[DRBD-user] Suggestion for DRBD User's Guide

Florian Haas florian.haas at linbit.com
Mon Mar 17 09:27:16 CET 2008

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


On Wednesday 12 March 2008 17:03:25 Whit Blauvelt wrote:
> That's reassuring! I haven't set the syncer rate at all. I followed the
> draft manual advice at www.drbd.org/users-guide/s-configure-resource.html,
> so my drbd.conf has no more in it that your "Example configuration" there.
> Maybe that's not a very useful example?

It is a *simple* example, and that is the intention. It is by no means an 
example that is expected to provide optimal performance. It is meant to 
illustrate a fairly minimal configuration required to get an example resource 
up and running.

> The sequence in the manual goes through doing the initial sync at
> www.drbd.org/users-guide/s-initial-full-sync.html without any reference to
> setting the syncer rate. It might improve the manual to advise reference to
> www.drbd.org/users-guide/s-configure-syncer-rate.html before starting the
> initial sync - or else have DRBD run with a more realistic default syncer
> rate when its not specified.

Well guess what, we might also advise to tune max-buffers, max-epoch-size, 
unplug-watermark, sndbuf-size and a half-dozen other settings, then to tune 
your SCSI subsystem, RAID level, I/O scheduler and so forth, all to match the 
performance characteristics of the system's hardware, because all those might 
affect your effective resync throughput. If it were Lars instead of myself 
writing this, the list would fill 3 more paragraphs. See my point?

What I do acknowledge though, is that the guide wasn't clear enough about what 
affects and limits your effective syncer rate. I hope to have made that 
clearer now; please read the items marked "Important" and "Tip" at 

> Some people read a whole manual closely before doing anything. Others of us
> assume that for a simple setup, following the steps as they're given should
> produce a usable first approximation. Since the manual's a draft, hopefully
> it can still be made better for the second sort of user.

The second sort of user does get a "usable first approximation" when following 
the example config. The second sort of user is also unlikely to read the 
information that will get him beyond that stage even when it is available, as 
you have so convincingly proved. ;-) 

> I'll reset the syncer rate. Thanks.

Good call.

What has also been added to the guide is a (trivial) formula for calculating 
the expected resync duration; see 


: Florian G. Haas
: LINBIT Information Technologies GmbH
: Vivenotgasse 48, A-1120 Vienna, Austria

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