From: | Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> |
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To: | Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> |
Cc: | ikedamsh(at)oss(dot)nttdata(dot)com, pgsql-hackers(at)postgresql(dot)org, tsunakawa(dot)takay(at)fujitsu(dot)com, magnus(at)hagander(dot)net |
Subject: | Re: New statistics for tuning WAL buffer size |
Date: | 2020-09-11 08:13:37 |
Message-ID: | afb99958-b4ad-e936-4001-400d6f3a22b3@oss.nttdata.com |
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Lists: | pgsql-hackers |
On 2020/09/11 16:54, Kyotaro Horiguchi wrote:
> At Fri, 11 Sep 2020 13:48:49 +0900, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote in
>>
>>
>> On 2020/09/11 12:17, Kyotaro Horiguchi wrote:
>>> Hello.
>>> At Wed, 09 Sep 2020 13:57:37 +0900, Masahiro Ikeda
>>> <ikedamsh(at)oss(dot)nttdata(dot)com> wrote in
>>>> I checked what function calls XLogBackgroundFlush() which calls
>>>> AdvanceXLInsertBuffer() to increment m_wal_buffers_full.
>>>>
>>>> I found that WalSndWaitForWal() calls it, so I added it.
>>>> Is it better to move it in WalSndLoop() like the attached patch?
>>> By the way, we are counting some wal-related numbers in
>>> pgWalUsage.(bytes, records, fpi). Since now that we are going to have
>>> a new view related to WAL statistics, wouln't it be more useful to
>>> show them together in the view?
>>
>> Probably yes. But IMO it's better to commit the current patch first,
>> and then add those stats into the view after confirming exposing them
>> is useful.
>
> I'm fine with that.
>
>> BTW, to expose the total WAL bytes, I think it's better to just save
>> the LSN at when pg_stat_wal is reset rather than counting
>> pgWalUsage.bytes. If we do that, we can easily total WAL bytes by
>> subtracting that LSN from the latest LSN. Also saving the LSN at the
>> reset timing causes obviously less overhead than counting
>> pgWalUsage.bytes.
>
> pgWalUsage is always counting so it doesn't add any overhead.
Yes. And I'm a bit concerned about the overhead by frequent message sent for WAL bytes to the stats collector.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
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