[nylug-talk] if I have 64GB of RAM, how much swap do I really need?

[H. G.]

On 8/31/07, Peter C. Norton <spacey-nylug at lenin.net> wrote:
>
> On Fri, Aug 31, 2007 at 10:37:25AM -0400, Heow Eide-Goodman wrote:
> >
> > It's a nice box SunFire x2200, SATA HDs and 64GB of RAM.
> >
> > It'll be used as a in-memory DB server (hence the large amount of RAM),
> > but how much swap would you suggest?  Seriously, simply accessing 96 to
> > 128GB of disk-space will take _time_
>
> At work, we've been investigating the behavior of swap on solaris and
> linux, and there are some painful realities. The rule, especially for
> a database server, would be this:
>
> 1) Don't size the database to more than 85% of the memory in the
>    box. The kernel will need a lot of the remaining space to do
>    accounting for shared memory and similar things.
>
> 2) Make sure you don't swap. Swapping is OK for small processes, but
>    for any processes or memory region > than about 100mb, you're going
>    to incurr performance penalties on swapin. The behavior you should
>    expect is to be able to swap back in one page (4k) per revolution
>    of the disk as a worst case, with a realistic rate of about 10/rev.
>
> 3) Allocate as little swap as possible so something large that you
>    care about doesn't actually swap out.
>
> Swap design hasn't kept up with the groth into multi-gb processes, and
> is still generally based on page sizes that are too small to make its
> use effecient.
>
> Some guidelines a couple of years back was to have 1x swap on larger
> systems, and no more, just so that the VM could reserve that swap when
> memory was allocated, but you never actually want any of that to be
> used when you're running in production.
>
> -Peter
>
> --
> The 5 year plan:
> In five years we'll make up another plan.
> Or just re-use this one.
>
>
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:D He'll be needing one of these(1) for swapping. :D

1: http://techreport.com/articles.x/9312/1