:0    0 111.8G  0 disk
├─sda1   8:1    0     1G  0 part /boot
└─sda2   8:2    0 110.8G  0 part /
sdb      8:16   0 465.8G  0 disk
└─sdb1   8:17   0   434G  0 part /home
sdc      8:32   1   1.9T  0 disk
└─sdc1   8:33   1   1.9T  0 part
sr0     11:0    1  1024M  0 rom
zram0  252:0    0   5.8G  0 disk [SWAP]              <=== there it is!

When you need and don’t need more swap space

If your system has a lot of memory, you may never need to use swap space. But it’s almost always a good idea to have it available. Storage space is relatively cheap compared to memory, and you never know when a process might add to its load. On the other hand, if your swap space is being used heavily most of the time, you might want to consider adding more RAM to the system as there is some performance penalty associated with using it.

Create swap file

If you need to create a paging file on a Linux system, use a command like this:

$ sudo dd if=/dev/zero of=/swapfile bs=1M count=8192
[sudo] password for me:
8192+0 records in
8192+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 147.893 s, 58.1 MB/s

After the file is created, change its file permissions, run the mkswap Command and use the swapon -a Command to make it available and the swapon –show Command to check if it was used.

$ sudo chmod 600 /swapfile
$ sudo mkswap /swapfile
Setting up swapspace version 1, size = 8 GiB (8589930496 bytes)
no label, UUID=3d060a1d-90d1-436f-97b6-4d1aebb15ce2
$ sudo swapon -a
$ swapon --show
NAME      TYPE SIZE USED PRIO
/swapfile file   8G   0B   -2

How to turn swapping off and on again

It is possible to switch the use of a paging file on and off and the use of the exchange and To deceive Commands, although you probably only want to turn off paging if you’ve added a swap partition and want to use that instead of the paging file.

$ sudo swapoff -v /swapfile
swapoff /swapfile
$ sudo swapon -v /swapfile
swapon: /swapfile: found signature [pagesize=4096, signature=swap]
swapon: /swapfile: pagesize=4096, swapsize=8589934592, devsize=8589934592
swapon /swapfile
$ swapon –show
NAME      TYPE SIZE USED PRIO
/swapfile file   8G   0B   -2

Wrap up

If your Linux system is running smoothly all the time, it likely doesn’t have any memory or swap issues. However, if it doesn’t, or you’re just curious about how to set up and use swap space, try some of the commands above to see what they can tell you.

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Copyright © 2021 IDG Communications, Inc.


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