Re: Offline enabling/disabling of data checksums

On Wed, Mar 13, 2019 at 11:54 AM Sergei Kornilov <sk@zsrv.org> wrote:

> Hi
>
> One new question from me: how about replication?
> Case: primary+replica, we shut down primary and enable checksum, and
> "started streaming WAL from primary" without any issue. I have master with
> checksums, but replica without.
> Or cluster with checksums, then disable checksums on primary, but standby
> think we have checksums.
>

Enabling or disabling the checksums offline on the master quite clearly
requires a rebuild of the standby, there is no other way (this is one of
the reasons for the online enabling in that patch, so I still hope we can
get that done -- but not for this version).

You would have the same with PITR backups for example. And especially if
you have some tool that does block or segment level differential.

Of course, we have to make sure that this actually fails.

I wonder if we should bring out the big hammer and actually change the
system id in pg_control when checksums are enabled/disabled by this tool?
That should make it clear to any tool that it's changed.


Also we support ./configure --with-blocksize=(not equals 8)? make check on
> HEAD fails for me. If we support this - i think we need recheck BLCKSZ
> between compiled pg_checksum and used in PGDATA
>

You mean if the backend and pg_checksums is built with different blocksize?
Yeah, that sounds like something which is a cheap check and should be done.

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 Magnus Hagander
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