On Fri, Mar 29, 2019 at 10:08 PM Michael Banck <michael.ba...@credativ.de> wrote:
> Hi, > > Am Freitag, den 29.03.2019, 16:52 +0100 schrieb Magnus Hagander: > > On Fri, Mar 29, 2019 at 4:30 PM Stephen Frost <sfr...@snowman.net> > wrote: > > > * Magnus Hagander (mag...@hagander.net) wrote: > > > > On Thu, Mar 28, 2019 at 10:19 PM Tomas Vondra < > tomas.von...@2ndquadrant.com> > > > > wrote: > > > > > On Thu, Mar 28, 2019 at 01:11:40PM -0700, Andres Freund wrote: > > > > > >On 2019-03-28 21:09:22 +0100, Michael Banck wrote: > > > > > >> I agree that the current patch might have some corner-cases > where it > > > > > >> does not guarantee 100% accuracy in online mode, but I hope the > current > > > > > >> version at least has no more false negatives. > > > > > > > > > > > >False positives are *bad*. We shouldn't integrate code that has > them. > > > > > > > > > > Yeah, I agree. I'm a bit puzzled by the reluctance to make the > online mode > > > > > communicate with the server, which would presumably address these > issues. > > > > > Can someone explain why not to do that? > > > > > > > > I agree that this effort seems better spent on fixing those issues > there > > > > (of which many are the same), and then re-use that. > > > > > > This really seems like it depends on which of the options we're talking > > > about.. Connecting to the server and asking what the current insert > > > point is, so we can check that the LSN isn't completely insane, seems > > > reasonable, but at least one option being discussed was to have > > > pg_basebackup actually *lock the page* (even if just for I/O..) and > then > > > re-read it, and having an external tool doing that instead of the > > > backend seems like a whole different level to me. That would involve > > > having an SQL function for "lock this page against I/O" and then > another > > > for "unlock this page", wouldn't it? > > > > Right. > > > > But what if we just added a flag to the BASE_BACKUP command in the > > replication protocol that said "meh, I really just want to verify the > > checksums, so please send the data to devnull and only feed me regular > > status updates on this connection"? > > I don't know whether BASE_BACKUP is the best interface for that (at > least right now) - backend/replication/basebackup.c's sendFile() gets > only an absolute filename to send, which is not adequate for more in- > depth server-based things like locking a particular page in a particular > relation of some particular tablespace. > ISTM that the fact that we had to teach it about different segment files > for checksum verification by splitting up the filename at "." implies > that it is not the correct level of abstraction (but maybe it could get > schooled some more about Postgres internals, e.g. by passing it a > RefFileNode struct and not a filename). > But that has to be fixed in pg_basebackup *regardless*, doesn't it? And if we fix it there, we only have to fix it once... //Magnus
