On Sat, Jun 6, 2015 at 4:51 PM, 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.
Here's a repro script and a suggested patch. (What I said about dirty pages in parentheses above was nonsense, I was confusing this with something else.) -- Thomas Munro http://www.enterprisedb.com
fix-bogus-subtrans-wraparound-error.patch
Description: Binary data
checkpoint-subtrans-boundary.sh
Description: Bourne shell script
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