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docs: Reframe track_io_timing related docs as wait time

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With AIO it does not make sense anymore to track the time for each individual
IO, as multiple IOs can be in-flight at the same time. Instead we now track
the time spent *waiting* for IOs.

This should be reflected in the docs. While, so far, we only do a subset of
reads, and no other operations, via AIO, describing the GUC and view columns
as measuring IO waits is accurate for synchronous and asynchronous IO.

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/5dzyoduxlvfg55oqtjyjehez5uoq6hnwgzor4kkybkfdgkj7ag@rbi4gsmzaczk
This commit is contained in:
Andres Freund 2025-03-30 18:04:40 -04:00
parent 12ce89fd07
commit b27f8637ea
2 changed files with 8 additions and 8 deletions
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4
doc/src/sgml/config.sgml
View File

@ -8568,7 +8568,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</term>
<listitem>
<para>
Enables timing of database I/O calls. This parameter is off by
Enables timing of database I/O waits. This parameter is off by
default, as it will repeatedly query the operating system for
the current time, which may cause significant overhead on some
platforms. You can use the <xref linkend="pgtesttiming"/> tool to
@ -8602,7 +8602,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</term>
<listitem>
<para>
Enables timing of WAL I/O calls. This parameter is off by default,
Enables timing of WAL I/O waits. This parameter is off by default,
as it will repeatedly query the operating system for the current time,
which may cause significant overhead on some platforms.
You can use the <application>pg_test_timing</application> tool to

12
doc/src/sgml/monitoring.sgml
View File

@ -2747,7 +2747,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<structfield>read_time</structfield> <type>double precision</type>
</para>
<para>
Time spent in read operations in milliseconds (if
Time spent waiting for read operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@ -2785,7 +2785,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<structfield>write_time</structfield> <type>double precision</type>
</para>
<para>
Time spent in write operations in milliseconds (if
Time spent waiting for write operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@ -2813,7 +2813,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<structfield>writeback_time</structfield> <type>double precision</type>
</para>
<para>
Time spent in writeback operations in milliseconds (if
Time spent waiting for writeback operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled, otherwise zero). This
includes the time spent queueing write-out requests and, potentially,
the time spent to write out the dirty data.
@ -2849,7 +2849,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<structfield>extend_time</structfield> <type>double precision</type>
</para>
<para>
Time spent in extend operations in milliseconds. (if
Time spent waiting for extend operations in milliseconds. (if
<xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@ -2923,7 +2923,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<structfield>fsync_time</structfield> <type>double precision</type>
</para>
<para>
Time spent in fsync operations in milliseconds (if
Time spent waiting for fsync operations in milliseconds (if
<xref linkend="guc-track-io-timing"/> is enabled and
<varname>object</varname> is not <literal>wal</literal>,
or if <xref linkend="guc-track-wal-io-timing"/> is enabled
@ -3010,7 +3010,7 @@ description | Waiting for a newly initialized WAL file to reach durable storage
<note>
<para>
Columns tracking I/O time will only be non-zero when
Columns tracking I/O wait time will only be non-zero when
<xref linkend="guc-track-io-timing"/> is enabled. The user should be
careful when referencing these columns in combination with their
corresponding I/O operations in case <varname>track_io_timing</varname>