- Linux divides its physical RAM (random access memory) into chucks 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.
Swap space will usually be a disk partition but can also be a file. Users may create a swap space during installation of Arch Linux or at any later time should it become necessary. Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support).
A swap partition can be created with most GNU/Linux partitioning tools (e.g.
cfdisk). Swap partitions are designated as type 82.
To set up a Linux swap area, the
mkswap command is used. For example:
# mkswap /dev/sda2
To enable the device for paging:
# swapon /dev/sda2
To enable this swap partition on boot, add an entry to fstab:
/dev/sda2 none swap defaults 0 0
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).
Swap file creation
As root use
fallocate to create a swap file the size of your choosing (M = Megabytes, G = Gigabytes) (
dd can also be used but will take longer). For example, creating a 512 MB swap file:
# fallocate -l 512M /swapfile # dd if=/dev/zero of=/swapfile bs=1M count=512
Set the right permissions (a world-readable swap file is a huge local vulnerability)
# chmod 600 /swapfile
After creating the correctly-sized file, format it to swap:
# mkswap /swapfile
Activate the swapfile:
# swapon /swapfile
/etc/fstab and add an entry for the swap file:
/swapfile none swap defaults 0 0
Remove swap file
To remove a swap file, the current swap file must be turned off.
# swapoff -a
# rm -rf /swapfile
Swap file resuming
Resuming the system from a swap file after hibernation requires an addition parameter to the kernel compared to resuming from a swap partition. The additional parameter is resume_offset=<Swap File Offset>.
The value of <Swap FIle Offset> can be obtained from the output of filefrag -v; The output is in a table format; the required value is located in the 'physical' column from the first row. Eg:
# filefrag -v /swapfile Filesystem type is: ef53 File size of /swapfile is 4290772992 (1047552 blocks, blocksize 4096) ext logical physical expected length flags 0 0 7546880 6144 1 6144 7557120 7553023 2048 2 8192 7567360 7559167 2048 ...
From the example <Swap FIle Offset> is 7546880.
Importand note: Please note that in kernel "resume" parameter you still have to type path to partition (e.g. resume=/dev/sda1) not to swapfile explicitly! Parameter resume_offset is for informing system where swapfile starts on hard disk.
Swap with USB deviceTemplate:Box YELLOW
Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. To add a a USB device to SWAP:
First, take a USB flash and partition it with a swap partition. To do this you can use graphical tools such as Gparted or console tools like fdisk. Make sure to label the partition as SWAP before writing the partition table.
Next edit the
# nano /etc/fstab
Now add a new entry, just under the current swap entry, which take the current swap partition over the new USB one
UUID=... none swap defaults,pri=10 0 0
where UUID is taken from the output of the command
ls -l /dev/disk/by-uuid/ | grep /dev/sdc1
Just replace sdc1 with your new USB swap partition.
in the original swap entry for teaching fstab to use HD swap only when USB is full
- access time is lower than traditional HD
- avoid power on HD when is in stand-by
- USB pendrives are cheaper than HD (meaning: you can't get a $5 HDD, price per gigabyte is obviously much higher for flash drives)
- flash memories have limited write cycles
- another device attached to the computer
This guide will work for other memory such as SD cards, etc.
Ensure that swap is activated with
$ swapon -s $ free -m
Swap values can be adjusted to help performance.
The swappiness sysctl parameter represents the kernel's preference (or avoidance) of swap space. Swappiness can have a value between 0 and 100. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems.
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 faster device. Priorities can be assigned in fstab via the
/dev/sda1 none swap defaults,pri=100 0 0 /dev/sdb2 none swap defaults,pri=10 0 0
Or via the
−−priority) parameter of swapon:
# swapon -p 100 /dev/sda1
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.