[DRBD-user] allow-two-primaries (hot migration) and three nodes...

[Stéphane Rivière]

Hi Anthony, Hi Gianni,

At first thanks for your friendly advices.

And I confirm I'm new in today hot migrating fields :) More precisely 
I've done some successful experiments almost ten years ago with xen drbd 
and remus (true HA). Very resource-intensive but also an interesting 
experience.

I've carefully checked each migrating steps with "drbdadm status 
<drbd-volume>".

To my understanding and observations (feel free to correct me) :

- The parameter "allows two primaries" is just a right to use two 
primaries at the same time, not a constant state with two primaries on.

- I always only saw one primary at a time on the three servers.

- When I launch a "xl migrate <domu> <destination_server>" a xen/drbd 
script is executed to manage (among others things) the primary state on 
<destination_server> when necessary.

I confirm I don't have to "drbdadm primary <drbd-volume>" on 
<destination_server> before a "xl migrate <domu> <destination_server>"

I migrate many times a test instance to server 1 > server 2 > server 3 > 
server 1 > server 2 > again and again with, a the same time a simple 
process running inside the instance and outputting text on a console.

Le 12/12/2020 à 20:47, Gianni Milo a écrit :
> AFAIK the recommendation is to have dual primary mode enabled _only_ 
> during the live migration stage and once that's completed, one must 
> revert back to the single primary mode. Linbit folks have multiple times 
> warned against using _permanently_ dual primary mode for obvious to 
> me reasons.

Sounds logical. Very hazardous. I understand that a "drbdadm primary 
<drbd-volume>" is like a mount with r/w rights.

The management of drbd and migrations is done only with scripts. So I 
hope to have a correct, error-free process. It's a hope, not a certainty :)

-- 
Stéphane Rivière
Ile d'Oléron - France