Re: Tuplesort merge pre-reading

On 12/09/16 12:16, Gavin Flower wrote:
[...]
>  two blocks would be logically adjacent (which means they are likely 
> to be physically close together, but not guaranteed!).
>
[...]

Actual disk layouts are quite complicated, the above is an over 
simplification, but the message is still valid.

There are various tricks of disc layout ( & low level handling) that can 
be used to minimise the time taken to read 2 blocks that are logically 
adjacent.  I had to know this stuff for discs that MainFrame computers 
used in the 1980's - modern disk systems are way more complicated, but 
the conclusions are still valid.

I am extremely glad that I no longer have to concern myself with 
understanding the precise low stuff on discs these days - there is no 
longer a one-to-one correspondence of what the O/S thinks is a disk 
block, with how the data is physically recorded on the disc.


Cheers,
Gavin