[DRBD-user] questions about protocol c

Adam Goryachev mailinglists at websitemanagers.com.au
Tue Nov 1 23:25:58 CET 2016

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

On 02/11/16 00:44, adream wrote:
> hi guys
>     I'm curious about implementation of drbd's protocol c, is there 
> any document describe the detail of the protocol c ?
> giving an situation,
> I have two node, node A works as primary, and node B works as 
> secondary, the drbd works in protocol c.
> then I write some date in node A, for example i vi create file named 
> t.txt, and write "hello drbd" into the node A.
> In the node B, the drbd has just created t.txt and receive "hello" 
> from node A, then some terrible thing happened, the the connections 
> between node A and node B broken, and node B will never receive the 
> rested message.
> so in this situation, if i mount the drbd in the node B, what will i see?
> a t.txt file with "hello " message, or nothing, or something else?
> looking forward to any reply.
> thanks

You mean like in the documentation:
Or in the source code?
Either way, I think this is pretty well documented, but I'll try and 
explain further by example...
In your scenario, if you create a file (touch t.txt) and then write 
"hello drbd" into the file (echo "hello drbd" > t.txt) then you have 
three possible outcomes. Either Node B will not have the file (if the 
crash happened before your touch command returned the success message), 
or it will have an empty file if the crash happened after the touch 
command returned the success message and before the echo command 
returned, or it will contain the content "hello drbd" if the crash was 
after the echo command completed.

So, whatever Node A says has been done, will have been done on Node B. 
In actual fact, this would be the same for Protocol B as well (assuming 
by crash you mean Node A vanished at that instant).

Hope that helps...


Adam Goryachev Website Managers www.websitemanagers.com.au

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