[DRBD-user] 40MBps Sync Limit 8.4.4 and Above

[David Mohr]

On 2014-11-19 14:29, Matt Kereczman wrote:
> On 11/18/2014 09:44 AM, Darren Ginter wrote:
>> Hello all,
>> 
>> With Ubuntu 14.04 (drbd 8.4.4), I cannot get more than 40MBps out of 
>> the
>> initial DRBD initialization sync no matter what settings (sync-rate,
>> c-max-rate, c-min-rate, etc).  I've been playing with this for tens of
>> hours now.  I've tried drbd 8.4.5 and above to no avail.
>> 
>> With Ubuntu 12.04 (drbd 8.4.3) I can max out a 10GbE NIC without issue 
>> on
>> the same hardware.
> 
> You may need to tune 'max-buffers' and 'max-epoch-size' to get some 
> more
> performance out of the secondary node. This is outlined in the user's
> guide (section 15.3.1).
> 
> I have the values below configured on a system running DRBD 8.4.5,
> resyncing at ~400MiB/s (almost the max write speed of the backing 
> disk):
> 
> resource <resource> {
>   net {
>     max-buffers 80k;
>     max-epoch-size 20k;
>     ...
>   }
>   ...
> }
> 
> Those settings are a little extreme; start at 8000, and tune 
> accordingly.

I'd suggest to add this info as a note to the docs @ 
http://www.drbd.org/users-guide-emb/s-configure-sync-rate.html .

Yes, the drbdsetup man page already explains that these parameters may 
be important when you can't saturate link speed, but finding them in the 
first place is not as easy as it should be IMHO.

Thanks,
~David