Re: [HACKERS] On markers of changed data - Mailing list pgsql-hackers

From Andrey Borodin
Subject Re: [HACKERS] On markers of changed data
Date
Msg-id A7F58A00-0EC2-4F5A-9B4A-CDFE92FFA62F@yandex-team.ru
Whole thread Raw
In response to Re: [HACKERS] On markers of changed data  (Andrey Borodin <x4mmm@yandex-team.ru>)
List pgsql-hackers
Hello!


> 9 окт. 2017 г., в 10:23, Andrey Borodin <x4mmm@yandex-team.ru> написал(а):
>
> Thanks, Stephen, this actually pointed what to look for
> VM is WAL-logged [0]
> FSM is not [1]
>
> [0]
https://github.com/postgres/postgres/blob/113b0045e20d40f726a0a30e33214455e4f1385e/src/backend/access/heap/visibilitymap.c#L315
> [1]
https://github.com/postgres/postgres/blob/1d25779284fe1ba08ecd57e647292a9deb241376/src/backend/storage/freespace/freespace.c#L593

After tests of binary equivalence before and after backup I've come to conclusion, that Visibility Map cannot be
backupedincrementally: it's bits are unset without page LSN bump. This can lead to wrong results of Index Only Scans
executedon freshly restored backups. 

In my implementation of incremental backup in WAL-G I will disable any FSM, VM and XACT\CLOG incrementation.

Posting this for the record, so that if someone goes this way info will be available. Thank you for your attention.

Best regards, Andrey Borodin.

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