Difference between revisions of "Swap"

Revision as of 04:15, 9 November 2012

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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

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).

To check swap status, use:

$ swapon -s

Or：

$ free -m

Swap partition

A swap partition can be created with most GNU/Linux partitioning tools (e.g. fdisk, 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

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 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
Or
# 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 /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.

As root:

# swapoff -a

Remove swapfile:

# rm -rf /swapfile

Swap file resuming

Resuming the system from a swap file after hibernation requires an addition kernel parameter 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 with USB device

Thanks to modularity offered by Linux, we can have multiple swap partitions spread over different devices. If you have a very full hard disk, USB device can be used as partition temporally. But this method has some severe disadvantage：

• USB device is slower than hard disk.
• flash memories have limited write cycles. Using it as swap partition will kill it quickly.
• when another device is attached to the computer, no swap can be used.

To add a a USB device to SWAP, first take a USB flash and partition it with a swap partition.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 fstab

# 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. sdb1

Last, add

pri=0

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

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

Performance Tuning

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 100. Setting this parameter to a low value will reduce swapping from RAM, and is known to improve responsiveness on many systems.

/etc/sysctl.conf

vm.swappiness=1
vm.vfs_cache_pressure=50

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 faster device. Priorities can be assigned in fstab via the pri parameter:

/dev/sda1 none swap defaults,pri=100 0 0
/dev/sdb2 none swap defaults,pri=10  0 0

Or via the −p (or −−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.