fsyncing data to disk

Hi,
this is not exactly a Postgresql question, but an input from hackers
list like this would be invaluable for me.
I am coding my own database engine, and I decided to do not implement
transaction engine because it implies too much code.
But to achieve the Durability of ACID I need a 100% reliable write to
disk. By design no record in my DB will be larger than 512 bytes, so I
am using the page size of 512 bytes, that matches the size of the disk
block, so every write() I will execute with the following fdatasync()
call will be 100% written, is that correct? It won't make a 300 byte
write if I tell it to write 512 and the power goes off or will it? I
am going to use the whole partition device for the DB (like /dev/sda1)
, so no filesystem code will be used. Also I am using asynchronous IO
(the aio_read and aio_write) and I don't know if they can be combined
with the fdatasync() syscall?

Will appreciate your comments

Regards

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The power of zero is infinite