[DRBD-user] slow sync speed of 12TB over 10GE

Lars Täuber taeuber at bbaw.de
Tue Jan 13 15:41:14 CET 2009


Lars Ellenberg <lars.ellenberg at linbit.com> schrieb:
> On Mon, Jan 12, 2009 at 04:30:03PM +0100, Stefan Seifert wrote:
> > But I'm curious: why does it hurt? A naive view would be that it won't help, 
> > but at least not hurt, since it would just not throttle anything. Or do you 
> > mean, that it will hurt normal operations which is quite understandable?
> 
> because the way it is implemented.
> sort of like this:
>   N = sync_rate / 32 kB * 0.1 second.
> that much requests (32kB each) will be requested in one batch. (ok, tried...)
> so if you request much more than it can handle,
> it may stumble over its own feet.
> 
> it is really meant only as a throttle.
> 
> if you don't want it to throttle,
> set it _slightly_ above the physical limit.
> 
> if you set it too large, it hurts.
> how much it hurts depends largely on the io backend,
> it may be quite noticeable, or only measurable.
> but it hurts.

This should go into the docs IMHO.

Lars



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