Unbuffered writes means that you are not using any of the operating systems file systems buffering techniques and writing the the raw file stream straight to the hardware.
The larger the block size means that you are making less frequent calls for data to be written.
One way to look at it is by comparing to money.
In your left hand you have a pile of coins, the block size is the denomination of the coin, a small block size is a penny, a large block size is a $1 coin.
You have to put $10 dollars into a machine. Which will do it quicker pennies or dollar coins?
Now, introduce a purse/wallet, that it your buffer, if you turn buffering on, each coin must be put into the purse before it can be then taken out and put into the machine.
As you are working with a large file and large block sizes, buffering doesn't help you. If you were using small files with small block sizes, the buffer allows the slow hard disk time to process each file and provides the faster software to offload its work to keep it moving.
Does that help?
Look at my two articles and you will see real world examples of the effect of block size;
QNAP NAS Memory Upgrade, Hardware Change and Performance Benefits[
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The Hybrid Alternative - Did it make a difference?[
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If you look at the graphs, the block size is along the X-axis, and in both benchmarks the buffering is disabled.
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