Basic IO concepts

Input from and output to disk (IO) involves your application interacting with the operating system to access the disk device. Since any system call actually interrupts your program (your process stops running for however-long), IO impedes performance to some extent. The system provides some assitance in lessening the impact but you should consider IO as something to used thoughtfully, particularly on the VPP.

Data transferred by IO handled in records. Each read or write handles at most one record (sometimes can handle