Question: It's an InnoDB database. The query cache has 1.2GB in size with 300k queries in the cache. Most of the cached queries are related to 3 major tables (260MB data length plus 600MB index length altogether). Everytime I tried to insert new entry or update existing entries in these 3 tables, MySQL slowed down a lot just like the server is dead. I got "Timeout" if I open my website in browser.

I check the Munin stats, it shows 95% of the query cache are cleared after the operation so I guess the sudden overload was caused by inserting entries to heavy-read tables. And during the overload, Munin just shows a white strip on the graph so you may get an idea on how bad the situation was.

How can I optimize this?

Answer: 
You can try iperf, it works as a client and server so you need to install it on two computers, here's how you use it

run on Server 

```
iperf -s

```

run on client

```
iperf -c <ip of server>

```

hope that helps,
RayQuang
Best practice is to have the cables certified from the person installing it. YOu pretty much need special equipment for doing so - then you get exact measurementy that your cabling adheres to a specific standard. SHould have bee done at install time and you should have gotten the measurement sips that the tester prints out.