Re: On markers of changed data

Michael Paquier wrote:

> That’s actually what pg_rman is doing for what it calls incremental
> backups (perhaps that would be differential backup in PG
> terminology?), and the performance is bad as you can imagine. We could
> have a dedicated LSN map to do such things with 4 bytes per page. I am
> still not convinced that this much facility and the potential bug
> risks are worth it though, Postgres already knows about differential
> backups if you shape it as a delta of WAL segments. I think that, in
> order to find a LSN map more convincing, we should find first other
> use cases where it could become useful. Some use cases may pop up with
> VACUUM, but I have not studied the question hard enough...

The case I've discussed with barman developers is a large database
(couple dozen of TBs should be enough) where a large fraction (say 95%)
is read-only but there are many changes to the active part of the data,
so that WAL is more massive than size of active data.

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Álvaro Herrera https://www.2ndQuadrant.com/
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