A doubt w.r.t WAL

Hi,

I have a doubt regarding WAL.

Let's say I have only one wAL segment of 16MB and in a single transaction I
put 20MB of data, say a text file dump inside a transaction.

How does WAL handles this situation from POV of using/reusing WAL space and
ensuring consistency of data in table?

Does it record committed/uncommitted transaction id in clog and safely start
pushing data into data files?

 Shridhar