Thanks for all replays.

I did a small test, here are the settings :
- FB Super Classic 2.5.1 64 bit
- Win7 pro 64 bit
- Database size about 640mb
- Select something like this :
select a.field1, b.field2, sum(c.field3)
from table1 a, table2 b, table3 c
where ....
group by ....
- temp directory set to disk
- temp cache set to 64mb

Here are the options :
1) Database put on a disk. Page Size 16KB, DefaultDbCache 8192.
Execution time is about 1 min.
2) Database put on a RAM. Page Size 4KB, DefaultDbCache 0.
(Although DefaultDbCache set to 0, flamerobin indicate the value of Page
Buffer = 50)
Execution time is about 35 second.
3) Database put on a disk. Page Size 16KB, DefaultDbCache 0.
Execution time is more than 2 min.

During each test, I reboot the comp.

Option 1 consumed 128 MB memory per connection, while option 2 consumed 200KB per connection and 640MB for database file.

So, I guess, by having database on RAM (if possible), is the top tweaking possible regarding file I/O.

Anto