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Re: Replication and HA - some basic questions
Posted by: Bernd Lentes
Date: March 23, 2016 04:52AM

Hi,

ok. Maybe my posting is too large. I will reduce it:


we have a web app which runs a MySQL DB and dynamic webpages with perl and apache httpd. Webpages serve reading and writing into the db. The db is important for our own work flow, so i'd like to make it HA.
I have two HP servers and will use SLES 11 SP4 64bit as OS. MySQL is 5.5.47. For HA i'd like to use pacemaker, which is available in SLES High Availibility Extension.
I have experience in linux, but i'm not a database administrator nor developer. HA is important for us, we don't have performance problems.
My first idea was to run the web app and the db in a virtual machine on the host and in case of a failure of one host pacemaker would run the vm on the other host. VM would be stored on a FC SAN. I stopped following this idea.
I have bought a book about HA: "Linux Hochverfügbarkeit" from Oliver Liebel. It's only available in german. But i can recommend it, it's very detailed and well explained.

He proposed to have two hosts, and on each is running a MySQL instance as master AND slave. But it's not a "real multi master solution", because pacemaker takes care that the IP for the web app just points to one master. So i don't have the multi-master problems with concurrent inserts (i believe).
His idea is that host A is master for the slave on host B, and host B is the master for the slave on host A. OK ?
Let's imagining that the IP to the web app points to host A, inserts are done to the master on host A and replicated to the slave on host B. Now host A has problems, pacemaker redirects the IP to host B, and everything should be fine.
What do you think about this setup ? Where is the advantage to a "classical Master/Slave Replication" ? How should i configure log-slave-updates in this scenario ? What is if the slave is not up-to-date ?

Let's imagine i have two hosts again: Host A is master, host B is slave. Nothing else. No real or pseudo "Multi-Master". IP points to host A. Host A has problems, pacemaker recognizes it, promotes B to master and pivot the IP. Everything should be fine. Where is the disadvantage of this setup compared to the "Multi-Master Replication" in the book ? The OCF ressource agent for mysql should be able to handle the mysql stuff and the RA for the IP pivots the IP.
What is if the slave is not up-to-date ?

Bernd

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