If there's anything I can try on my servers to help diagnose the issues, please let me know. If desired, I can arrange access for debugging.
On Sat, Jun 6, 2015 at 12:51 AM, Thomas Munro <thomas.mu...@enterprisedb.com > wrote: > On Sat, Jun 6, 2015 at 1:25 PM, Alvaro Herrera <alvhe...@2ndquadrant.com> > wrote: > > Thomas Munro wrote: > > > >> My idea was that if I could get oldestXact == next XID in > >> TruncateSUBSTRANS, then TransactionIdToPage(oldestXact) for a value of > >> oldestXact that happens to be immediately after a page boundary (so > >> that xid % 2048 == 0) might give page number that is >= > >> latest_page_number, causing SimpleLruTruncate to print that message. > >> But I can't figure out how to get next XID == oldest XID, because > >> vacuumdb --freeze --all consumes xids itself, so in my first attempt > >> at this, next XID is always 3 ahead of the oldest XID when a > >> checkpoint is run. > > > > vacuumdb starts by querying pg_database, which eats one XID. > > > > Vacuum itself only uses one XID when vac_truncate_clog() is called. > > This is called from vac_update_datfrozenxid(), which always happen at > > the end of each user-invoked VACUUM (so three times for vacuumdb if you > > have three databases); autovacuum does it also at the end of each run. > > Maybe you can get autovacuum to quit before doing it. > > > > OTOH, if the values in the pg_database entry do not change, > > vac_truncate_clog is not called, and thus vacuum would finish without > > consuming an XID. > > I have manage to reproduce it a few times but haven't quite found the > right synchronisation hacks to make it reliable so I'm not posting a > repro script yet. > > I think it's a scary sounding message but very rare and entirely > harmless (unless you really have wrapped around...). The fix is > probably something like: if oldest XID == next XID, then just don't > call SimpleLruTruncate (truncation is deferred until the next > checkpoint), or perhaps (if we can confirm this doesn't cause problems > for dirty pages or that there can't be any dirty pages before cutoff > page because of the preceding flush (as I suspect)) we could use > cutoffPage = TransactionIdToPage(oldextXact - 1) if oldest == next, or > maybe even always. > > -- > Thomas Munro > http://www.enterprisedb.com >
