Writing a Tool to simulate non-sequential disk I/O (simulate db file sequential read) in C POSIX

I have over the years come across the same issue a couple of times, and it normally is that the read speed on SAN is absolutely atrocious when doing non-sequential I/O to the disks. Problem being of course that most databases will be doing non-sequential I/O to disks, databases most common read process is db file sequential reads, which would not cause a sequential read of the actual blocks on the device.

My second issue is that it is normally tricky to segregate the different processes enough to be able to clearly test or even show the exact issue with the non-sequential reads and writes, so I end up in very lengthy discussions about possibilities of changing computing theory rather than actually changing the SAN to be able to handle these type of requests, and while that allows for very creative use of similes it is not very efficient use of my time, and I really have little need for more overtime.

So my thought was, how would one go about writing a utility that takes a file that opens a large file and read random blocks of data throughout the file, that way simulating the same effect in a controlled environment.

The general layout I was thinking is

Input for program

name [file to read] [block size] [number of reads]

set block size
set number of reads

get file size

open file

for I << number of reads
set random block address
read random block address from file

close


My problem is, how would I go about reading a random block address from a file.
And is there any way to get the time in milliseconds the operation took

And the POSIX bit, basically the systems I need to use this code on are locked down pretty heavily, and installing a new compiler is a couple of months worth of work, so I want a tool that will be able to be compiled on almost any old compiler.