Apparent strange disk behaviour in 6.0

[Julian Elischer]

Julian Elischer wrote:

>
>
> I've been playing around with some raid arrays.
> I've notived some odd things.
>
>
[stuff]

Ok I've done some researching..

it APPEARS that teh system is swapping out running programs in order to 
store more write data!

experiment:
boot to single user mode.
type:
mount {big partition}
dd if=/dev/zero bs=128K of=/$bigpartition}/bigfile count=1000000

notice that after a short while your dd is killed because the system is 
out of swapspace.
(it doesn't have any)
Why the F*ck does it need swapspace.? there are exactly 2 proceses 
running in userspace
and one of them s in wait4(). dd shows a resident size of about 170KB
leaving about a GIGABYTE of unused RAM.

The system should make dd wait rather than trying to swap its pages out..


if you then do
swapon (your swap device)
and repeat teh command in the background,
vmstat 1 will show you pages being faulted in and out...
no WONDER IO goes to hell in a handbasket..

Outgoing IO should never be able to force running programs out!
It should start re-using old pages from the same file!

4.11 gives a consinstent 65MB/sec with this array, for as long as I run it..
6.0 gives me 65MB for 15 seconds and then it drops to 20MB/sec and then 
10MB/sec
and the swap disk bursts into life.

the array goes from all the lights solidly on, to bursts of activity 
with large gaps in between them.