How to Add Swap Space

How To Add Swap Space to a Linux System

There are two ways to go about adding swap space to a linux system.

1. Use of a hard disk partition
2. Create a swap file on an existing file system

To view current swap utilization, a user can use the commands free or swapon. Swap utilization may also be viewed within the /proc file system:

[root@linux01 ~]# free -m
             total       used       free     shared    buffers     cached
Mem:         48297      34139      14157          0        556      25966
-/+ buffers/cache:       7616      40681
Swap:        10236         49      10187

[root@linux01 ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/sda3                               partition       10482404        50364   -1

[root@linux01 ~]# cat /proc/swaps 
Filename                                Type            Size    Used    Priority
/dev/sda3                               partition       10482404        50364   -1

Clearly, we see swap -s and /proc/swaps provides the same output

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Method 1

Disk partition

Use of a hard disk partition is very simple to setup. If a spare hard disk is available, or there’s unpartitioned space available on an existing hard disk, use fdisk to create a partition. In my case, I had neither immediately available to me, but what I did have was plenty of EMC SAN, so I created a LUN, presented it to the server, and created a partition via fdisk just as if it were a regular disk. While creating the partition, don’t forget to set the partition type to Linux swap ==Hex code 82== via the ==t== option in fdisk:

t change a partition's system id

In the case of VMware, a virtual disk can be added. In this case, or in the case of adding a new physical disk to a server, here’s how to add the disk to the system without rebooting, minus the need to create the ext3 filesystem of course.

Here is what mine looked like after creating the partition via fdisk:

[root@linux01 ~]# fdisk -l /dev/emcpoweraj

Disk /dev/emcpoweraj: 32.2 GB, 32212254720 bytes
64 heads, 32 sectors/track, 30720 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes

          Device Boot      Start         End      Blocks   Id  System
/dev/emcpoweraj1               1       30720    31457216   82  Linux swap

Next, use the mkswap command to set up a Linux swap area, and swapon to enable the device for swapping

[root@linux01 ~]# mkswap /dev/emcpoweraj1
[root@linux01 ~]# swapon /dev/emcpoweraj1

In order to make this swap partition available across reboots, don’t forget to add it to the /etc/fstab file.

!!! note “fstab entry”

## xtra swap via EMC LUN
/dev/emcpoweraj1        swap    swap    defaults        0 0

Verify the new swap space is available to the system:

[root@linux01 ~]# cat /proc/swaps 
Filename                                Type            Size    Used    Priority
/dev/sda3                               partition       10482404        50360   -1
/dev/emcpoweraj1                        partition       31457208        0       -2

[root@linux01 ~]# free -m
             total       used       free     shared    buffers     cached
Mem:         48297      32580      15717          0        562      26390
-/+ buffers/cache:       5626      42671
Swap:        40956         49      40907

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Method 2

Free disk space

If you don’t have additional, physical hardware or non-partitioned space available, but have plenty of free disk space within an existing filesystem, you can create a file and use that for swap space. Here’s an example of adding 2 GB of swap via a swap file:

[root@linux01 ~]# dd if=/dev/zero of=/root/xtraswap bs=1M count=2048
2048+0 records in
2048+0 records out

[root@linux01 ~]# ls -lh /root/xtraswap 
-rw-r--r--  1 root root 2.0G Jul 23 14:08 /root/xtraswap

Permissions should be changed such that only root has access, then setup the linux swap area via mkswap and enable it

[root@linux01 ~]# chmod 600 /root/xtraswap
[root@linux01 ~]# mkswap /root/xtraswap
Setting up swapspace version 1, size = 2147479 kB
[root@linux01 ~]# swapon /root/xtraswap

Just as in method 1 above, verify the extra swap space has been added by looking at /proc/swaps or using the swap -s command. In addition, add the newly created swap space to /etc/fstab to ensure it’s available on reboot.

!!! note “fstab entry”

## xtra swap via swap file
/root/xtraswap        swap    swap    defaults        0 0