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Linux Swap partition and Swap file

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Introduction

You can provide swap space in Linux iether via swap partition or swap file (or combination):

You can have multiple swap partitions and combine usage of swap file with swap partition.

Procedure to add a swap file

You need to use dd command to create swap file. Next you need to use mkswap command to set up a Linux swap area on a device or in a file. All operations should be done as root. Here is step-by-step the procedure:

With /etc/fstab entry after reboot the new swap will be mounted automatically.

How do I verify swap is activated or not

You can use several commands:  


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HowTo Check Swap Usage in Linux

October 28, 2012 | 8 comments
How do I check swap (paging) usage under Linux operating systems using command bash/ksh line options?

Swap space (also known as paging) is nothing but computer memory management involving swapping regions of memory to and from storage. You can see swap usage summary by device using any one of the following commands. You may have to login as root user to use the following commands.

The maximum useful size of a swap area depends on the architecture and the kernel version. For Linuux kernels after v2.3.3+ there is no such limitation on swap size.

Option #1: /proc/swaps file

Type the following command to see total and used swap size:
# cat /proc/swaps
Sample outputs:

Filename				Type		Size	Used	Priority
/dev/sda3                               partition	6291448	65680	0

Option #2: swapon command

Type the following command:
# swapon -s
Sample outputs:

Filename				Type		Size	Used	Priority
/dev/sda3                               partition	6291448	65680	0

Option #3: free command

Use the free command as follows:
# free -g
# free -k
# free -m

Sample outputs:

             total       used       free     shared    buffers     cached
Mem:         11909      11645        264          0        324       8980
-/+ buffers/cache:       2341       9568
Swap:         6143         64       6079

Option #4: vmstat command

Type the following vmstat command:
# vmstat
# vmstat 1 5

Sample outputs:

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 1  9 1209512 101352   1504 127980    0    3    11    20   60   55  3  1 95  1
 2 11 1209640 101292   1508 134132  844  424  5608   964 23280 15012  2  8 20 70
 0 10 1210052 108132   1532 125764  648  660 10548   916 22237 18103  3 10 11 77
 1 13 1209892 106484   1500 128052  796  240 10484   980 24024 12692  2  8 24 67
 1  9 1209332 113412   1500 124028 1608  168  2472   620 28854 13761  2  8 20 70

Note down the following output from swap field:

  1. si: Amount of memory swapped in from disk (/s).
  2. so: Amount of memory swapped to disk (/s).

Option #5: top/atop/htop command

Type the following commands:
# atop
# htop
# top

Sample outputs (from top command):

top - 02:54:24 up 15:24,  4 users,  load average: 0.45, 4.84, 6.75
Tasks: 266 total,   1 running, 264 sleeping,   0 stopped,   1 zombie
Cpu(s):  3.2%us,  1.4%sy,  0.0%ni, 94.4%id,  1.0%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:   8120568k total,  7673584k used,   446984k free,     4516k buffers
Swap: 15859708k total,  1167408k used, 14692300k free,  1151972k cached
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
13491 vivek     20   0 1137m 279m 6692 S   10  3.5  19:17.47 firefox
 5663 vivek     10 -10 1564m 1.1g  59m S    8 14.5   5:10.94 vmware-vmx
 2661 root      20   0  352m 185m 8604 S    6  2.3  65:40.17 Xorg
 3752 vivek     20   0 3566m 2.6g  12m S    6 33.6  63:44.35 compiz
 4798 vivek     20   0  900m  50m 4992 S    2  0.6   0:11.04 chrome
 5539 vivek     20   0 1388m 838m 780m S    2 10.6   1:45.78 VirtualBox
 6297 root      20   0     0    0    0 S    2  0.0   0:00.15 kworker/2:0
 6646 root      20   0 19252 1404  936 R    2  0.0   0:00.01 top
    1 root      20   0  8404  644  608 S    0  0.0   0:03.32 init
    2 root      20   0     0    0    0 S    0  0.0   0:00.03 kthreadd
    3 root      20   0     0    0    0 S    0  0.0   0:02.30 ksoftirqd/0
    6 root      RT   0     0    0    0 S    0  0.0   0:00.00 migration/0
    7 root      RT   0     0    0    0 S    0  0.0   0:00.24 watchdog/0
   37 root       0 -20     0    0    0 S    0  0.0   0:00.00 cpuset
   38 root       0 -20     0    0    0 S    0  0.0   0:00.00 khelper
   39 root      20   0     0    0    0 S    0  0.0   0:00.00 kdevtmpfs
   40 root       0 -20     0    0    0 S    0  0.0   0:00.00 netns

[Dec 03, 2007] All about Linux swap space by Gary Sims

There are some errors. Not having swap doesn't mean that your kernel will crash.  Recommendation of allocating 1 GB swap for 128 MB of RAM is questionable. The swap management needs physical RAM, and the more swap you have, the more RAM you need. Linus have said that you should generally aviod hving more than twice your amount of RAM for swap partition. And for most application servers (for example Oracle application server) half-memory swap is adequate.
December 03, 2007 |  Linux.com

Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files.

