Обсуждение: doc: Improve wal_level and effective_wal_level GUC around logical replication
doc: Improve wal_level and effective_wal_level GUC around logical replication
От
"David G. Johnston"
Дата:
Hi,
In evaluating the proposed change to wal_level docs in [1] I found it to be undesirable, but the discussion and my own review discovered some other changes that should be considered.
Since effective_wal_level now exists, Logical Replication Getting Started needs to be updated.
Add links to the main topics that the different levels apply to.
Be explicit about the ordering of minimal, replica, and logical.
Move the behavior of effective_wal_level to the GUC for it, leaving behind just the pointer that wal_level is now just a "minimum".
David J.
Вложения
Re: doc: Improve wal_level and effective_wal_level GUC around logical replication
От
Andreas Karlsson
Дата:
On 3/4/26 4:55 PM, David G. Johnston wrote: > In evaluating the proposed change to wal_level docs in [1] I found it to > be undesirable, but the discussion and my own review discovered some > other changes that should be considered. While I am not happy with the text before I am not sure your changes here really make it much better, just different, hmm. > Since effective_wal_level now exists, Logical Replication Getting > Started needs to be updated. > Add links to the main topics that the different levels apply to. > Be explicit about the ordering of minimal, replica, and logical. > Move the behavior of effective_wal_level to the GUC for it, leaving > behind just the pointer that wal_level is now just a "minimum". Shouldn't it be: "Ensure that <xref linkend="guc-wal-level"/> is at least replica" and "wal_level = replica" as the logical replication slot may be created later? -- Andreas Karlsson Percona
> On Mar 4, 2026, at 23:55, David G. Johnston <david.g.johnston@gmail.com> wrote: > > Hi, > > In evaluating the proposed change to wal_level docs in [1] I found it to be undesirable, but the discussion and my ownreview discovered some other changes that should be considered. > > Since effective_wal_level now exists, Logical Replication Getting Started needs to be updated. > Add links to the main topics that the different levels apply to. > Be explicit about the ordering of minimal, replica, and logical. > Move the behavior of effective_wal_level to the GUC for it, leaving behind just the pointer that wal_level is now justa "minimum". > > David J. > > [1] https://www.postgresql.org/message-id/CA%2Bv5N42vKtY17653eBHFx%2BCw06E2fSmPrXmX2qKYG23%3D4Ycd7A%40mail.gmail.com > <v1-0001-doc-Add-some-clarity-around-wal_level.patch> A few comments: 1 ``` --- a/doc/src/sgml/logical-replication.sgml +++ b/doc/src/sgml/logical-replication.sgml @@ -3508,11 +3508,12 @@ pg_ctl -D /opt/PostgreSQL/data2_upgraded start -l logfile <title>Quick Setup</title> <para> - First set the configuration options in <filename>postgresql.conf</filename>: + Ensure that <xref linkend="guc-effective-wal-level"/> is at least replica + (this is the default). <programlisting> -wal_level = logical +SHOW effective_wal_level; </programlisting> - The other required settings have default values that are sufficient for a + The other required settings also have default values that are sufficient for a basic setup. </para> ``` I think wal_level should be at least “logical”. I don’t get why did you change to “replica”? It’s clear that we still need“logical” to support logical replication. 2 ``` + <literal>replica</literal>, which writes enough data to support + WAL archiving (see <xref linkend="continuous-archiving"/>) and + physical replication (see <xref linkend="streaming-replication"/>) + Running read-only queries on a standby server. Below that is <literal>minimal</literal>, ``` Here, “Running read-only …” sentence looks not smooth, and there is not a period for the last sentence before “Running".The old wording was “including running read-only ….”. Best regards, -- Chao Li (Evan) HighGo Software Co., Ltd. https://www.highgo.com/
doc: Improve wal_level and effective_wal_level GUC around logical replication
От
"David G. Johnston"
Дата:
On Tuesday, April 7, 2026, Chao Li <li.evan.chao@gmail.com> wrote:
> On Mar 4, 2026, at 23:55, David G. Johnston <david.g.johnston@gmail.com> wrote:
>
> Hi,
>
> In evaluating the proposed change to wal_level docs in [1] I found it to be undesirable, but the discussion and my own review discovered some other changes that should be considered.
>
> Since effective_wal_level now exists, Logical Replication Getting Started needs to be updated.
> Add links to the main topics that the different levels apply to.
> Be explicit about the ordering of minimal, replica, and logical.
> Move the behavior of effective_wal_level to the GUC for it, leaving behind just the pointer that wal_level is now just a "minimum".
>
> David J.
>
> [1] https://www.postgresql.org/message-id/CA%2Bv5N42vKtY17653eBH Fx%2BCw06E2fSmPrXmX2qKYG23%3D4 Ycd7A%40mail.gmail.com
> <v1-0001-doc-Add-some-clarity-around-wal_level.patch>
A few comments:
1
```
--- a/doc/src/sgml/logical-replication.sgml
+++ b/doc/src/sgml/logical-replication.sgml
@@ -3508,11 +3508,12 @@ pg_ctl -D /opt/PostgreSQL/data2_upgraded start -l logfile
<title>Quick Setup</title>
<para>
- First set the configuration options in <filename>postgresql.conf</filename>:
+ Ensure that <xref linkend="guc-effective-wal-level"/> is at least replica
+ (this is the default).
