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Subject: Re: max_slot_wal_keep_size and wal_keep_segments
To: Fujii Masao <masao.fujii@oss.nttdata.com>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>
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From: David Steele <david@pgmasters.net>
Message-ID: <8b385d28-cb4e-25b2-4fcd-c5b30dd5a9a3@pgmasters.net>
Date: Mon, 20 Jul 2020 08:21:46 -0400
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On 7/20/20 6:02 AM, Fujii Masao wrote:
> 
> 
> On 2020/07/20 13:48, Fujii Masao wrote:
>>
>>
>> On 2020/07/17 20:24, David Steele wrote:
>>>
>>> On 7/17/20 5:11 AM, Fujii Masao wrote:
>>>>
>>>>
>>>> On 2020/07/14 20:30, David Steele wrote:
>>>>> On 7/14/20 12:00 AM, Fujii Masao wrote:
>>>>>>
>>>>>> The patch was no longer applied cleanly because of recent commit.
>>>>>> So I updated the patch. Attached.
>>>>>>
>>>>>> Barring any objection, I will commit this patch.
>>>>>
>>>>> This doesn't look right:
>>>>>
>>>>> +   the <xref linkend="guc-wal-keep-size"/> most recent megabytes
>>>>> +   WAL files plus one WAL file are
>>>>>
>>>>> How about:
>>>>>
>>>>> +   <xref linkend="guc-wal-keep-size"/> megabytes of
>>>>> +   WAL files plus one WAL file are
>>>>
>>>> Thanks for the comment! Isn't it better to keep "most recent" part?
>>>> If so, what about either of the followings?
>>>>
>>>> 1. <xref linkend="guc-wal-keep-size"/> megabytes of WAL files plus
>>>>      one WAL file that were most recently generated are kept all time.
>>>>
>>>> 2. <xref linkend="guc-wal-keep-size"/> megabytes + <xref 
>>>> linkend="guc-wal-segment-size"> bytes
>>>>      of WAL files that were most recently generated are kept all time.
>>>
>>> "most recent" seemed implied to me, but I see your point.
>>>
>>> How about:
>>>
>>> +   the most recent <xref linkend="guc-wal-keep-size"/> megabytes of
>>> +   WAL files plus one additional WAL file are
>>
>> I adopted this and pushed the patch. Thanks!
>>
>> Also we need to update the release note for v13. What about adding the 
>> following?
>>
>> ------------------------------------
>> Rename configuration parameter wal_keep_segments to wal_keep_size.
>>
>> This allows how much WAL files to retain for the standby server, by 
>> bytes instead of the number of files.
>> If you previously used wal_keep_segments, the following formula will 
>> give you an approximately equivalent setting:
>>
>> wal_keep_size = wal_keep_segments * wal_segment_size (typically 16MB)
>> ------------------------------------

I would rework that first sentence a bit. How about:

+ This determines how much WAL to retain for the standby server,
+ specified in megabytes rather than number of files.

The rest looks fine to me.

Regards,
-- 
-David
david@pgmasters.net