I would say caching, on multiple levels (CPU, DB, File System). By splitting at least some of the load, it's possible for parts of the cache to become old and get flushed. When everything is on one machine, the box has a complete picture of the traffic patterns and can optimize better.


On Jul 31, 2007, at 8:17 AM, nigel wood wrote:


A puzzler for you guys...... all plausible explanations (and suggestions for proving them) gratefully received.

We run several MySQL database servers in the traditional master- slave configuration and attempt (rather poorly) to spread select queries between them. Normally the slave gets 1/3 of the master load. Both machines have identical configurations, hardware specifications and network connectivity. The main clients of these databases are PHP websites without persistent connections. A fail- over pair of machines in a separate building replicates from the master.

Today (as a result of replication failure) we directed all the traffic normally sent to the reporting server back to the master server adding a 1/3 to its load. Several areas of the websites got FASTER afterwards and I'm currenlty at a loss to explain why.


Nigel Wood

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