On Fri, Mar 29, 2019 at 4:30 PM Stephen Frost <sfrost@snowman.net> wrote:
Greetings,
* Magnus Hagander (magnus@hagander.net) wrote: > On Thu, Mar 28, 2019 at 10:19 PM Tomas Vondra <tomas.vondra@2ndquadrant.com> > wrote: > > > On Thu, Mar 28, 2019 at 01:11:40PM -0700, Andres Freund wrote: > > >Hi, > > > > > >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"?