Swap size

[Paul Schmehl]

--On Friday, August 17, 2007 11:07:14 -0400 Andy Greenwood 
<greenwood.andy at gmail.com> wrote:

> Jerry McAllister wrote:
>> On Fri, Aug 17, 2007 at 02:05:57AM +0200, Nicholas Wieland wrote:
>>
>>
>>> I was reading tuning(7), and I found that I should size my swap
>>> double the size of my physical memory.
>>> AFAIK that was true some years ago, when memory was not as cheap as
>>> now, and following that guideline I should set my swap to 2GB, which
>>> seems far too much for swap (at least to me ...). I will never need
>>> this much memory as 1GB RAM and 2GB swap.
>>> Is it still correct ? How can I resize with bsdlabel if I already
>>> used all my disk space during install ?
>>>
>>
>> Remember, disk sizes have shot up too.
>> No, 2 GB is not excessive.   You can get by with less, but you're
>> not likely to be using proportionately as much disk now as you used
>> to by going with 2X - I aim for a little over 2X.
>>
>> Remember that swap gets used for crash dumps and also for paging.
>> Now, you may think that you want to keep your machine from paging
>> and in one sense that is true.   If you are so memory bound that
>> it has to page just to run, you're going to be so slow that it
>> seems to have froze (by today's standards).   But, the system does
>> write stuff to page space and for processes that are often called
>> it can speed things up.
>>
>> So, it is not really a waste to assign that much to swap.
>>
>> ////jerry
>>
>>
>>> TIA,
>>>   ngw
>>>
>>> --
>>> Nicholas Wieland
>>> nicholas.wieland at gmail.com
>>>
>>>
>>>
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> My understanding was that you should estimate swap size based on the
> sizes of the programs which might be paged out. However, when I first set
> up my system, I didn't know this and created 1G swap slices (one on each
> disk) but I am not convinced that this was the best thing to do, since my
> system almost never uses a noticible percentage of the swap space. right
> now, I've got
>
> [andy at zeus fusefs-sshfs]$ swapinfo
> Device          1K-blocks     Used    Avail Capacity
> /dev/ad0s1b.eli   1048576     1148  1047428     0%
> /dev/ad1s1b.eli   1048576     1096  1047480     0%
> Total             2097152     2244  2094908     0%
>
> And the system is under normal load. This system has 1G of RAM. Is there
> any sense in having this much swap space when it's not being used?

Yes.  As was stated earlier, you will need that much space to save a core 
file if the system crashes.  If you don't care about troubleshooting major 
system crashes, then don't worry about it.  OTOH, disk sizes have grown so 
large that 2GB of swap is negligible use of space.  I always configure swap 
to be 2xRAM plus 200MB.  On a 300GB drive, that's less than 1% of the space 
available.

-- 
Paul Schmehl (pauls at utdallas.edu)
Senior Information Security Analyst
The University of Texas at Dallas
http://www.utdallas.edu/ir/security/