Swap is a type of filesystem and is a virtual memory. Whenever your RAM is full, your operating system will look for further memory in your swap space. For this reason, you reserve some part of the hard disk to create a swap partition.
Identifying Current Swap Space Usage
[[email protected] ~]# cat /proc/swaps Filename      Type      Size      Used     Priority /dev/sda7   partition   1951740    4         -1
Alternatively, use the swapon command:
[[email protected] ~]# swapon -s Filename       Type      Size      Used    Priority /dev/sda7     partition   1951740     4       -1
Finally, the free command may also be used:
[[email protected] ~]# free         total     used    free   shared    buffers   cached Mem:  895112   721656   173456    0     36592    310156 -/+ buffers/cache:    374908   520204 Swap:  1952736   4   1952732
Adding a Swap File
Additional swap may be quickly added to the system by creating a file and assigning it as swap. This is achieved as follows. The following dd command example creates a swap file with the name swap with a size of 1Gb.
Create the swap file using the dd command:
[[email protected] ~]# dd if=/dev/zero of=/swap bs=1M count=1024 1024+0 records in 1024+0 records out 1073741824 bytes (1.1 GB) copied, 17.4283 s, 61.6 MB/s
Configure the file as swap
Change the permission of the swap file so that only root can access it
[[email protected] ~]# chmod 600 /root/swap [[email protected] ~]# mkswap /swap Setting up swapspace version 1, size = 1048572 KiB Enable the newly created swapfile : [[email protected] ~]# swapon /swap
Finally, modify the /etc/fstab file to automatically add the new swap at system boot time by adding the following line:
# cat /etc/fstab /swap none swap sw 0 0
Once the swap space has been activated, verify that it is in use using the swapon –s command:
[[email protected] ~]# swapon -s Filename Type Size Used Priority /dev/sda7 partition 1951740 142884 -1 /swap file 1048572 0 -2
[[email protected] ~]# free -k       total     used     free    shared    buffers    cached Mem:    895112   828484   66628    0     2144     539552 -/+ buffers/cache:   286788   608324 Swap:    3000312  142876   2857436
If you don’t want to reboot to verify whether the system takes all the swap space mentioned in the /etc/fstab, use following to enable or disable swap.
[[email protected] ~]# swapoff -a
[[email protected] ~]# swapon -a
De-activate the additional swap space at any time using the swapoff command as follows:
[[email protected] ~]# swapoff /newswap
