回复： Why is XLOG_FPI_FOR_HINT always need backups?

Thank you for reply.
You are right and the PostgreSQL server writes the entire content of each disk page to WAL during the first modification of that page after a
checkpoint while data checksum is on.

But I wonder whether it is necessary or not while my file system can protect the blocks of database to be torn. And I read a comment in function MarkBufferDirtyHint:
```
/*
 * If we need to protect hint bit updates from torn writes, WAL-log a
 * full page image of the page. This full page image is only necessary
 * if the hint bit update is the first change to the page since the
 * last checkpoint.
 *
 * We don't check full_page_writes here because that logic is included
 * when we call XLogInsert() since the value changes dynamically.
 */

```
However, the code tell me it has nothing to do with full_page_writes. I can't figure it out. 

--
Zhang Wenjie

------------------ 原始邮件 ------------------
发件人: "Japin Li" <japinli(at)hotmail(dot)com&gt;;
发送时间:&nbsp;2021年7月6日(星期二) 晚上7:04
收件人:&nbsp;"zwj"<757634191(at)qq(dot)com&gt;;
抄送:&nbsp;"pgsql-hackers"<pgsql-hackers(at)lists(dot)postgresql(dot)org&gt;;
主题:&nbsp;Re: Why is XLOG_FPI_FOR_HINT always need backups?

On Tue, 06 Jul 2021 at 17:58, zwj <757634191(at)qq(dot)com&gt; wrote:
&gt; Hi all,
&gt;
&gt; When I read the source code file src/backend/access/transam/xloginsert.c, I get something confused me.
&gt; In the function XLogSaveBufferForHint, the flags are always REGBUF_FORCE_IMAGE which means it is always need backups.
&gt; Is it right? Why do not check the full_page_writes?

The documentation [1] says:

------------------------------------------------------------------------------
wal_log_hints (boolean)
When this parameter is on, the PostgreSQL server writes the entire content of
each disk page to WAL during the first modification of that page after a
checkpoint, even for non-critical modifications of so-called hint bits.
------------------------------------------------------------------------------

Does that mean whether the full_page_writes enable or not, if the wal_log_hints
enabled, we always write the entire content of each disk page to WAL? If I'm
right, should we mention this in wal_log_hints?

[1] https://www.postgresql.org/docs/current/runtime-config-wal.html

--
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.