How to create linux swap

How to Create Swap Partition on Linux

Swap memory is required when the system requires more memory than it is physically available. The kernel swaps out less used pages and gives memory to the current process that needs the memory immediately. So a page of memory is copied to the preconfigured space on the hard disk. Disk speed is much slower compared to memory speed. Swapping pages give more space for current applications in the memory (RAM) and make the application run faster.

Swap space is located on hard drives, which have a slower access time than physical memory. Swap space can be a dedicated swap partition or swap file, or a combination of both.

In this tutorial, we learn how to create a swap partition on a Linux system.

Create a swap partition

Lets first check disk space, then create a partition and followed by enabling swap.

1) Check disk space

Check if you have enough space on disk to create a new partition for swap using fdisk or parted command.

We will use /dev/sdb disk for our swap. Check the swap with free -m command; we have:

You can see that we don’t have a swap partition. We can also use the command below for verification

You see that we don’t have a return. It means that there is no swap

2) Create a partition for swap

As we saw we have enough unallocated space on the disk, we can create a new partition using tools like parted or fdisk.

Here I am going to use fdisk command to create a partition:

You can type m command for the help which will list you different possibilities. We will create a new partition for our swap with n command

To define now our partition as swap type, we will use t command

The Hex code for swap partition on Linux is 82 . Now we will save the changes with w command

Make the new partition as swap. Change toggle id to 82 (for swap). Let’s check with fdisk -l command:

You can see the mention ‘Linux swap’ on the last line.

Note: On the latest version Ubuntu and Centos it uses create swap file instead of a swap partition. Let’s see how to create a swap file.

Just use dd command or fallocate to create a file (say 1 GB or 2GB).

Then follow the below steps.

3) Format to swap mode

After defining our partition, we need to format it for «swap mode» so run mkswap command on the newly created swap partition. Make sure to choose the correct partition number which you need to enable swap. You may use -L option to set LABEL on the swap partition.

Run the following command define /dev/sdb1 as swap partition:

4) Enable Swap space

Now that our swap partition is formatted, we need to enable the swap space so run swapon command to enable it:

5) Verify swap space

Verify the newly added swap space using the command below:

6) Add to fstab file

Then add newly created swap partition to /etc/fstab file. It should look as below:

How to Create swap partition for lvm

You can have an LVM installation on your server and you need to create a swap partition. The procedure is not exactly the same because of «lvm mode»

We must first create the LVM2 logical volume of size 8 GB:

After creating the logical volume, we need to format the new swap space:

To be sure that our swap partition will be mounted automatically even if we restart the server, we need to add the following entry to the /etc/fstab file:

Now we need to enable the extended logical volume:

To test if the logical volume was successfully created, use swapon -s or free -m command to inspect the swap space.

Extend swap partition for lvm

You can need to extend your swap partition because the actual swap size doesn’t satisfy your job. With lvm, it is possible to directly increase the size of an existing partition as below.

You must first identify the swap volume group which is ‘/dev/rootvg/swapvol’ in our case. You need first to disable the current swapping

Now you must resize the volume group to indicate the space to increase

We want to increase from 8 GB to 16 GB

Now we need to format the space

Now we need to activate the swap for devices marked as swap in /etc/fstab

Remove swap partition

For some reason, you can need to remove your swap partition in lvm mode.

To remove a swap partition, you first need to disable the swapping for the associated logical volume whether it is lvm or something else there

The second principle is to remove the volume so you need to delete the swap partition entirely.

Now we need to remove the following entry from the /etc/fstab file

How to adjust swappiness property

Swappiness value defines how often system swaps data out of RAM to the swap space. The current swappiness value is stored in «/proc/sys/vm/swappiness’ file. This is a value between 0 and 100. A low value say close to zero will make the kernel to try to avoid swapping. A server can have a value closer to 0 and Desktop 60 should be okay.

Conclusion

In this tutorial, we learned how to create a swap partition and enable swap on Linux. The modern computer comes with high memory and if you think your application will exhaust memory then it’s advised to add a bit swap.

For old computers with small memory, it is always good to give twice the RAM size for your swap space.

I hope you enjoyed reading this and please provide your suggestions on the below comment section.

