Re: 6.0 VFS/VM over-runs. Was: Apparent strange disk behaviour in 6.0

I thought the original description was good..

file data written to cache to await output to disk is allowed to grow to 
the extent that running programs are swapped out.
(or at least paged out).


here's a simple experiment that takes 1 minute to show the problem.
Best with a system with 2 disk drives (or a disk and a raid in my case)

boot to single user.

mount a large partition  that has some room.. hopefully several times 
the space of all RAM.
.

do:

dd if=/dev/zero of=/${mountpoint}/bigfile bs=128k count=1000000 & iostat 1


notice that dd dies with "not enough swapspace" after some number of 
seconds.
(there is no swap attached).  also notice that until the crash, IO speed 
was max for the
disk.

then add swap and do it again.

notice that at teh point where dd previously crashed, the swap disk
springs into life and disk IO on your main disk goes into the toilet.
notice that dd gets swapped out. (!!!!)

when the disk recovers enough, dd pages back in and creates more IO but 
in teh mean time there is no IO
going on to the disk.. (watch the LEDs).



Scott Long wrote:

> Julian Elischer wrote:
>
>> This is an absolute show stopper for 6.0
>>
>
> Can you distill this down to a problem description that we can track?
>
> Scott