Re: fr1 speed calculation tricks...

"Also sprach denis bonnenfant:"
> First, I set speed_limit_min and max to respectively 1 000 000 and 2 000 000.
> this way, i can test the real speed of the link during reconstruction.
> I forced full reconstruction of nda, and got a speed of 38 000 KB/s, with a 
> system load of 8!

Well, the load is just the number of processes waiting in i/o, so that
probably means 4 daemons, or servers, or something like that.

The "real speed" will probably be different from what you think, because
on write enbd can skip writing blocks that are/will be the same both
sides. Measuring read and write separately may show you the effect.

> So I set min to 1000, hoping that md will go down to this limit. No effect !

It's a minimum - when raid sees the speed is below that it increases
the priority of the daemons, so as to bring the speed up again,
hopefully.

> I set max to 10 000, this time it works, speed fall to this value, and load fall 
> to 1. Setting min value seemed to have no effect on real reconstruction speed, 
> weird!

I don't think you'd expect it to. You can look at the throttle code in
md.c to see how it works.

> Then i try the same test with an intelligent resync of a clean disk :
> 
> set min to 1000, max to 10000. Reconstruction speed falls to 1000 ! I raise max 
> to 1 000 000, no effect. I raise min to 1 000 0000, speed goes to this large 
> value !

That's right.


> So it looks like
>   - max is corresponding to the maximum speed to be set for true reconstruction,

That's what it means! The raid code will insert a wait when speed goes
above it. That will likely slow the resync down :-).

>   - min is corresponding to the minimum speed for intelligent resync.
> So min must be set much higher than max

Well, yes. Priority of resync thread is increased when speed falls
below here.

> Am I right ?

Well, it's not really a mystery! The throttle loop is in md.c.


Peter