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

On Sun, Dec 26, 2021 at 10:36 AM SATYANARAYANA NARLAPURAM <
satyanarlapuram(at)gmail(dot)com> wrote:

>
>> Actually all the WAL insertions are done under a critical section (except
>> few exceptions), that means if you see all the references of XLogInsert(),
>> it is always called under the critical section and that is my main worry
>> about hooking at XLogInsert level.
>>
>
> Got it, understood the concern. But can we document the limitations of the
> hook and let the hook take care of it? I don't expect an error to be thrown
> here since we are not planning to allocate memory or make file system calls
> but instead look at the shared memory state and add delays when required.
>
>
Yet another problem is that if we are in XlogInsert() that means we are
holding the buffer locks on all the pages we have modified, so if we add a
hook at that level which can make it wait then we would also block any of
the read operations needed to read from those buffers. I haven't thought
what could be better way to do this but this is certainly not good.

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