Re: Online checksums patch - once again

On Thu, Jan 7, 2021 at 3:05 PM Daniel Gustafsson <daniel(at)yesql(dot)se> wrote:
>
> > On 5 Jan 2021, at 18:19, Michael Banck <michael(dot)banck(at)credativ(dot)de> wrote:
>
> >> diff --git a/doc/src/sgml/ref/initdb.sgml b/doc/src/sgml/ref/initdb.sgml
> >> index 385ac25150..e3b0048806 100644
> >> --- a/doc/src/sgml/ref/initdb.sgml
> >> +++ b/doc/src/sgml/ref/initdb.sgml
> >> @@ -219,6 +219,7 @@ PostgreSQL documentation
> >> failures will be reported in the
> >> <link linkend="monitoring-pg-stat-database-view">
> >> <structname>pg_stat_database</structname></link> view.
> >> + See <xref linkend="checksums" /> for details.
> >> </para>
> >> </listitem>
> >> </varlistentry>
> >
> >> diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
> >> index f4bc147b10..5dcfcdd2ff 100644
> >> --- a/doc/src/sgml/wal.sgml
> >> +++ b/doc/src/sgml/wal.sgml
> >> @@ -230,6 +230,103 @@
> >> </para>
> >> </sect1>
> >>
> >> + <sect1 id="checksums">
> >> + <title>Data Checksums</title>
> >> + <indexterm>
> >> + <primary>checksums</primary>
> >> + </indexterm>
> >> +
> >> + <para>
> >> + Data pages are not checksum protected by default, but this can optionally be
> >> + enabled for a cluster. When enabled, each data page will be assigned a
> >> + checksum that is updated when the page is written and verified every time
> >> + the page is read. Only data pages are protected by checksums, internal data
> >> + structures and temporary files are not.
> >> + </para>
> >> +
> >> + <para>
> >> + Checksums are normally enabled when the cluster is initialized using <link
> >> + linkend="app-initdb-data-checksums"><application>initdb</application></link>.
> >> + They can also be enabled or disabled at a later time, either as an offline
> >> + operation or in a running cluster. In all cases, checksums are enabled or
> >> + disabled at the full cluster level, and cannot be specified individually for
> >> + databases or tables.
> >> + </para>
> >> +
> >> + <para>
> >> + The current state of checksums in the cluster can be verified by viewing the
> >> + value of the read-only configuration variable <xref
> >> + linkend="guc-data-checksums" /> by issuing the command <command>SHOW
> >> + data_checksums</command>.
> >> + </para>
> >> +
> >> + <para>
> >> + When attempting to recover from corrupt data it may be necessary to bypass
> >> + the checksum protection in order to recover data. To do this, temporarily
> >> + set the configuration parameter <xref linkend="guc-ignore-checksum-failure" />.
> >> + </para>
> >
> > I think the above is rather informative about checksums in general and
> > not specific to online activation of checksusm, so could pretty much be
> > committed verbatim right now, except for the "either as an offline
> > operation or in a running cluster" bit which would have to be rewritten.
>
> That might indeedbe useful regardless of the state of this patch. Robert and
> Heikki: being committers who have both reviewed recent versions of the patch,
> would you prefer these sections be broken out into a separate patch?

I think it would be ;)

Obviously in that case adapted so that it has to be changed offline,
and the patch would have to change that to say it can be changed
online.

--
Magnus Hagander
Me: https://www.hagander.net/
Work: https://www.redpill-linpro.com/