WAL Performance Improvements

Hi,
I've attached a patch which should improve the performance of WAL by
reducing the fsync time
and write time by 50%(if OS page size is 4k) , if the transaction
generate the WAL data less then 4k. Instead of
writing every time 8k data in to the WAL file it will write only the
portion of the data which
as changed from the last time(Example : if transaction generates 150
bytes of WAL data ,then it writes
only 150 bytes instead of 8k).

please apply for 7.3.

Regards
jana