How to increase swap size on CentOS 8 / RHEL 8

Swap space is the second type of memory in modern Linux systems. The primary function of swap space is to substitute disk space for RAM memory when real RAM fills up and more space is needed. Linux uses swap space to increase the amount of virtual memory available to a host. It can use one or more dedicated swap partitions or a swap file on a regular filesystem or logical volume.
Now we are going to check:
•             Current swap space in the system.
•             How to identify swap volumes in the system.
•             How to increase the size of swap volume.
•             How to add one more swap device to system.

This documentation is based on the test done in a CentOS 4.18.0-80.el8.x86_64 machine. The same procedure can be applied on RHEL 8 as well. We follow the LVM method to manage the volumes here though a normal partition also can be used for swap. We will see both the options here when we add another device as a swap volume.

How to check the current swap size in the system?

We can see the current swap size either by using the command “free” or fetching the details from file “/proc/meminfo“. Some examples given below.
[root@labsrv1 ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          981Mi       759Mi        82Mi        13Mi       139Mi        76Mi
Swap:         511Mi       0.0Ki       511Mi
[root@labsrv1 ~]#

[root@labsrv1 ~]# cat /proc/meminfo |grep Swap
SwapCached:            4 kB
SwapTotal:        524284 kB
SwapFree:         524016 kB
[root@labsrv1 ~]#
The above commands output show that the system has 512MB space allocated for swap and at this point only 4KB of that is in use and rest of the size is free (SwapTotal:        524284 kB).
Sometimes we see that the swap utilization is more, and we need to allocate more space to swap device to allocate more pages. Either we extend the existing swap volume (in case of LVM) or add a new volume to swap. We will see both the options later. If we want to extend the existing swap volume size, we need to identify the device allocated for swapping

How to identify the swap volumes in the system?

Some set of commands like “swapon”, “lsblk” etc. will give us the details to identify the swap volumes. Let’s see how to identify the device from these commands. Please look at the highlighted areas in the below output.
[root@labsrv1 dev]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       524284  268     -2
[root@labsrv1 dev]#

Ok. Above command shows that our swap device is “/dev/dm-1”.
[root@labsrv1 dev]# ls -l /dev/dm-1
brw-rw---- 1 root disk 253, 1 Dec 29 22:52 /dev/dm-1
[root@labsrv1 dev]#

This command output shows that the device “dev/dm-1” has major and minor numbers as 253 and 1.
[root@labsrv1 dev]# lsblk
NAME              MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vda               252:0    0    8G  0 disk
├─vda1            252:1    0  500M  0 part /boot
└─vda2            252:2    0  7.5G  0 part
  ├─rootvg-rootlv 253:0    0    7G  0 lvm  /
  └─rootvg-swaplv 253:1    0  512M  0 lvm  [SWAP]
vdb               252:16   0    4G  0 disk
[root@labsrv1 dev]#

This command output shows that the numbers 253 and 1 belongs to the volume “swaplv” in “rootvg”. So our swap device is “/dev/rootvg/swaplv”. If you want to confirm this see the output from below command.
[root@labsrv1 dev]# ls -l /dev/rootvg/swaplv
lrwxrwxrwx 1 root root 7 Dec 29 22:52 /dev/rootvg/swaplv -> ../dm-1
[root@labsrv1 dev]#

Since the swap device to be enabled at the system reboot, the entries for swap device will be available in “/etc/fstab” file as well as below.
[root@labsrv1  ~]# cat /etc/fstab |grep swap
/dev/mapper/rootvg-swaplv swap                    swap    defaults        0 0
[root@labsrv1  ~]#

We can see that, as we figured out earlier the swap device is “swaplv” in “rootvg”.

How to increase the size of swap volume?


Now we have the details of swap volumes. Let’s see how to increase the size of it. To increase the size, you need to:
a.            Increase the size of swap device (if it is an LV).
b.            Disable the swap in that volume.
c.             Make swap FS on the device once again.
d.            Enable the swap o the device.

