What should be the ideal number of parallel java threads for copying a large set of files from a quad core linux box to an external shared folder? I can see that with a single thread it is taking a hell lot of time to move the files one by one. Multiple threads is improving the copy performance, but I don't know what should be the exact number of threads.
I am using Java executor service to create the thread pool.
Just try different variants and measure performance. There are also such bottlenecks as HDD and network speed, so I guess there is no definite answer.
Absolutely you should work this out by testing. As Dmitry points out, there are many factors involved. In fact, CPU is almost certainly not the bottleneck with an IO- and network-based operation. You'll probably find things level off and start to get worse before you have too many threads, but if you want to minimise testing, use a graph of a few results to interpolate a good guess for the ideal value.
To get a better understanding of what's taking all the time, use metrics tools to measure utilisation of your resources - in this case that should include network interface traffic and disk queue length.