View source for Swap

Swap is a secondary storage for [[memory]] that is swapped or paged out, usually residing on your hard disk.  Hard drives are a lot slower to read and write to than [[RAM]], so ideally you will want to increase your RAM if financially possible if you see that your swap is being used, assuming speed as priority. 

It is also used for saving the [[kernel]] [[core]]file when the system [[panic]]s. Some systems compress their corefile to swap and others don't (such as [[BSD]]), this is why it's a good idea to have 1.5 to 2 times as much swap as physical memory in a machine (as a rule of thumb), so that you can get a good corefile.

Don't forget to ensure there is always enough room in your swap if your RAM is being used to its maximum capacity:
 # swapinfo
 Device          1K-blocks     Used    Avail Capacity
 /dev/ad0s1b        105288    68596    36692    65%

otherwise you'll start getting errors to syslog, running applications will be killed, and it will create a general
mess of things:
  
 Oct 27 09:51:38 mail swap_pager_getswapspace(9): failed
 Oct 27 09:51:38 mail swap_pager_getswapspace(5): failed
 Oct 27 09:51:38 mail swap_pager_getswapspace(16): failed
 Oct 27 09:51:40 mail pid 95400 (perl5.8.7), uid 125, was killed: out of swap space

If you are running out of swapspace frequently, or just want to increase the capacity, you can [http://www.freebsd.org/handbook/adding-swap-space.html add a swapfile ]very easily on [[FreeBSD]].


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[http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/swap-encrypting.html Encrypting swap in FreeBSD] has existed since version 5.3





== Encrypting your swap in GNU/[[Linux]] ==

This was done on [[Ubuntu]] feisty fawn. If you're doing this on your live system, first deactivate your current swap:

 # swapoff -a

I added this to /etc/crypttab:
 
 swap /dev/[[Variables|sda2]]  /dev/urandom  cipher=aes-cbc-essiv:sha256,hash=sha256,size=256,swap

and then added the following to /etc/fstab:

 /dev/mapper/swap       none    swap     0    0

finally, I ran:

 $ sudo  /etc/init.d/cryptdisks start
 $ swapon -a

to see if it's working:

 $ swapon -s 

or 

 $ cat /proc/swaps


== Encrypting swap in MacOSX ==

System Preferences -> Security pane and choose "Use secure virtual memory"