Greetings, * Tomas Vondra (tomas.von...@2ndquadrant.com) wrote: > On 08/08/2018 04:08 PM, David Steele wrote: > >On 8/7/18 12:05 PM, Stephen Frost wrote: > >>>All I'm saying is that (assuming my understanding of RestoreArchivedFile is > >>>correct) we can't just do that in the current restore_command. We do need a > >>>way to ask the archive for some metadata/checksums, and restore_command is > >>>too late. > >> > >>Yeah, I see what you mean there. An alternative might be to *not* > >>unlink the file, in case the restore command wants to check if it's > >>valid and matches what's in the archive, and instead restore the > >>requested file somewhere else and then perform an unlink/mv after > >>the restore command has been run. > >I don't see any cases (in master, at least) where RestoredArchivedFile() > >would unlink an existing WAL segment. If you look at the call sites > >they all pass "RECOVERYHISTORY" or "RECOVERYXLOG" to the recovername > >parameter. > > > >RestoreArchivedFile() does overwrite the existing file after we decide > >that we prefer the restored version over the one in pg_wal, but that > >happens after restore_command returns. > > > >So as far as I can see, it could be possible for the restore_command to > >do something clever here. > > Oh, I see - I really was reading RestoredArchivedFile() wrong ... > > Yes, it seems possible to do this in restore_command. Currently it'd require > constructing the local segment path (replacing RECOVERYXLOG with the actual > segment filename), but that's solvable by adding a new pattern similar to > %p.
Nice, but I don't know that there's any need to add another %-flag, really. David, do you think we'd really need that? > I'm not sure if packing all this into a single restore_command is a good > idea, because I'm pretty sure it'll lead to abhorrently monstrous bash > commands jammed into the restore_command. But then again, we kinda already > have that, and all archival systems already provide simple and correct > commands doing the right thing. And extending that would not be a big deal. Running a bash snippet as restore_command is a bad idea already, imv. The real PG backup tools already have much more complicated restore commands written in C, Perl, or Python, and could definitely implement this. There's still a bit of a question as to if it'd really be worth it, but perhaps it would be in certain cases. Thanks! Stephen
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