Change swap file linux

Change swap file linux

This page provides an introduction to swap space and paging on GNU/Linux. It covers creation and activation of swap partitions and swap files.

Linux divides its physical RAM (random access memory) into chunks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

Support for swap is provided by the Linux kernel and user-space utilities from the util-linux package.

Contents

Swap space

Swap space can take the form of a disk partition or a file. Users may create a swap space during installation or at any later time as desired. Swap space can be used for two purposes, to extend the virtual memory beyond the installed physical memory (RAM), and also for suspend-to-disk support.

If it is beneficial to extend the virtual memory with swap depends on the amount of installed physical memory. If the amount of physical memory is less than the amount of memory required to run all the desired programs, then it may be beneficial to enable swap. This avoids out of memory conditions, where the Linux kernel OOM killer mechanism will automatically attempt to free up memory by killing processes. To increase the amount of virtual memory to the required amount, add the necessary difference (or more) as swap space.

The biggest drawback of enabling swap is its lower performance, see section #Performance. Hence, enabling swap is a matter of personal preference: some prefer programs to be killed over enabling swap and others prefer enabling swap and slower system when the physical memory is exhausted.

To check swap status, use:

Or to show physical memory as well as swap usage:

Swap partition

The factual accuracy of this article or section is disputed.

A swap partition can be created with most GNU/Linux partitioning tools. Swap partitions are typically designated as type 82 . Even though it is possible to use any partition type as swap, it is recommended to use type 82 in most cases since systemd will automatically detect it and mount it (see below).

To set up a partition as Linux swap area, the mkswap(8) command is used. For example:

To enable the device for paging:

To enable this swap partition on boot, add an entry to /etc/fstab :

where the device_UUID is the UUID of the swap space.

See fstab for the file syntax.

Activation by systemd

systemd activates swap partitions based on two different mechanisms. Both are executables in /usr/lib/systemd/system-generators . The generators are run on start-up and create native systemd units for mounts. The first, systemd-fstab-generator , reads the fstab to generate units, including a unit for swap. The second, systemd-gpt-auto-generator inspects the root disk to generate units. It operates on GPT disks only, and can identify swap partitions by their type GUID, see systemd#GPT partition automounting for more information.

Disabling swap

To deactivate specific swap space:

Alternatively use the -a switch to deactivate all swap space.

Since swap is managed by systemd, it will be activated again on the next system startup. To disable the automatic activation of detected swap space permanently, run systemctl —type swap to find the responsible .swap unit and mask it.

Swap file

As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD).

Manually

Swap file creation

Use dd to create a swap file the size of your choosing. For example, creating a 512 MiB swap file:

Set the right permissions (a world-readable swap file is a huge local vulnerability):

After creating the correctly sized file, format it to swap:

Activate the swap file:

Finally, edit the fstab configuration to add an entry for the swap file:

For additional information, see fstab#Usage.

Remove swap file

To remove a swap file, it must be turned off first and then can be removed:

Finally remove the relevant entry from /etc/fstab .

Automated

zram-generator

The aim of this tool is the creation of zram devices. It is written in Rust and resides in systemd’s GitHub. It can be installed with the zram-generator package. Configuration is straightforward and explained in the README.

systemd-swap

systemd-swap is a script for creating hybrid swap space from zram swaps, swap files and swap partitions. It is not affiliated with the systemd project.

Install the systemd-swap package. Uncomment and set swapfc_enabled=1 in the Swap File Chunked section of /etc/systemd/swap.conf . Start/enable the systemd-swap service.

Visit the authors GitHub page for more information and setting up the recommended configuration.

Swap encryption

Performance

Swap operations are usually significantly slower than directly accessing data in RAM. Disabling swap entirely to improve performance can sometimes lead to a degradation, since it decreases the memory available for VFS caches, causing more frequent and costly disk I/O.

Swap values can be adjusted to help performance:

Swappiness

The swappiness sysctl parameter represents the kernel’s preference (or avoidance) of swap space. Swappiness can have a value between 0 and 200 (max 100 if Linux /sys/fs/cgroup/memory/memory.swappiness or /proc/sys/vm/swappiness can be read in order to obtain the raw integer value.

To temporarily set the swappiness value:

To set the swappiness value permanently, create a sysctl.d(5) configuration file. For example:

To test and more on why this may work, take a look at this article.

VFS cache pressure

Another sysctl parameter that affects swap performance is vm.vfs_cache_pressure , which controls the tendency of the kernel to reclaim the memory which is used for caching of VFS caches, versus pagecache and swap. Increasing this value increases the rate at which VFS caches are reclaimed[1]. For more information, see the Linux kernel documentation.

Priority

If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk ( /dev/sda ) and a slower disk ( /dev/sdb ), assign a higher priority to the swap area located on the fastest device. Priorities can be assigned in fstab via the pri parameter:

Or via the —priority parameter of swapon:

If two or more areas have the same priority, and it is the highest priority available, pages are allocated on a round-robin basis between them.

Using zswap or zram

Zswap is a Linux kernel feature providing a compressed write-back cache for swapped pages. This increases the performance and decreases the IO-Operations. ZRAM creates a virtual compressed Swap-file in memory as alternative to a swapfile on disk.

Striping

There is no necessity to use RAID for swap performance reasons. The kernel itself can stripe swapping on several devices, if you just give them the same priority in the /etc/fstab file. Refer to The Software-RAID HOWTO for details.

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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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