Re: Throttling WAL inserts when the standby falls behind more than the configured replica_lag_in_bytes

Поиск
Список
Период
Сортировка
От SATYANARAYANA NARLAPURAM
Тема Re: Throttling WAL inserts when the standby falls behind more than the configured replica_lag_in_bytes
Дата
Msg-id CAHg+QDdeHo6ydZBs+T55fD8YYAHA8TOyRvK+c8EYSC9-wMjWXw@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Throttling WAL inserts when the standby falls behind more than the configured replica_lag_in_bytes  (Dilip Kumar <dilipbalaut@gmail.com>)
Ответы Re: Throttling WAL inserts when the standby falls behind more than the configured replica_lag_in_bytes  (Dilip Kumar <dilipbalaut@gmail.com>)
Re: Throttling WAL inserts when the standby falls behind more than the configured replica_lag_in_bytes  (Andres Freund <andres@anarazel.de>)
Список pgsql-hackers
Дерево обсуждения


On Wed, Dec 29, 2021 at 10:38 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
On Thu, Dec 30, 2021 at 1:09 AM Andres Freund <andres@anarazel.de> wrote:
Hi,

On 2021-12-29 11:34:53 -0800, SATYANARAYANA NARLAPURAM wrote:
> On Wed, Dec 29, 2021 at 11:31 AM Andres Freund <andres@anarazel.de> wrote:
> Andres, thanks for the comments. Agreed on this based on the previous
> discussions on this thread. Could you please share your thoughts on adding
> it after SyncRepWaitForLSN()?

I don't think that's good either - you're delaying transaction commit
(i.e. xact becoming visible / locks being released).

Agree with that.
 
That also has the danger
of increasing lock contention (albeit more likely to be heavyweight locks /
serializable state). It'd have to be after the transaction actually committed.

Yeah, I think that would make sense, even though we will be allowing a new backend to get connected insert WAL, and get committed but after that, it will be throttled.  However, if the number of max connections will be very high then even after we detected a lag there a significant amount WAL could be generated, even if we keep long-running transactions aside.  But I think still it will serve the purpose of what Satya is trying to achieve.

I am afraid there are problems with making the RPO check post releasing the locks. By this time the transaction is committed and visible to the other backends (ProcArrayEndTransaction is already called) though the intention is to block committing transactions that violate the defined RPO. Even though we block existing connections starting a new transaction, it is possible to do writes by opening a new connection / canceling the query. I am not too much worried about the lock contention as the system is already hosed because of the policy. This behavior is very similar to what happens when the Sync standby is not responding. Thoughts?


 

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

В списке pgsql-hackers по дате отправления:

Предыдущее
От: "houzj.fnst@fujitsu.com"
Дата:
Сообщение: RE: Confused comment about drop replica identity index
Следующее
От: Dilip Kumar
Дата:
Сообщение: Re: Per-table storage parameters for TableAM/IndexAM extensions