UNIX / Linux: 2 Ways to Add Swap Space Using dd, mkswap and swapon
Question: I would like to add more swap space to my Linux system. Can you explain with clear examples on how to increase the swap space?
Answer: You can either use a dedicated hard drive partition to add new swap space, or create a swap file on an existing filesystem and use it as swap space.
How much swap space is currently used by the system?
Free command displays the swap space. free -k shows the output in KB.
# free -k
total used free shared buffers cached
Mem: 3082356 2043700 1038656 0 50976 1646268
-/+ buffers/cache: 346456 2735900
Swap: 4192956 0 4192956
Swapon command with option -s, displays the current swap space in KB.
# swapon -s
Filename Type Size Used Priority
/dev/sda2 partition 4192956 0 -1
Swapon -s, is same as the following.
# cat /proc/swaps
Filename Type Size Used Priority
/dev/sda2 partition 4192956 0 -1
Method 1: Use a Hard Drive Partition for Additional Swap Space
If you have an additional hard disk, (or space available in an existing disk), create a partition using fdisk command. Let us assume that this partition is called /dev/sdc1
Now setup this newly created partition as swap area using the mkswap command as shown below.
# mkswap /dev/sdc1
Enable the swap partition for usage using swapon command as shown below.
# swapon /dev/sdc1
To make this swap space partition available even after the reboot, add the following line to the /etc/fstab file.
# cat /etc/fstab
/dev/sdc1 swap swap defaults 0 0
Verify whether the newly created swap area is available for your use.
# swapon -s
Filename Type Size Used Priority
/dev/sda2 partition 4192956 0 -1
/dev/sdc1 partition 1048568 0 -2 # free -k
total used free shared buffers cached
Mem: 3082356 3022364 59992 0 52056 2646472
-/+ buffers/cache: 323836 2758520
Swap: 5241524 0 5241524
Note: In the output of swapon -s command, the Type column will say “partition” if the swap space is created from a disk partition.
Method 2: Use a File for Additional Swap Space
If you don’t have any additional disks, you can create a file somewhere on your filesystem, and use that file for swap space.
The following dd command example creates a swap file with the name “myswapfile” under /root directory with a size of 1024MB (1GB).
# dd if=/dev/zero of=/root/myswapfile bs=1M count=1024
1024+0 records in
1024+0 records out # ls -l /root/myswapfile
-rw-r--r-- 1 root root 1073741824 Aug 14 23:47 /root/myswapfile
Change the permission of the swap file so that only root can access it.
# chmod 600 /root/myswapfile
Make this file as a swap file using mkswap command.
# mkswap /root/myswapfile
Setting up swapspace version 1, size = 1073737 kB
Enable the newly created swapfile.
# swapon /root/myswapfile
To make this swap file available as a swap area even after the reboot, add the following line to the /etc/fstab file.
# cat /etc/fstab
/root/myswapfile swap swap defaults 0 0
Verify whether the newly created swap area is available for your use.
# swapon -s
Filename Type Size Used Priority
/dev/sda2 partition 4192956 0 -1
/root/myswapfile file 1048568 0 -2 # free -k
total used free shared buffers cached
Mem: 3082356 3022364 59992 0 52056 2646472
-/+ buffers/cache: 323836 2758520
Swap: 5241524 0 5241524
Note: In the output of swapon -s command, the Type column will say “file” if the swap space is created from a swap file.
If you don’t want to reboot to verify whether the system takes all the swap space mentioned in the /etc/fstab, you can do the following, which will disable and enable all the swap partition mentioned in the /etc/fstab
# swapoff -a # swapon -a