[DRBD-user] remote buffer cache

Thilo Uttendorfer tuttendorfer at linux-ag.com
Fri Nov 2 10:58:00 CET 2012

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,

Am Donnerstag 25 Oktober 2012 11:24:05 schrieb Lars Ellenberg:
> On Tue, Oct 23, 2012 at 11:03:14AM +0200, Thilo Uttendorfer wrote:
> > Hi,
> >
> > the man-page of drbd.conf says about protocol B "...completed, if it has
> > reached local disk and remote buffer cache".
> >
> > How can I determine the size of the remote buffer cache? Is this cache
> > also limited by the "max-buffers" option?
> DRBDs max-buffers settings has something to do with it,
> but that does not change between DRBD protocols.
> There are pictures somewhere in those "papers" linked from the
> documentation page. iirc.

thanks for the explanation!

What I'm interested in is a setup with 2 different systems: One with a very 
fast SSD and one with a much slower regular disk. I understand protcol "B" 
like that: If you write lots of data on the fast (drbd primary) system 
the "remote buffer cache" on the slow (drbd secondary) system will be filled 
because writing to the local disk will take longer. If this cache is full, 
the write performace will decrease on the primary. Reading from disk will 
always be fast because the slow system is not involved.

Is my understanding of protocol "B" correct? Now, to decide if this setup 
makes sense, it is important to know how big the "remote buffer cache" is or 
how I can increase it. Can you give me a hint how to determine and/or change 
the size of the remote buffer cache?

Thank you very much,

Thilo Uttendorfer
Linux Information Systems AG
Putzbrunner Str. 71, 81739 München

Fon: +49 89 993412-11, Fax: +49 89 993412-99
tuttendorfer at linux-ag.com, http://www.linux-ag.com

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