On Tue, Nov 15, 2016 at 7:51 AM, Tsunakawa, Takayuki <tsunakawa.ta...@jp.fujitsu.com> wrote: > From: pgsql-hackers-ow...@postgresql.org >> [mailto:pgsql-hackers-ow...@postgresql.org] On Behalf Of Amit Kapila >> It looks like the code in 9.3 or later version uses the recptr as the target >> segment location >> (targetSegmentPtr) whereas 9.2 uses recptr as beginning of segment (readOff >> = 0;). If above understanding is right then it will set different values >> for latestPagePtr in 9.2 and 9.3 onwards code. >> > > In 9.2, the relevant variable is not recptr but recaddr. recaddr in 9.2 and > recptr in later releases point to the beginning of a page just read, which is > not always the beginning of the segment (targetSegmentPtr). >
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. ..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? -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