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Linux Add a Swap File Tutorial

I need additional swap space to improve my Linux server/desktop system performance. How do I add a swap file to Linux system using command line options without creating a new partitions?

In Linux, as in most other Unix-like operating systems, it is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions, although I recommend using a swap partition. The administrative flexibility of swap files outweighs that of partitions; since modern high capacity hard drives can remap physical sectors, no partition is guaranteed to be contiguous. You can add swap file as a dedicated partition or use following instructions to create a swap file.

Tutorial details
Difficulty level	Intermediate
Root privileges	Yes
Requirements	None
Est. reading time	5m

Procedure To Add a Swap File Under Linux

Step 1 – Login as the Root User

Open a terminal window (select Applications > Accessories > Terminal) or login to remote server using the ssh client. Switch to the root user by typing su — (or sudo -s ) and entering the root password, when prompted:
$ su —
OR
$ sudo -s

Step 2 – Create Storage File

Type the following command to create 512MB swap file (1024 * 512MB = 524288 block size):
# dd if=/dev/zero of=/swapfile1 bs=1024 count=524288
Sample outputs:

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1. if=/dev/zero : Read from /dev/zero file. /dev/zero is a special file in that provides as many null characters to build storage file called /swapfile1.
2. of=/swapfile1 : Read from /dev/zero write storage file to /swapfile1.
3. bs=1024 : Read and write 1024 BYTES bytes at a time.
4. count=524288 : Copy only 523288 BLOCKS input blocks.

Step 3 – Secure swap file

Setup correct file permission for security reasons, enter:
# chown root:root /swapfile1
# chmod 0600 /swapfile1
A world-readable swap file is a huge local vulnerability. The above commands make sure only root user can read and write to the file.

Step 4 – Set up a Linux swap area

Type the following command to set up a Linux swap area in a file:
# mkswap /swapfile1
Sample outputs:

Step 5 – Enabling the swap file

Finally, activate /swapfile1 swap space immediately, enter:
# swapon /swapfile1

Step 6 – Update /etc/fstab file

To activate /swapfile1 after Linux system reboot, add entry to /etc/fstab file. Open this file using a text editor such as vi:
# vi /etc/fstab
Append the following line:
/swapfile1 none swap sw 0 0
Save and close the file. Next time Linux comes up after reboot, it enables the new swap file for you automatically.

How do I verify Linux swap file is activated or not?

Simply use the free command:
$ free -m

How can I display swap usage summary on Linux?

Type the following swapon command:
# swapon -s
Sample outputs:

Another option is to view /proc/meminfo file:
$ less /proc/meminfo
$ grep -i —color swap /proc/meminfo
Sample outputs:

You can also use top command, atop command, and/or htop command to display information about swap usage:
# top
# atop
# htop
Sample outputs from a database server running on a CentOS Linux server:

How can I disable devices and files for paging and swapping on Linux?

You need to use the swapoff command:
# swapoff /swapfile1
# swapon -s

How do I set swappiness on a Linux server?

The syntax is:
# sysctl vm.swappiness=VALUE
# sysctl vm.swappiness=20
OR
# echo VALUE > /proc/sys/vm/swappiness
# echo 30 > /proc/sys/vm/swappiness

The value in /proc/sys/vm/swappiness file controls how aggressively the kernel will swap memory pages. Higher values increase agressiveness, lower values descrease aggressiveness. The default value is 60. To make changes permanent add the following line to /etc/sysctl.conf :

Conclusion

You learned how to add swap file under Linux operating systems.

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Comments on this entry are closed.

TYPO: vi /etc/fstatb
It should be fstab

Thanks for heads up!

If you don’t want to reboot the machine to enable the new swapfile, after step f) you can issue these commands:

swapoff -a
swapon -a

to first stop and then start all swaps in /etc/fstab

this was really helpful for debugging my automounting issue. Turns out I had a typo on the swapfile name.
Thnx some_guy

Thanks for the help. But how would I go about deleting this swapfile? Also, how does creating a swapfile this way differ from creating a separate partition for one? I only ask because I would like to have multiple installs of linux on my system, and I would like them to share the same swapfile.

