Greetings, * chenhj (chjis...@163.com) wrote: > > At 2018-01-23 09:56:48, "Stephen Frost" <sfr...@snowman.net> wrote: > >I've only read through the thread to try and understand what's going on > >and the first thing that comes to mind is that you're changing > >pg_rewind to not remove the WAL from before the divergence (split) > >point, but I'm not sure why. As noted, that WAL isn't needed for > >anything (it's from before the split, after all), so why keep it? Is > >there something in this optimization that depends on the old WAL being > >there and, if so, what and why? > > After run pg_rewind, the first startup of postgres will do crash recovery. > And crash recovery will begin from the previous redo point preceding the > divergence. > So, the WAL after the redo point and before the divergence is needed.
Right. > Of course, the WAL before the redo point is not needed, but in my point of > view, > recycling unwanted WAL does not have to be done by pg_rewind. That's what pg_rewind has been doing though, isn't it? And it's not like that WAL is useful for anything, is it? That's also how pg_basebackup works. > >That's also different from how pg_basebackup works, which I don't think > >is good (seems like pg_rewind should operate in a pretty similar manner > >to pg_basebackup). > > Thanks for your comments! > I also considered copy WAL just like how pg_basebackup does,but a implement > similar to pg_basebackup's manner may be not so simple. Using the replication protocol to fetch WAL would be a good thing to do (actually, making pg_rewind entirely work through a connection to the current primary would be great) but that's independent of what I'm asking for here. Here I'm just suggesting that we not change what pg_rewind is doing today when it comes to the existing WAL on the old-primary. > And the WAL which contains the previous redo point preceding the divergence > may be only exists in target server and had been recycled in source. That's > different between pg_rewind and pg_basebackup. Hm, pg_rewind was removing that and expecting it to be on the new primary? If that's the case then I could see an argument for keeping WAL that's from the divergence point onward, but I still don't think we should have pg_rewind just leave all of the prior WAL in place. Thanks! Stephen
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