Swap Space

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.

Swap Space

Swap space will usually be a disk partition but can also be a file.
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).

Check Swap Status

cat /proc/swap

$ cat /proc/swap
Filename                                Type            Size    Used    Priority
/dev/sda2                               partition       2048276 112     -1

Shows swap partition /dev/sda2 with allocated size 2GB and 112KB used space.

swapon

swapon, swapoff – enable/disable devices and files for paging and swapping

$ swapon -s
 Filename                                Type            Size    Used    Priority
 /var/swapfile.img                       file            2097144 5256    -1

Show swap file /var/swapfile.img with allocated size 2GB and with 5MB used space.

free

free – Display amount of free and used memory in the system

$ free
             total       used       free     shared    buffers     cached
Mem:       5986512    3210580    2775932          0      80016    1043436
-/+ buffers/cache:    2087128    3899384
Swap:      5996540          0    5996540


$ free -m
             total       used       free     shared    buffers     cached
Mem:          5846       3113       2732          0         78        997
-/+ buffers/cache:       2038       3808
Swap:         5855          0       5855

Here we got about 6GB allocated and unused space.

vmstat

vmstat – Report virtual memory statistics

$ vmstat 1 5
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
1  1   8392  20800    268  15568    0    2   136   309   50  137  2  2 95  1
1  1   8492 118864    268  14680    0  100   768   100  374  300 42 58  0  0
1  1   8616  27064    164  13628    0  124   640   124  344  311 44 56  0  0
1  0   9020  79260    164  12396    0  404   768   404  363  332 49 51  0  0
1  1   9464  89268    164  11328    0  444   640   444  387  320 41 59  0  0
  • swpd: the amount of virtual memory used. (up to 9.5MB)
  • si: Amount of memory swapped in from disk (/s). (up to 444KB)
  • so: Amount of memory swapped to disk (/s). (nothing: 0KB)

Setting Up Swap Space

Normally, there are only two steps to setting up swap space, creating the partition and adding it to /etc/fstab. A typical fstab entry for a swap partition at /dev/hda6 would look like this:

/dev/hda6   swap    swap    defaults    0   0

The next time you reboot, the initialization scripts will activate it automatically and there’s nothing more to be done.

However, if you want to make use of it right away, you’ll need to activate it maually. As root, type:

$ mkswap -f /dev/hda6
$ swapon /dev/hda6

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