On 29 March 2018 at 19:02, Robert Haas <robertmh...@gmail.com> wrote: > >>> Also, I will say it once more: this change DOES decrease robustness. >>> It's like blockchain without the chain aspect, or git commits without >>> a parent pointer. We are not only interested in whether individual >>> WAL records are valid, but whether they form a consistent series. >>> Cross-checking xl_prev provides some measure of confidence about that; >>> xl_curr offers none. >> >> Not sure. >> >> 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. Which I think is enough to rebuild the >> chain of WAL records. >> >> To defeat this, this would need to happen: >> >> a) the WAL record gets written to a different location >> b) the xl_curr gets corrupted in sync with (a) >> c) the WAL checksum gets corrupted in sync with (b) >> d) the record overwrites existing record (same size/boundaries) >> >> That seems very much like xl_prev. > > I don't think so, because this ignores, for example, timeline > switches, or multiple clusters accidentally sharing an archive > directory.
I'm not hearing any actual technical problems. > Given where we are > in the release cycle, I'd prefer to defer that question to next cycle. Accepted, on the basis of risk. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
