Can someone tell me if this is a High Performance Compute or High Available mySQL cluster?
There is a picture of the setup.
This is part of the config.ini they talk about
[ndbd default]
NoOfReplicas=2 # Number of replicas
Is it correct understood that NoOfReplicas determines if I have a HPC or a HA cluster?
With a setup like the one pictured on that page you get HPC and some elements of HA. If one of the NDBD servers fails, the SQL servers will connect to another and usually continue without intervention. If one of the SQL servers fail the application has to know to connect to another SQL server (or there needs to be a layer of abstraction) in order to maintain HA.
Replicas are for redundancy across servers, which in-turn may provide higher availability. The number of replicas directly correlates with the number of servers in a partition that may fail without bringing MySQL Cluster offline.
Partitions are to split processing across servers, which in-turn may provide higher performance. The number of partitions directly correlates with the number of servers that queries to MySQL Cluster will be divided between.
Let me see if I can make this clear. If you have two servers with two replicas, write performance might be reduced because the data has to be replicated. In both cases, with either a single replica or two replicas, read performance might be enhanced because there are two partitions (two servers) to split the query across.
MySQL Cluster does not provide redundancy of the MySQL Server front-end, so it is not an out-of-the-box, instant HA solution. You will need either application logic to locate an available SQL server, or you will need service monitoring / network logic to switch out a failed SQL server with one that functions. (You could also try installing the MySQL Server front-end directly on the application servers.)
Also, performance hits or benefits from MySQL Cluster will vary based on how your application is using the database, and what bottlenecks exist. Switching from local sockets to network connections will increase latency. Simple queries for cached data will bottleneck at the SQL server. On the other hand, some processing that is either bound by the processor or my disk I/O will benefit heavily. The best way to know the exact performance hit or benefit that MySQL Cluster will have on your application might just be to test it.
First of all, most HPC clusters will contain some aspects of HA (the converse is not true, though). That's because to work faster, each node has to work without much coordination with other nodes; so it typically there are many pieces that can fail without stopping the others.
But in the end, i'd say that MySQL clustering is mostly intended for HPC, not HA.
