[DRBD-user] DRBD terrible sync performance on 10GigE

Harald Dunkel harald.dunkel at aixigo.de
Fri Dec 11 09:04:28 CET 2015

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Hi Lars,

On 12/04/2015 05:04 PM, Lars Ellenberg wrote:
> You are not supposed to disable the resync controller,
> you are supposed to correctly use it.
> https://blogs.linbit.com/p/443/drbd-sync-rate-controller-2/
> ... you should:
>     * set c-plan-ahead to 20 (default with 8.4),
> 	or more if there’s a lot of latency on the connection (WAN
> 	link with protocol A); or less, if you want to have it
> 	react faster to changes
>     * leave the fixed resync rate (the initial guess for the controller)
>       at about 30% or less of what your hardware can handle;
>     * set c-max-rate to 100% (or slightly more) of
>       what your hardware can handle;
>       (the default is 100M, which was the effective limitation in this case)
>     * set c-fill-target to the minimum (just as high as necessary)
>       that gets your hardware saturated, if the system is otherwise idle.
> Respectively, figure out the maximum possible resync rate in your
> setup while the system is idle, then set c-fill-target to the
> minimum setting that still reaches that rate.
> And finally, while checking application request
> latency/responsiveness, tune c-min-rate to the maximum that still
> allows for acceptable responsiveness.
> You may need to adjust max-buffers and/or tcp send/receive buffer
> sizes as well.

Sorry to say, but this is way too complex for an ordinary user
(speaking just for myself, of course). It gets even more complex
if you have several drbd resources. The man page doesn't help in
this case, either.

How about some recommendations with real numbers?

BTW, the man page for version 8.9.4 mentions that the default for
c-plan-ahead is 0. If I got this correctly this means that the
dynamic resync speed controller is disabled by default. Is this


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