[colug-432] Swap science, was: Install and Manually Partition Debian

[R P Herrold]

On Sat, 15 Mar 2014, Rob Funk wrote:

> I don't know what Debian's default is these days, but I 
> generally recommend a big swap combined with /tmp 

Why as to a large swap?  In the former times a swap 2X 
physical ram 'ruled the roost'.  More recent lore is 'if 
you are into swap', you are paying a high price for r/w 
speeds onto spining rust

We have been testing SAN, and adding an invisible cache layer 
of SSD 'just makes sense', save that TANSTAAFL, and that layer 
is not free ;)  Seagate also offers a drive with 64G of front 
end SSD, in front of the spinning platters.  I ordered a 
clutch of these today, for production on an NFS (and 
managed iscsi / LVM) server.  I'll be pulling numbers off to 
see how this affects compile times (I save stats on all 
builds, and will be able to see if / how it helps/ hurts, 
from some log reduction)

A lot of computer science is invalidating caches well, and the 
'self learning' tactic which the Seagate device implies may 
work well in my Linux environment, or it may fail 
conspicuously.  We'll see ...

Last week, I was asked swap sizing strategy, but did and do 
not know of a CURRENT and evidence based 'general answer' -- 
with VM's from the Hypervisor perspective, we KNOW from 
statistics reduction, that our choke point is drive RW, and so 
avoiding swap just makes sense.  VM's would pay that penalty 
as well

The rule, from the times of the Medes and the Persians, is: 
'If you cannot put numbers on it, one is being offered an 
opinion, and not science'

Does anyone have some recent 'science' as to swap sizing 
considerations?

-- Russ herrold