[DRBD-user] tuning DRBD

[Lars Ellenberg]

On Thu, Jun 07, 2007 at 10:46:31AM -0700, Ben wrote:
> Does anybody have any advice for tuning DRBD 0.7? I'm not seeing much  
> out there except for some un-explained bullet points for a hacked  
> version for use with OpenSSI, and I don't know how much of that  
> advice is due to assumptions that are only true for OpenSSI. Some  
> specific questions I have are:
> 
> 1. What are the tradeoffs with changing al-extents?

smaller al-extents
	* more frequent meta data updates (synchonous sector writes), 
	  introducing more frequent maximum latency writes.
	* smaller area to resync after primary crash, 
	  less mimimal resync time.

> 2. Can I eck out more throughput by increasing sndbuf-size? Does that  
> increase my write latency when using Protocol C?

most likely not.

> 3. What's a reasonable formula for determining max-buffers? Does  
> increasing them imply I should increase something else too?

if you go too high, you should increase your physical RAM :)

but don't tune that up too high.
that number is basically in "pages", iirc, so if you compare
(max-buffers * 4k * number of drbd's in use) to your physical RAM,
it should be some not too impressive number.
or you might say hello to the oom killer :)


> 4. Does anybody know if the Areca raid cards like high or low unplug- 
> watermark settings?

sorry. you have to try...

> 5. Does decreasing max-epoch-size reduce my write latency?

most likely not.

> Does anybody have any experience playing with the schedulers in the
> 2.6 kernel?

yes of course.
but the answers here depend very much on what you specifically ask for.

-- 
: Lars Ellenberg                            Tel +43-1-8178292-0  :
: LINBIT Information Technologies GmbH      Fax +43-1-8178292-82 :
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