Recently, I’ve been trying to create a swapfile partition and it’s not being recognized, so I’ve been having trouble.

Thanks, I have been looking for the last step for months heh.

Thank you for the guide; I’d like to add another detail. In this line one may choose to use other units to make things more simple:

# dd if=/dev/zero of=/swapfile1 bs=1024 count=524288

You can turn it into:

# dd if=/dev/zero of=/swapfile1 bs=1M count=512

This means that the block size is 1 MB, so count=512 means “I need 512 megs”, there is no need to do any other calculations.

hi,
i’m about to install ubuntu karmic on a new computer with thre sata hard drives.

I usually allocate a little more of current ram in swap partition (5 gigas) so I can safely hibernate, but on a post I read in FreeBSD forums there’s a link to FreeBSD manual where states there should be a swap partition for every disk, not just one swap for the whole system.

So, I know Linux is not *BSD but I wonder if the same is applicable here because installing a swap of 5 gigs in all three sata seems a waste of space to me!

Set swap priority:

• Swapiness is the priority of input/output for swap. To look the current value:
cat /proc/sys/vm/swappiness

To change the swap priority (lower value means less swapping):
sysctl vm.swappiness=10

To have this value set at boot add it to /etc/sysctl.conf
vm.swappiness=0

Hi!
First I must say thanks for a great how to.
And secondly, I’m referencing it in Arch Linux wiki how to create swap file.
Hope you agree with it.
If not please contact me.
Link: http://wiki.archlinux.org/index.php/HOWTO_Create_swap_file

Thanks! I’m newbie to Linux, It helped me in time.

After creating the swap file its permissions should be set so that only root can access the file:

chmod 600 /swapfile1

+1 to Thomas suggestion.

You MUST chmod 600 /swapfile1. Otherwise your box will get owned!

So, after following these steps I received the notification that my root partition is full. And then, after rebooting, I cannot log into gnome. It will start gdm but will not go into gnome from there, it will only bring me back to gdm. startx does not work as well. So how do I do the oposite of this?

You just have to:
-make sure the swap file is not active
swapon -s
-if it is active
swapoff /swapfile (or whatever you called it)
-comment out the line in your /etc/fstab file (Place a hashmark # in front of it or just delete the line)
-remove the swapfile:
rm /swapfile (or whatever you called it)

Reboot and you should be away unless you want to create a smaller swap file in which case you should just make sure it isn’t in use, delete the file and recreate it using a smaller size.

Since kernel 2.6.31, you can use the util fallocate instead of dd on btrfs, ext4, ocfs2, and xfs filesystems. It’s *much* faster than dd on really big swap files.

Hi Vivek,
5 years on and the page is still useful. Thanks 🙂

dd if=/dev/zero of=/swapfile1 bs=1024 count=524288
pls tell me the meaning and function of this command..why we are using if=/dev/zero and of=/swapfile..why this command started with dd..

dd is a command that does a low level copy of data from a to b. ‘if’ is the input file for the copy. /dev/zero is a device that generates as many null characters as it is asked to create and passes them on. ‘of’ is your output file or what will become your new (or additional) swapfile. the ‘/swapfile’ could be any name that you want it to be as long as you don’t forget what it is for the further manipulations. ‘bs’ is the block size of the file and ‘count’ is the number of bytes in the file. So, what the command does is takes 524288 null bytes from /dev/zero and puts them into /swapfile thus initially populating the file.

How do you increase the size of a swap file?

either create a new pagefile that’s larger mount it as swap, then umount the old swap and delete. then modify fstab as appropriate.

or if it’s a lightly loaded machine you can unmount the swap create a larger swap and remount. This has the benefit that you can reuse the name 😉

or…Of course you can create a 1gig file and just add that as swap as well. The only problem here is ensuring that the file is contiguous,

i.e. 1x 3gb swap or 3x 1gb swap (swapfile1+swapfile2+swapfile3) or indeed 1x 2gb swap + 1x1gb swap.

@Sergio, add another one. Or unmount the swap file and recreate a larger file.

“Or unmount the swap file and recreate a larger file.”

