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@hotmail.com>;
发送时间: 2021年7月6日(星期二) 晚上7:04
收件人: "zwj"<757634191@qq.com>;
抄送: "pgsql-hackers"<pgsql-hackers@lists.postgresql.org>;
主题: Re: Why is XLOG_FPI_FOR_HINT always need backups?
On Tue, 06 Jul 2021 at 17:58, zwj <757634191@qq.com> wrote:
> Hi all,
>
> When I read the source code file src/backend/access/transam/xloginsert.c, I get something confused me.
> In the function XLogSaveBufferForHint, the flags are always REGBUF_FORCE_IMAGE which means it is always need backups.
> 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.
