<para>

There are two internal functions to write WAL data to disk:

<function>XLogWrite</function> and <function>issue_xlog_fsync</function>.

When <xref linkend="guc-track-wal-io-timing"/> is enabled, the total

When <xref linkend="guc-track-io-timing"/> is enabled, the total

amounts of time <function>XLogWrite</function> writes and

<function>issue_xlog_fsync</function> syncs WAL data to disk are counted as

<literal>wal_write_time</literal> and <literal>wal_sync_time</literal> in

<xref linkend="pg-stat-wal-view"/>, respectively.

<varname>write_time</varname> and <varname>sync_time</varname> in

<xref linkend="pg-stat-io-view"/> for the <varname>object</varname>

<literal>wal</literal>, respectively.

<function>XLogWrite</function> is normally called by

<function>XLogInsertRecord</function> (when there is no space for the new

record in WAL buffers), <function>XLogFlush</function> and the WAL writer,

<literal>fsync</literal>, or <literal>fsync_writethrough</literal>,

the write operation moves WAL buffers to kernel cache and

<function>issue_xlog_fsync</function> syncs them to disk. Regardless

of the setting of <varname>track_wal_io_timing</varname>, the number

of the setting of <varname>track_io_timing</varname>, the number

of times <function>XLogWrite</function> writes and

<function>issue_xlog_fsync</function> syncs WAL data to disk are also

counted as <literal>wal_write</literal> and <literal>wal_sync</literal>

in <structname>pg_stat_wal</structname>, respectively.

counted as <varname>writes</varname> and <varname>fsyncs</varname>

in <structname>pg_stat_io</structname> for the <varname>object</varname>

<literal>wal</literal>, respectively.

</para>

<para>