Improve configurability for IO related behavoir

[浩辰 何 <hehaochen@xxxxxxxxxxx>]

Dear developers:

I have some suggestions on PostgreSQL's IO behavior. I am not sure my opinions are right. Many thanks for your time!

It is documented that :

wal_sync_method​ (enum): The open_*​ options also use O_DIRECT if available.

Shall PostgreSQL consider making O_DIRECT configurable? In MySQL, innodb_flush_method​ can change if use  O_DIRECT or not. 

I made some benchmarking on O_DIRECT of MySQL, and I find that sometimes, using O_DIRECT may hurt performance. 

https://ibb.co/BVWnwnh

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Comparing the blue (buffered IO) and green (direct IO) lines, I find buffered IO is faster. So I think having such parameter in PostgreSQL 
is reasonable.

On the other hand, wal_sync_method​ only controls how WAL is written to devices, while for data file I notice that PostgreSQL uses 
sync_file_range(2)​. So shall we also make it configurable? I also find that in some systems, open​ with O_SYNC is much more faster
than write​ + fsync​ :

https://ibb.co/f1VsCC1 

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Furthermore, the results above are also related to IO API supported by OS. MySQL support synchronized IO and Linux libaio​. It seems

that PostgreSQL only supports synchronized IO, so shall we support more IO engines? like io_uring​ which is very popular in recent years.

Thanks & best,

Haochen