[MacDV] Re: how and why to benchmark

[Peter van der Linden]

On Jan 8, 2004, at 11:31 AM, Jan E. Schotsman wrote:
> I find it hard to believe there is no relation between sector numbers
> reported by a controller and actual position on disk. Surely it is
> important for disk performance that a controller tries to realize a 
> request
> for contiguous disk space as a set of sectors that is close to each 
> other
> in some sense? Please explain, Dmytro.

Jan -- here is another way of thinking about it that may help.
Inside an individual MacOS (Unix) process, the address space runs 
sequentially from 0 to 2^N -1.  However, there is no relationship 
whatsoever between the addresses used in your program, and the actual 
physical addresses assigned by the kernel.  Nor can you see those 
actual physical memory addresses in your program.  The kernel has 
virtualized the address space.

You understand and agree with the foregoing, right?   Now read it again 
making these substitutions:
"address" = disk sector
"kernel" = disk controller
"program" = kernel's view of disk layout.

This is why programs that draw pretty pictures of disk blocks are at 
best misleading.

The disk tutorial I mentioned is at
http://www.acmqueue.org/modules.php?name=Content&pa=showpage&pid=46

     Peter