I don't get still.
Suppose we have a data file with blocks with important (non-empty) data:
A B C D
1. I call pg_start_backup().
2. Tar starts to copy A block to the destination archive...
3. During this copying, somebody removes data from a table which is situated in B block. So this data is a subject for vacuuming, and the block is marked as a free space.
4. Somebody writes data to a table, and this data is placed to a free space - to B block. This is also added to the WAL log (so the data is stored at 2 places: at B block and at WAL).
5. Tar (at last!) finishes copying of A block and begins to copy B block.
6. It finishes, then it copies C and D to the archive too.
7. Then we call pg_stop_backup() and also archive collected WAL (which contains the new data of B block as we saw above).
The question is - where is the OLD data of B block in this scheme? Seems it is NOT in the backup! So it cannot be restored. (And, in case when we never overwrite blocks between pg_start_backup...pg_stop_backup, but always append the new data, it is not a problem.) Seems to me this is not documented at all! That is what my initial e-mail about.
(I have one hypothesis on that, but I am not sure. Here is it: does vacuum saves ALL deleted data of B block to WAL on step 3 prior deletion? If yes, it is, of course, a part of the backup. But it wastes space a lot...)