From: | Craig Ringer <craig(at)2ndquadrant(dot)com> |
---|---|
To: | Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com> |
Cc: | Andres Freund <andres(at)anarazel(dot)de>, Thom Brown <thom(at)linux(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Logical decoding on standby |
Date: | 2017-03-30 03:34:01 |
Message-ID: | CAMsr+YGKJ8AtcwkGUnjn_gJho-z+0L3mQWg8xT8YvC+Sa4nDOw@mail.gmail.com |
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Lists: | pgsql-hackers |
On 29 March 2017 at 23:13, Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com> wrote:
> On 29 March 2017 at 10:17, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>> On 29 March 2017 at 16:44, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>>
>>> * Split oldestCatalogXmin tracking into separate patch
>>
>> Regarding this, Simon raised concerns about xlog volume here.
>>
>> It's pretty negligible.
>>
>> We only write a new record when a vacuum runs after catalog_xmin
>> advances on the slot with the currently-lowest catalog_xmin (or, if
>> vacuum doesn't run reasonably soon, when the bgworker next looks).
>
> I'd prefer to slow things down a little, not be so eager.
>
> If we hold back update of the catalog_xmin until when we run
> GetRunningTransactionData() we wouldn't need to produce any WAL
> records at all AND we wouldn't need to have VACUUM do
> UpdateOldestCatalogXmin(). Bgwriter wouldn't need to perform an extra
> task.
>
> That would also make this patch about half the length it is.
>
> Let me know what you think.
Good idea.
We can always add a heuristic later to make xl_running_xacts get
emitted more often at high transaction rates if it's necessary.
Patch coming soon.
--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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