Cluster performance is going to be limited by the "weakest" machine hosting a data node (the "bottleneck" effect that Mikael spoke of). This is *especially* true with regard to memory -- all nodes are limited to the amount of RAM on the machine with the least amount of memory. In other words, if you have 3 machines running data nodes each having 1GB RAM and a fourth with only 500 MB, then *none* of the data nodes may use more than 500 MB each for storing Cluster data. This is a direct effect of MySQL Cluster's architecture, and there's really no way round it other than to add more memory to the "weak" machine.
I don't believe this is true to the same degree with regard to processor speed, but it is likely to be a factor as well, since the replication of data between Cluster nodes is synchronous. If you've 3 machines with 2 GHz CPUs and one with only a 400 MHz, the host with the 400 MHz processor is definitely going to exert a "drag" on the others because it can't process data as quickly.
Another thing to consider is that MySQL Cluster is intended to provide high availability and not necessarily high speed.
In addition to increasing the available RAM and using faster CPUs, you will get better performance with faster networking between data nodes -- you'll obtain faster response times with gigabit Ethernet than with 100 Mbps, and even better performance using the SCI protocol. However, Cluster is not as fast as MyISAM. Processor and RAM are not particularly critical for management or even MySQL servers, but for the nodes actually storing the data, the more you have, and the more evenly CPU and memory are distributed between data node hosts, the better the performance is going to be.
Re MySQL 4.1 vs. MySQL 5.0: That's correct, we're not likely to be introducing any new features in 4.1 at this point (none that I'm aware of are planned) and future 4.1 releases will be focused on bug and security fixes -- the new features and other real improvements are going into 5.0.
Re PHP: I would seriously recommend switching to PHP 5 since you're already using MySQL 4.1 -- then you can use ext/mysqli, and so take advantage of a native object-oriented API, prepared statements, multiple statements, and more secure client authentication. And of course, PHP 5 provides much better class and object support, introduces exceptions, has new and improved XML capabilities, etc. I would recommend migrating to PHP 5 whether or not you're going to use MySQL Cluster.
MySQL Documentation Team @ Oracle
Orlando, Florida, USA
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MySQL Server Documentation