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Date: Mon, 16 Nov 2020 18:24:10 +0900
From: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
To: lchch1990@sina.cn
Cc: Amit Kapila <amit.kapila16@gmail.com>, pgsql-hackers
 <pgsql-hackers@postgresql.org>
Subject: Re: Add statistics to pg_stat_wal view for wal related parameter
 tuning
In-Reply-To: <2020111311322320943423@sina.cn>
References: <35ef960128b90bfae3b3fdf60a3a860f@oss.nttdata.com>,
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 <06bc2d405b086a72fb14657de9befae8@oss.nttdata.com>,
 <2020102114544806663477@sina.cn>,
 <ac8b1f5e589932254efe373e252e5dcb@oss.nttdata.com>
 <2020111311322320943423@sina.cn>
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On 2020-11-13 12:32, lchch1990@sina.cn wrote:
>> Now, pg_stat_wal supports reset all informantion in WalStats
>> using pg_stat_reset_shared('wal') function.
>> Isn't it enough?
> Yes it ok, sorry I miss this infomation.

OK.

>>> 3. I do not think it's a correct describe in document for
>>> 'wal_buffers_full'.
> 
>> Where should I rewrite the description? If my understanding is not
>> correct, please let me know.
> Sorry I have not described it clearly, because I can not understand
> the meaning of this
> column after I read the describe in document.
> And now I read the source code of walwrite and found the place where
> 'wal_buffers_full'
> added is for a backend to wait a wal buffer which is occupied by other
> wal page, so the
> backend flush the old page in the wal buffer(after wait it can).
> So i think the origin decribe in document is not so in point, we can
> describe it such as
> 'Total number of times WAL data written to the disk because a backend
> yelled a wal buffer
> for an advanced wal page.
> 
> Sorry if my understand is wrong.

Thanks for your comments.

You're understanding is almost the same as mine.
It describes when not only backends but also other backgrounds 
initialize a new wal page,
wal buffer's space is already used and there is no space.

> 'Total number of times WAL data written to the disk because a backend
> yelled a wal buffer for an advanced wal page'

Thanks for your suggestion.
I wondered that users may confuse about how to use "wal_buffers_full" 
and how to tune parameters.

I thought the reason which wal buffer has no space is
important for users to tune the wal_buffers parameter.

How about the following comments?

'Total number of times WAL data was written to the disk because WAL 
buffers got full
  when to initialize a new WAL page'


Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION