iowait 理解

1 What exactly is "iowait"?

'iowait' is the percentage of time the CPU is idle 
AND there is at least one I/O in progress.

If the CPU is idle, the kernel then determines if there is
at least one I/O currently in progress to either a local disk
or a remotely mounted disk (NFS) which had been initiated
from that CPU. If there is, then the 'iowait' counter is
incremented by one. If there is no I/O in progress that was
initiated from that CPU, the 'idle' counter is incremented
by one.

%iowait = (time  cpu is  idle but have I/O in process) / (all cpu time)

2、iowait 有时候是有意义的，有时候没有意义。

2.1  优化磁盘I/O，更换更快的硬盘 -- 降低 iowait

Example 1: 
Let's say that a program needs to perform transactions on behalf of
a batch job. For each transaction, the program will perform some
computations which takes 10 milliseconds and then does a synchronous
write of the results to disk. Since the file it is writing to was
opened synchronously, the write does not return until the I/O has
made it all the way to the disk. Let's say the disk subsystem does
not have a cache and that each physical write I/O takes 20ms.
This means that the program completes a transaction every 30ms.
Over a period of 1 second (1000ms), the program can do 33
trans