Question about "PBO"

I found the “PBO” ext can speed up the I/O transfer rate,it is help of in my app,but I also have some questions:
-If I allocate many “PBO” objects(every pbo is 1024x1024*4 bytes,and allocate 10 or more objects),will it hurt the system performance?

-Can I write data to pbo in the thread that does not create it(means can I use “glMapBuffer”,“glUnmapBuffer”,“memcpy” in different thread)?If it is,does the different pbo can be written data in parallelly?

-Does the pbo’s data write & read can be executing in parallelly?