Re: Online checksums patch - once again - Mailing list pgsql-hackers

On Thu, Jan 7, 2021 at 3:05 PM Daniel Gustafsson <daniel@yesql.se> wrote:
>
> > On 5 Jan 2021, at 18:19, Michael Banck <michael.banck@credativ.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/