On 03/29/2018 11:18 PM, Tom Lane wrote: > Tomas Vondra <tomas.von...@2ndquadrant.com> writes: >> If each WAL record has xl_curr, then we know to which position the >> record belongs (after verifying the checksum). And we do know the size >> of each WAL record, so we should be able to deduce if two records are >> immediately after each other. > > Per my point earlier, XLOG_SWITCH is sufficient to defeat that argument.
But the SWITCH record will be the last record in the WAL segment (and if there happens to be a WAL record after it, it will have invalid xl_curr pointer). And the next valid record will be the first one in the next WAL segment. So why wouldn't that be enough information? > Also consider a timeline fork. It's really hard to be sure which record > in the old timeline is the direct ancestor of the first one in the new > if you lack xl_prev: > > A1 -> B1 -> C1 -> D1 > \ > B2 -> C2 -> D2 > > If you happened to get confused and think that C2 is the first in its > timeline, diverging off the old line after B1 not A1, there would be > nothing about C2 to disabuse you of your error. Doesn't that mean the B1 and B2 have to be exactly the same size? Otherwise there would be a gap between B1/C2 or B2/C1, and the xl_curr would be enough to detect this. And how could xl_prev detect it? AFAIK XLogRecPtr does not include TimeLineID, so xl_prev would be the same for both B1 and B2. I admit WAL internals are not an are I'm particularly familiar with, though, so I may be missing something utterly obvious. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