<programlisting>
-wal_level = logical
+SHOW effective_wal_level;
</programlisting>
- The other required settings have default values that are sufficient for a
+ The other required settings also have default values that are sufficient for a
basic setup.
</para>
```
I think wal_level should be at least “logical”. I don’t get why did you change to “replica”? It’s clear that we still need “logical” to support logical replication.
A minimum setting of replica is automatically interpreted (i.e., effective WAL level) as logical when necessary so the default setting of replica doesn’t need to be changed anymore. Seeing replica in effective WAL level means no logical replication has been setup and the dba hasn’t reduced the system to minimal. If one sees logical there, that works too.
2
```
+ <literal>replica</literal>, which writes enough data to support
+ WAL archiving (see <xref linkend="continuous-archiving"/>) and
+ physical replication (see <xref linkend="streaming-replication"/>)
+ Running read-only queries on a standby server. Below that is <literal>minimal</literal>,
```
Here, “Running read-only …” sentence looks not smooth, and there is not a period for the last sentence before “Running". The old wording was “including running read-only ….”.
Yeah, I still stand by the whole of it but need to do another pass.
I do get that there likely wasn’t any ambiguity - even presented replica, minimal, logical - as to which was highest/middle/lower and being explicit here maybe seems overly verbose.
Also, I added a qualifier regarding “primary servers” but am now unsure why. Kinda concerned about logical where publisher and subscriber are not mutually exclusive.
Re: doc: Improve wal_level and effective_wal_level GUC around logical replication
От
"David G. Johnston"
Дата:
On Tuesday, April 7, 2026, Andreas Karlsson <andreas@proxel.se> wrote:
Since effective_wal_level now exists, Logical Replication Getting Started needs to be updated.Shouldn't it be:
"Ensure that <xref linkend="guc-wal-level"/> is at least replica"
Given “at least” it is immaterial whether one inspects wal_level or effective_wal_level. It seems more intuitive to tell someone to look at the value that represents what the system presently is using, not the defined minimum level.
David J.
Re: doc: Improve wal_level and effective_wal_level GUC around logical replication
От
shveta malik
Дата:
Few comments: 1) + Running read-only queries on a standby server. Looks incomplete and disconnected from previous sentence. 2) + level used by the system. Namely, whether replica has been promoted to + logical (minimal never promotes). We shall replace replica, logical, minimal with below <literal>replica</literal> <literal>logical</literal> <literal>minimal</literal> Also shall we add 'wal_level'? Otherwise, 'replica' on its own could be understood as a replica server: Namely, whether wal_level replica has been promoted to .... 3) + The <xref linkend="guc-effective-wal-level"/> parameter reports the actual + level used by the system. Namely, whether replica has been promoted to + logical (minimal never promotes). + This parameter can only be set at server start. "This" here is misleading. Does this refer to wal_level or effective_wal_level? 4) Below that is <literal>minimal</literal>, Shall we change "that" to <literal>replica</literal> to avoid any confusion? thanks Shveta
Re: doc: Improve wal_level and effective_wal_level GUC around logical replication
От
"David G. Johnston"
Дата:
Thank you, and the others, for the reviews.
Few comments:
1)
+ Running read-only queries on a standby server.
Looks incomplete and disconnected from previous sentence.
Yeah, I missed that one.
2)
+ level used by the system. Namely, whether replica has been promoted to
+ logical (minimal never promotes).
We shall replace replica, logical, minimal with below
<literal>replica</literal>
<literal>logical</literal>
<literal>minimal</literal>
Yeah, I need to do a markup pass too apparently. Though I do question whether we are being too rote about these. In this sentence yes I use the actual labels but it’s more about concept than talking about something you’d literally type somewhere. One doesn’t promote a literal, they promote the operating mode of the server which is then reflected by the change of a value.
Also shall we add 'wal_level'? Otherwise, 'replica' on its own could
be understood as a replica server:
Namely, whether wal_level replica has been promoted to ....
The preceding sentence to which “namely” links talks about the levels. But I’ll give it a second look. From my reply above, writing “WAL level has been promoted to logical from replica”, is probably a better structure.
3)
+ The <xref linkend="guc-effective-wal-level"/> parameter
reports the actual
+ level used by the system. Namely, whether replica has been promoted to
+ logical (minimal never promotes).
+ This parameter can only be set at server start.
"This" here is misleading. Does this refer to wal_level or effective_wal_level?
When talking about where/when a setting can specified, “this” always refers to the setting being documented. I’m doubtful of the need to reword things so that some other setting being talked about doesn’t get confused instead. The fact that we say effective WAL level is basically a runtime derived setting in the prior sentence reinforces this belief. And we try to keep this sentence consistently at the end of the description. Maybe it needs its own paragraph though? I’ll consider that.
4)
Below that is <literal>minimal</literal>,
Shall we change "that" to <literal>replica</literal> to avoid any confusion?
I’m on the fence at the moment. This entire section needs a relook anyway and I’ll keep all these in mind.
David J.