Swap

From ArchWiki
Revision as of 16:13, 21 September 2011 by James Eder (Talk | contribs) (Add "Performance Tuning" section for "Swappiness" and "Priority" (new))

Jump to: navigation, search

Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary end

From All about Linux swap space:

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).

Swap partition

A swap partition can be created with most GNU/Linux partitioning tools (e.g. Template:Codeline, Template:Codeline). Swap partitions are designated as type 82.

To set up a Linux swap area, the Template:Codeline command is used. For example:

# mkswap /dev/sda2
Warning: All data on the specified partition will be lost.

To enable the device for paging:

# swapon /dev/sda2

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

/dev/sda2 swap swap defaults 0 0

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).

Swap file creation

As root, use Template:Codeline to create a file the size of your choosing. For example, creating a 512 MB swap file:

# 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

Edit Template:Filename and add an entry for the swap file:

/swapfile swap swap defaults 0 0

Ensure proper swap file size and activation with Template:Codeline:

$ free -m

Swap file removal

To remove a swap file, the current swap file must be turned off.

As root:

# swapoff -a

Remove swapfile:

# rm -rf /swapfile

Resume from Swap file

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.

Swap over USB

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.

Make sure you are writing the partition to the correct disk!

Next edit the Template:Codeline

# 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=... swap 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. Template:Codeline

We use UUID because when you attach other devices to the computer it could modify the device order

Last, add

pri=0

in the original swap entry for teaching fstab to use HD swap only when USB is full

ADVANTAGES

  • access time is slower than traditional HD
  • avoid power on HD when is in stand-by
  • USB are cheaper than HD

DISADVANTAGES

  • flash memories have limited write cycles
  • another device attached to the computer

To check if the tweak works, run

swapon -s

and see if your new swap partition is listed.

Naturally this guide will work for other memory such as SD cards, etc.

Performance Tuning

Swappiness

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.

Template:File

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 (Template:Filename) and a slower disk (Template:Filename), assign a higher priority to the swap area located on the faster device. Priorities can be assigned in fstab via the Template:Codeline parameter:

/dev/sda1 swap swap defaults,pri=100 0 0
/dev/sdb2 swap swap defaults,pri=10  0 0

Or via the Template:Codeline (or Template:Codeline) parameter of swapon:

# swapon -p 100 /dev/sda1