Why, when (as you first suggested) adding another one is so simple?

how to i increase swapfile to 2gig… or more im confused

You did not really say where you confusion lies. You have one of two options. You can either add a second swap file with whatever size will bring the total up to 2 GB or you can delete the current swapfile and establish a new one that is 2 GB in size. I believe both scenarios are covered in the comments above as well as the initial article so you will have to be a little more explicit if you want help …

thank u for a clarification of this doubt

We tried to add a swap file on one server and now using above steps but now it is growing big.

root@server [/]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 4.9G 3.9G 710M 85% /
tmpfs 1.9G 0 1.9G 0% /dev/shm
/dev/sda1 485M 86M 374M 19% /boot
/dev/sda6 176G 31G 136G 19% /home
/dev/sda5 20G 6.6G 12G 36% /usr
/dev/sda3 29G 6.3G 22G 23% /var
/usr/tmpDSK 485M 16M 444M 4% /tmp
root@server [/]# free -m
total used free shared buffers cached
Mem: 3810 3545 264 0 425 2288
-/+ buffers/cache: 831 2979
Swap: 2999 325 2674
root@server [/]# grep swap /etc/fstab
/swapfile swap swap defaults 0 0
root@server [/]# du -sch /swapfile
3.0G /swapfile
3.0G total
root@server [/]#

Thank you for this useful post 🙂

Thanks alot
I was wondering to create my swap space due to out of space for oracle installation
so the precheck was failed
this gave me the correct result to over come my problem
no need to restart the OS, it will apply after creating the swap file
again very big thank for the support list

Thanks a lot brother ! Saved a lot of effort on my tomcat dying silently.
ps (I’m a colleague of ashish gite)

It help me to solve a problem using Riak, the OS was installed without swap partition.

Unless something else has an express need to access /swapfile1 by a block device mount point “swap” this presents MORE of a security risk NOT LESS. The second ‘/etc/fstab’ field should be “none”. Consider ‘/media/sda1/pagefile.sys none swap defaults 0 0’.
You have used the example ‘/swapfile1 swap swap defaults 0 0’. During init, otherwise unmounted TRUE block devices REQUIRE mounting “swap”–designated in the second field ‘/dev/sda3 swap auto defaults 0 0’. A generic swap-file (not on the root device) requires patching: that it’s device has been previously mounted in some rc script ‘mount /dev/sda1’ (assuming valid entry in /etc/fstab) along with an existing contiguous /swapfile1 (else create–use dd), ‘delay 1’ (must wait for completion), ‘mkswap /mnt/sda1/swapfile1’ must be rw (defaults) and compatible permissions for the device file system, and of course ‘swapon /mnt/sda1/swapfile1’.

Thank you very much. I moved wordpress to AWS ec2 and started getting Database establishing error. I found that out of 1.7GB only 30-40 mb was left. Now with these steps I created additional 512 mb. Hope now I do not get any memory issue.

A much faster way to create a 1GB swapfile is…

dd if=/dev/zero of=/swapfile bs=1024 count=1 seek=1M

This skips to the end of the swapfile and writes just one block leaving a giant “hole” that will be read as zeros. Disk space is allocated as the file is written to. Beware, you are allowed to create a swap file larger than you have free disk space.

Nuts, ignore my last message. If you try to turn on swap using the above you will get…

# swapon /swapfile
swapon: /swapfile: skipping – it appears to have holes.

You can use simple way

# dd if=/dev/zero of=/opt/swapfile bs=1M count=1024

1. block size is 1 MB
2. count=1024 means 1GB (swap file Size)
3. /opt/swapfile (location as you want swap file)
then
# mkswap /opt/swapfile
Do entry in Fstab
# swanon -a

#Like?#
swapon /swapfile
swapoff -a
dd if=/dev/zero of=/opt/swapfile bs=1M count=5700
mkswap /opt/swapfile
swanon -a
sysctl vm.swappiness=100
swapon /swapfile
#

swapon -s is useful in showing how much swap space and on what drives.

Normally I would prefer to make a swap partition rather then making a swap file. That way your swap space does not crash if your filesystem does. (ie if / has an issue, at least your swap memory is safer…. )

Very helpful article.

Ultimate ….i have no words for these documentation

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