>> First step, to increase the swap LV size by “lvextend” command. In our lab we have swap device with 512MB size, and we have 4GB free space available in the “rootvg”. So, we are good to increase the swap LV size easily. In case you do not have enough free space in the system, you may need to extend the VG by adding an additional disk. You may need to refer the LVM guide for extending VG by adding a new disk.
For your reference I have given the output of “lvs” and “vgs” command to see the size of LV and free space in VG.
[root@labsrv1 ~]# lvs
  LV     VG     Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  rootlv rootvg -wi-ao----   7.00g
  swaplv rootvg -wi-ao---- 512.00m
[root@labsrv1 ~]#

[root@labsrv1 ~]# vgs
  VG     #PV #LV #SN Attr   VSize  VFree
  rootvg   2   2   0 wz--n- 11.50g 4.00g
[root@labsrv1 ~]#

Since we have enough free space in the system, I am going to add one more 512MB to the swap LV.
[root@labsrv1 ~]# lvextend -L +512M /dev/rootvg/swaplv
  Size of logical volume rootvg/swaplv changed from 512.00 MiB (128 extents) to 1.00 GiB (256 extents).
  Logical volume rootvg/swaplv successfully resized.
[root@labsrv1 ~]#

Now the size of swap LV increased to 1GB.
[root@labsrv1 ~]# lvs
  LV     VG     Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  rootlv rootvg -wi-ao---- 7.00g
  swaplv rootvg -wi-ao---- 1.00g
[root@labsrv1 ~]#

Still you will see the size of swap as 512MB by “free” command as we did not increase the swap size but only the swap LV size.

>> Second step, to disable the existing swap volume for which the size of LV we increased. Before disabling the swap, please make sure that the system is swapping very minimal by using that device as an important factor to be considered especially when you are running the system with a single swap device.
[root@labsrv1 ~]# swapoff -v /dev/rootvg/swaplv
swapoff /dev/rootvg/swaplv
[root@labsrv1 ~]#

[root@labsrv1 ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          981Mi       730Mi        86Mi        13Mi       164Mi        99Mi
Swap:            0B          0B          0B
[root@labsrv1 ~]#

After disabling the swap, as in the above output we can see that the swap space size reduced to 0B. This again depends on the configuration of your system. If you have more than one swap devices configured in your system and you disabled only one device then it will show the size of remaining device in instead of 0B.

>> Third step, to recreate the swap formatting in the entire LV. Earlier we had it only on first 512MB and now we need to have the same on remaining 512MB too.
[root@labsrv1 ~]# mkswap /dev/rootvg/swaplv
mkswap: /dev/rootvg/swaplv: warning: wiping old swap signature.
Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
no label, UUID=b1243731-c803-4dce-9c54-e29d0c7436aa
[root@labsrv1 ~]#

Last step, to re-enable the swapping.
[root@labsrv1 ~]# swapon -v /dev/rootvg/swaplv
swapon: /dev/mapper/rootvg-swaplv: found signature [pagesize=4096, signature=swap]
swapon: /dev/mapper/rootvg-swaplv: pagesize=4096, swapsize=1073741824, devsize=1073741824
swapon /dev/mapper/rootvg-swaplv
[root@labsrv1 ~]#

Now you will see the swap space increased as 1G as below.
[root@labsrv1 ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          981Mi       731Mi        85Mi        13Mi       164Mi        98Mi
Swap:         1.0Gi          0B       1.0Gi
[root@labsrv1 ~]#

Since the system is configured with LVM, it was quite easy for us to increase the size. Suppose we have whole disk configuration in the system, then what do we do? Yes, it is possible to add a partition as swap device. We are going to see that in the next step.

How to add one more swap device for the system?


Two ways to add:
a.            Add a whole disk  or disk partition as swap device
b.            Add a new LV as swap device.

