Swap space

Bob Johnson fbsdlists at gmail.com
Tue Nov 1 07:50:44 PST 2005

On 11/1/05, Ian Lord <mailing-lists at msdi.ca> wrote:
> Hi,
> I just bought 4 servers with 4 gigs of ram, the documentation
> proposes to use 2 to 3 times the amount of ram for swap... I don't
> think 12 gigs of swap would be useful lol, but do I really need to
> put 4 gigs of ram. (It might be useful for kernel dump but...)

That's the original reason for the suggestion.  You need more than
twice the amount of RAM if you need to capture a dump for debugging. 
If you won't ever be doing that, you may not need so much swap.

My experience is that if you have much more than twice the RAM size
swapped out, things start to get so sluggish that it is better to find
a way to reduce your memory requirement or provide more physical
memory.  Of course, that is characteristic of MY usage and may not
apply at all to your usage, but it may be the reason that 3xRAM is the
upper limit of what is routinely recommended.

> What do you guys do with swap space in this scenario ?

Provide what you think you will need.  It depends on what you expect
to be doing with your memory.  A busy mail server that will be using
huge amounts of temporary storage to manipulate the messages may not
need a lot of swap, simply because you might prefer to throttle
incoming mail when the system gets heavily loaded rather than get deep
into swap and slow the system down.

I configured my new 4GB servers with 4GB of swap.  That's more than I
expect to ever need, but I have oodles of disk space.   If you have
multiple drives, you may want to spread that out among the drives for
more efficiency (but to do a dump you need enough contiguous space on
one drive).

- Bob

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