Because standby is running in syncronous replication, whereby wal archiver is asynchronous. Therefore there is a small window where slave has received the data but master has not pushed it yet to wal archive.
Regards, Sasa Am 28.02.2017 02:48 schrieb "Adrian Klaver" <adrian.kla...@aklaver.com>: > On 02/27/2017 05:29 PM, Sasa Vilic wrote: > >> Master is streaming directly to standby. Both master and standby are >> pushing WALs to archive. >> >> My point is that in case that master crashed completely (and we failover >> to standby) and wal archiver on master didn't push everything to wal >> archive, we would still have a wal pushed from slave. Therefore there is >> no interruption in WAL stream. >> > > Still failing to see how the standby can have more information then what > the master had sent to it at the time of the crash. > > >> Regards, >> Sasa >> >> On 28 February 2017 at 01:57, Adrian Klaver <adrian.kla...@aklaver.com >> <mailto:adrian.kla...@aklaver.com>> wrote: >> >> On 02/27/2017 04:40 PM, Sasa Vilic wrote: >> >> Hallo, >> >> I am trying to setup shared WAL archive between master and >> standby. >> Standby is synchronously streaming from master and both servers >> run with >> archive_mode = always. The ideas is that when promoting standby to >> master we would not missed WALs. >> >> >> I seem to be missing the point of duplicating your effort. >> >> You are doing this, correct?: >> >> Master WAL --> WAL archive <-- >> | >> Master stream --> Standby --> | >> >> I can't see how the Standby contributes anything to the archive that >> it does not already have from the Master? >> >> >> >> My problem is that sometimes WAL uploaded from master and from >> slave are >> not 100% identical. In most cases they are but occasionally they >> are >> not. I have written small script that ensures that upload is free >> of >> race condition and I log md5 sum of each WAL. Aren't WALs from >> master >> and standby supposed to be identical? After all, standby is just >> consuming WAL that it is receiving from master ... >> >> Or do you have any better suggestion on how to achieve continuous >> incremental backup? >> >> Thanks in advance >> >> >> >> -- >> Adrian Klaver >> adrian.kla...@aklaver.com <mailto:adrian.kla...@aklaver.com> >> >> >> > > -- > Adrian Klaver > adrian.kla...@aklaver.com >
