From the postgresql docs
[full page writes are] needed because a page write that is in process during an operating system crash might be only partially completed, leading to an on-disk page that contains a mix of old and new data. The row-level change data normally stored in WAL will not be enough to completely restore such a page during post-crash recovery. Storing the full page image guarantees that the page can be correctly restored...
How come the WAL logs are not enough to do a full restore? My understanding is that they contain all page updates since (atleast) the last checkpoint.
Can someone give an example of how simply replaying the WAL logs from the last checkpoint could still result in data loss?