Let’s see both the ways to add swap device. We are planning to add 1GB device with each method which are whole disk or disk partition and LV device.
Add a  whole disk  or disk partition as swap device:
Before that lets have an agreement of the current size and number of the swap device.
[root@labsrv1 ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          981Mi       730Mi        85Mi        13Mi       164Mi        98Mi
Swap:         1.0Gi          0B       1.0Gi
[root@labsrv1 ~]#

[root@labsrv1 ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       1048572 0       -2
[root@labsrv1 ~]#

Current swap space : 1GB
Number of swap devices : 1
Expected swap size after new device addition : 2GB
Expected number of swap devices : 2

I have added one 1GB disk to the system. The name of the device is “/dev/vdc” as shown below.
[root@labsrv1 ~]# fdisk -l|grep vd
Disk /dev/vda: 8 GiB, 8589934592 bytes, 16777216 sectors
/dev/vda1  *       2048  1026047  1024000  500M 83 Linux
/dev/vda2       1026048 16777215 15751168  7.5G 8e Linux LVM
Disk /dev/vdb: 4 GiB, 4294967296 bytes, 8388608 sectors
Disk /dev/vdc: 1 GiB, 1073741824 bytes, 2097152 sectors
[root@labsrv1 ~]#

•             Create swap format in /dev/vdc.
[root@labsrv1 ~]# mkswap /dev/vdc
Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
no label, UUID=ab8f7372-4473-47a7-92ca-3119179a5fee
[root@labsrv1 ~]#

•             Enable new swap device.
[root@labsrv1 ~]# swapon -v /dev/vdc
swapon: /dev/vdc: found signature [pagesize=4096, signature=swap]
swapon: /dev/vdc: pagesize=4096, swapsize=1073741824, devsize=1073741824
swapon /dev/vdc
[root@labsrv1 ~]#

Current swap size is 2GB.
[root@labsrv1 ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          981Mi       729Mi        75Mi        12Mi       176Mi       101Mi
Swap:         2.0Gi       8.0Mi       2.0Gi
[root@labsrv1 ~]#

Number of devices for swap are 2 now.
[root@labsrv1 ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       1048572 8192    -2
/dev/vdc                                partition       1048572 0       -3
[root@labsrv1 ~]#

Since we added a new device, we need to add the corresponding entries in “/etc/fstab” file to get the swap enabled at system reboot for this device too. Add the below entry in “/etc/fstab” file.
/dev/vdc swap defaults        0 0

Add a new LV as swap device:
Current swap space : 2GB
Number of swap devices : 2
Expected swap size after new device addition : 3GB
Expected number of swap devices : 3

•             Create a new LV with 1GB size. The new LV name is “swaplv2”.
[root@labsrv1 ~]# lvcreate -L 1GB -n swaplv2 rootvg
  Logical volume "swaplv2" created.
[root@labsrv1 ~]#

The new LV “swplv2” is mapping to “/dev/dm-2” as shown in the below output. We expect ”/dev/dm-2“ as the third swap device.
[root@labsrv1 ~]# ls -l /dev/rootvg/swaplv2
lrwxrwxrwx 1 root root 7 Dec 30 00:14 /dev/rootvg/swaplv2 -> ../dm-2
[root@labsrv1 ~]#

•             Create swap format on /dev/rootvg/swaplv2.
[root@labsrv1 ~]# mkswap /dev/rootvg/swaplv2
Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
no label, UUID=a75a8d74-61f8-49da-8a45-d216344487d2
[root@labsrv1 ~]#

•             Enable the swap device /dev/rootvg/swaplv2.
[root@labsrv1 ~]# swapon -v /dev/rootvg/swaplv2
swapon: /dev/mapper/rootvg-swaplv2: found signature [pagesize=4096, signature=swap]
swapon: /dev/mapper/rootvg-swaplv2: pagesize=4096, swapsize=1073741824, devsize=1073741824
swapon /dev/mapper/rootvg-swaplv2
[root@labsrv1 ~]#

Now we can see our system is running with 3GB swap space.
[root@labsrv1 ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          981Mi       731Mi        67Mi        12Mi       182Mi        99Mi
Swap:         3.0Gi       8.0Mi       3.0Gi
[root@labsrv1 ~]#

Number of swap devices became 3.
 [root@labsrv1 ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       1048572 8192    -2
/dev/vdc                                partition       1048572 0       -3
/dev/dm-2                               partition       1048572 0       -4
[root@labsrv1 ~]#

As I mentioned above. Add the below entry to “/etc/fstab” file to bring this swap device enabled during system reboot.
/dev/rootvg/swaplv2 swap defaults        0 0

That is all about swap device configuration. If you found this useful or you have any suggestions/questions, then please leave a comment below. You may get in touch with us by an email too.



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