I want to know how many bytes per second can I write to the disk and read from it.
How can I do that on linux machine?
Use a benchmark tool like bonnie(++). It's easy to install on about every distribution, and since it measures different aspects, you get quite a good picture how the system performs in a given situation.
If you just want to use basic tools, you could use dd:
For write speed:
(The product of bs and count should be at least twice your RAM size)
For read speed:
Remember that this is a very rough estimate and measures a situation that is unlikely to occur in normal operations.
Josh Berkus gave a pretty detailed talk at pgCon 2009 on performance tuning; the first half or so is just dedicated to measuring disk I/O and solutions. It's big and long, but you only need to watch it once to get an idea of what sorts of things to think about.
Also take a look at the benchmarking video, which covers much of the same content.
I would just use hdparm to measure the read speeds of the drives:
you can test read speeds on formatted drives with data on it but be careful with writing as if wrongly used can corrupt data.
Hope that helps, RayQUang