To see what swap space you have, use the command swapon -s. The output will look something like this:

Filename        Type            Size    Used    Priority
/dev/sda5       partition       859436  0       -1

Each line lists a separate swap space being used by the system. Here, the 'Type' field indicates that this swap space is a partition rather than a file, and from 'Filename' we see that it is on the disk sda5. The 'Size' is listed in kilobytes, and the 'Used' field tells us how many kilobytes of swap space has been used (in this case none). 'Priority' tells Linux which swap space to use first. One great thing about the Linux swapping subsystem is that if you mount two (or more) swap spaces (preferably on two different devices) with the same priority, Linux will interleave its swapping activity between them, which can greatly increase swapping performance.

To add an extra swap partition to your system, you first need to prepare it. Step one is to ensure that the partition is marked as a swap partition and step two is to make the swap filesystem. To check that the partition is marked for swap, run as root:

fdisk -l /dev/hdb

Replace /dev/hdb with the device of the hard disk on your system with the swap partition on it. You should see output that looks like this:

 Device Boot    Start   End     Blocks  Id      System
/dev/hdb1       2328    2434    859446  82      Linux swap / Solaris

If the partition isn't marked as swap you will need to alter it by running fdisk and using the 't' menu option. Be careful when working with partitions -- you don't want to delete important partitions by mistake or change the id of your system partition to swap by mistake. All data on a swap partition will be lost, so double-check every change you make. Also note that Solaris uses the same ID as Linux swap space for its partitions, so be careful not to kill your Solaris partitions by mistake.

Once a partition is marked as swap, you need to prepare it using the mkswap (make swap) command as root:

mkswap /dev/hdb1

If you see no errors, your swap space is ready to use. To activate it immediately, type:

swapon /dev/hdb1

You can verify that it is being used by running swapon -s. To mount the swap space automatically at boot time, you must add an entry to the /etc/fstab file, which contains a list of filesystems and swap spaces that need to be mounted at boot up. The format of each line is:

<file system>     <mount point>     <type>     <options>        <dump>    <pass>

Since swap space is a special type of filesystem, many of these parameters aren't applicable. For swap space, add:

/dev/hdb1       none    swap    sw      0       0

where /dev/hdb1 is the swap partition. It doesn't have a specific mount point, hence none. It is of type swap with options of sw, and the last two parameters aren't used so they are entered as 0.

To check that your swap space is being automatically mounted without having to reboot, you can run the swapoff -a command (which turns off all swap spaces) and then swapon -a (which mounts all swap spaces listed in the /etc/fstab file) and then check it with swapon -s.

... ... ...
 

How big should my swap space be?

It is possible to run a Linux system without a swap space, and the system will run well if you have a large amount of memory -- but if you run out of physical memory then the system will crash, as it has nothing else it can do, so it is advisable to have a swap space, especially since disk space is relatively cheap.

A rule of thumb is as follows:

  1. For a desktop system, use a swap space of double system memory, as it will allow you to run a large number of applications (many of which may will be idle and easily swapped), making more RAM available for the active applications;
  2. For a server, have a smaller amount of swap available (say half of physical memory) so that you have some flexibility for swapping when needed, but monitor the amount of swap space used and upgrade your RAM if necessary;

... ... ...

The Linux 2.6 kernel added a new kernel parameter called swappiness to let administrators tweak the way Linux swaps. It is a number from 0 to 100. In essence, higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Kernel maintainer Andrew Morton has said that he runs his desktop machines with a swappiness of 100, stating that "My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful."

One downside to Morton's idea is that if memory is swapped out too quickly then application response time drops, because when the application's window is clicked the system has to swap the application back into memory, which will make it feel slow.

The default value for swappiness is 60. You can alter it temporarily (until you next reboot) by typing as root:

echo 50 > /proc/sys/vm/swappiness

If you want to alter it permanently then you need to change the vm.swappiness parameter in the /etc/sysctl.conf file.

Conclusion

Managing swap space is an essential aspect of system administration. With good planning and proper use swapping can provide many benefits. Don't be afraid to experiment, and always monitor your system to ensure you are getting the results you need.



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