From: pgsql-hackers-ow...@postgresql.org > [mailto:pgsql-hackers-ow...@postgresql.org] On Behalf Of Amit Kapila > I think it beginning of segment (aka the first page of the segment), even > the comment indicates the same. > > /* > * Whenever switching to a new WAL segment, we read the first page of > * the file and validate its header, even if that's not where the > * target record is. ... > .. > */ > > However, on again looking at the code, it seems that part of code behaves > similarly for both 9.2 and 9.3.
Yes, the code behaves similarly in 9.2 and later. FYI, ValidXLogHeader() is called at two sites. The earlier one checks the first page of a segment when the real target page is different, and the latter one checks any page including the first page of a segment. > ..Because node3 found a WAL > ! * record fragment at the end of segment 10, it expects to find the ! > * remaining fragment at the beginning of WAL segment 11 streamed from ! > * node2. But there was a fragment of a different WAL record, because ! * > node2 overwrote a different WAL record at the end of segment 10 across ! > * to 11. > > How does node3 ensure that the fragment of WAL in segment 11 is different? > Isn't it possible that when node2 overwrites the last record in WAL segment > 10, it writes a record of slightly different contents but which is of the > same size as an original record in WAL segment 10? That case is detected by checking the CRC value in the XLOG record header. Regards Takayuki Tsunakawa -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
