Swap Space in Linux / RHEL / CENTOS / Fedora

What is a Swap space?

• Swap space is hard disk space that extends system RAM.

Swap space or virtual memory is hard disk space that acts as an extension of system RAM. Of course, due to the relative differential in data access on RAM versus hard disk, we prefer not to use swap space if it can be avoided. Nonetheless, it is vital to the proper functioning of a typical Linux system that some swap space be made available.

Swap space is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory.

Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files (a swap file can be used, provided the space has been pre-allocated using a tool such as dd).

The size of your swap should be equal to twice your computer’s physical RAM for up to 2 GB of physical RAM. For physical RAM above 2 GB, the size of your swap should be equal to the amount of physical RAM above 2 GB. The size of your swap should never less than 32 MB.

Using this basic formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one with 3 GB of physical RAM would have 5 GB of swap.

Creating Swap space As mentioned before, a swap space can be a partition or a file with pre-allocated space.

Setting up a Swap file

Suppose your system RAM is 2GB. Then you want to allocate 2 GB for swap space. So create a file anywhere for example in / as /swapfile.

Step 1: Open a shell, login as root

Step 2: Use dd command to create the file called /swapfile. We can use dd many ways to create the file.

[root@server ~]# dd if=/dev/zero of=/swapfile bs=1k count=2M

This will dump 2 million blocks of 1KB each into the /swapfile.

or

[root@server ~]# dd if=/dev/zero of=/swapfile bs=2k count=1M

This will dump 1 million blocks of 2KB each into the /swapfile.

or

[root@server ~]# dd if=/dev/zero of=/swapfile bs=1024 count=2097152

This will dump 2097152 blocks of 1024 each into the /swapfile.

Use any of these as you.

Step 3: Create the signature using mkswap.

[root@server ~]# mkswap /swapfile

Step 4: Add the swap space to the /etc/fstab file to make it persistent at system reboots.

[root@server ~]# vim /etc/fstab 

Add the following line

/swapfile     swap     swap     defaults     0 0

Step 5: Activate the new swap space using swapon -a.

[root@server ~]# swapon -a 

Step 6: Check the swap partition’s status using swapon -s or free -m.

[root@server ~]# swapon -s

[root@server ~]# free -m

Setting  up a Swap partition

Use fdisk or some other partitioning program to add a partition. Set the partition id type to 0×82 (Linux Swap / Solaris). Create the signature on the partition using mkswap and set a signature label to identify it consistently if disk device pathnames change.

Step 1: Open a shell, login as root

Step 2: Open fdisk to create the partition and change the system id.

[root@server ~]# fdisk -cu /dev/sda

Press n to add a new partition.

Now change the new partition's system id by pressing the t option.

Now save the table and exit. Restart for it it to work properly.

Step 3: Create the signature using mkswap.

[root@server ~]# mkswap -L SWAP2 /dev/sda2

 mkswap: /dev/sda2: warning: don't erase bootbits sectors(dos partition table detected). Use -f to force.Setting up swap space version 1, size = 10485756 KiB LABEL=SWAP2, UUID=986049e1-c454-484e-866c-22b38cb16e7b

Step 4: Add the swap space to the /etc/fstab file to make it persistent at system reboots.

 [root@server ~]# vim /etc/fstab 

Add the following line

LABEL=SWAP2     swap     swap     defaults     0 0

Step 5: Activate the new swap space using swapon -a.

 [root@server ~]# swapon -a 

Step 6: Check the swap partition’s status using swapon -s or free -m.

 [root@server ~]# swapon -s

 [root@server ~]# free -m