Re: Standby trying "restore_command" before local WAL - Mailing list pgsql-hackers
From | Stephen Frost |
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Subject | Re: Standby trying "restore_command" before local WAL |
Date | |
Msg-id | 20180806193239.GP27724@tamriel.snowman.net Whole thread Raw |
In response to | Re: Standby trying "restore_command" before local WAL (Tomas Vondra <tomas.vondra@2ndquadrant.com>) |
Responses |
Re: Standby trying "restore_command" before local WAL
|
List | pgsql-hackers |
Greetings, * Tomas Vondra (tomas.vondra@2ndquadrant.com) wrote: > On 08/06/2018 06:11 PM, Stephen Frost wrote: > >* Tomas Vondra (tomas.vondra@2ndquadrant.com) wrote: > >>On 08/06/2018 05:19 PM, Stephen Frost wrote: > >>>* David Steele (david@pgmasters.net) wrote: > >>>>I think for the stated scenario (known good standby that has been > >>>>shutdown gracefully) it makes perfect sense to trust the contents of > >>>>pg_wal. Call this scenario #1. > >>>> > >>>>An alternate scenario (#2) is that the data directory was copied using a > >>>>basic copy tool and the pg_wal directory was not excluded from the copy. > >>>> This means the contents of pg_wal will be in an inconsistent state. > >>>>The files that are there might be partials (not with the extension, > >>>>though) and you can easily have multiple partials. You will almost > >>>>certainly not have everything you need to get to consistency. > >> > >>Yeah. But as Simon said, we do have fairly strong protections about applying > >>corrupted WAL - every record is CRC-checked. So why not to fall-back to the > >>restore_command only if the locally available WAL is not fully consistent? > > > >"Corrupted" doesn't necessairly only mean "the data file was munged by > >the storage somehow." In this case, corrupted could be an old and only > >partial WAL file, in which case we'd possibly be missing WAL that needs > >to be replayed to bring the cluster back to a valid state, no? > > Why wouldn't that be detected by checksums? Also, why wouldn't this be > handled correctly by the logic I proposed, i.e. falling-back to remote WAL > segment if the file is incomplete/broken in some sense? WAL checksums are per WAL record, not across the whole file... And, yes, this seems like something we could probably write code to ensure we're doing correctly, possibly even without changing the existing WAL format or maybe we would have to, but either way, seems like additional code that would need to be written and some serious thought put into it to make sure it's correct. I'm all for that, just to be clear, but what I don't think we can do is move forward with a change to just prefer pg_wal over restore command. > >>>>But there's another good scenario (#3): where the pg_wal directory was > >>>>preloaded with all the WAL required to make the cluster consistent or > >>>>all the WAL that was available at restore time. In this case, it would > >>>>be make sense to prefer the contents of pg_wal and only switch to > >>>>restore_command after that has been exhausted. > >>>> > >>>>So, the choice of whether to prefer locally-stored or > >>>>restore_command-fetched WAL is context-dependent, in my mind. > >>> > >>>Agreed. > >> > >>Maybe, not sure. > > > >The argument that David makes above in scenario #2 certainly looks > >entirely likely to me and I don't think we've got any real protections > >against that. The current common use-cases happen to work around the > >risk because tools like pg_basebackup ignore the existing pg_wal > >directory when doing the backup and instead populate it with exactly the > >correct WAL that's needed, and in cases where a restore command is > >specified will always pull back only valid WAL, but I don't think we can > >decide that this scenario (#2 from above): > > I'm probably missing something, but why couldn't we detect that using CRC. > Or make sure we can detect that, e.g. by adding some additional info into > each WAL segment? CRC's are per WAL record, and possibly some WAL records might not be ok to replay, or at least we need to make sure that we replay the right set of WAL in the right order even when there are partial WAL files being given to PG (that aren't named that way...). The more I think about this, I think we really need to avoid partial WAL files entirely- what are we going to do when we get to the end of one? We'd need to request the full one from the restore command anyway, so seems like we should just go ahead and get it from the archive, the question is if there's an easy/cheap way to detect partial WAL files in pg_wal. > >>>>Ideally we could have a default that is safe in each scenario with > >>>>perhaps an override if the user knows better. Scenario #1 would allow > >>>>WAL to be read from pg_wal by default, scenario #2 would prefer fetched > >>>>WAL, and scenario #3 could use a GUC to override the default fetch behavior. > >>> > >>>Not sure how we'd be able to automatically realize which scenario we're > >>>in though..? > >> > >>But do we need to know it? I mean, can't we try the local WAL first, use it > >>if it passes the CRC checks (and possibly some other checks), and only > >>fallback to the remote WAL if it's identified as broken? > > > >Maybe- but I think we need to be quite sure about that and I don't > >believe that just checking the CRCs is enough. > > Sure. So what else would we need to add to each WAL segment to make that > possible? It needs to be cheap enough not to cause perf regression, but e.g. > a CRC of each segment + amount of data actually stored in it should be > sufficient. As I mention above, seems like what we should really be doing is having a way to know when a WAL file is full but still in pg_wal for some reason (hasn't been replayed yet due to intential lag on the replica, or unintentional lag on the replica, etc), and perhaps we even only do that on replicas or have tools like pg_basebackup and pgbackrest do that when they're doing a restore, so that it's clear to PG that all these files are full WAL and they can replay them completely. > Another idea - what if we instead allow fetching additional information of > the archived WAL, and make decision based on that? For example, before > restore_command we could request md5 checksum from the archive, compare it > to the local WAL, and if they match then use the local one. Otherwise > request the remote one. That'd still be a win. I was actually just thinking about having pgbackrest do exactly that. :) We already have the full sha256 checksum of every WAL file in the pgbackrest repository, it seems like it wouldn't be too hard to calculate the checksum of the files in pg_wal and ask the repo what the checksums are for those WAL files and then use the files in pg_wal if they checksums match. I can already imagine the argument coming back though- that'd require additional testing to ensure it's done correctly and I'm really not sure that it'd end up being all that much better except in some very special circumstances where there's a low bandwidth connection to the repo and often a lot of WAL left over in pg_wal. Thanks! Stephen
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