Do checkpoints flush all data from shared buffers ?

Hello,
    I was wondering if each checkpoint would flush all transactions from
the shared buffers or if there could be some left at the end of the
checkpoint ?
Because i was trying to lower I/Os of the disks, i got the checkpoint
timeout lowered to 150 seconds so i get twice the number the checkpoint.
I was hoping it would reduce the number of  I/Os on the disks because
there would be less data to write in datafiles...

Thanks in advance for your help

regards,
             Fabrice Franquenk