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

Hi,

On 2021-12-29 11:31:51 -0800, Andres Freund wrote:
> That's pretty much the same - XLogInsert() can trigger an
> XLogWrite()/Flush().
> 
> I think it's a complete no-go to add throttling to these places. It's quite
> possible that it'd cause new deadlocks, and it's almost guaranteed to have
> unintended consequences (e.g. replication falling back further because
> XLogFlush() is being throttled).

I thought of another way to implement this feature. What if we checked the
current distance somewhere within XLogInsert(), but only set
InterruptPending=true, XLogDelayPending=true. Then in ProcessInterrupts() we
check if XLogDelayPending is true and sleep the appropriate time.

That way the sleep doesn't happen with important locks held / within a
critical section, but we still delay close to where we went over the maximum
lag. And the overhead should be fairly minimal.


I'm doubtful that implementing the waits on a transactional level provides a
meaningful enough amount of control - there's just too much WAL that can be
generated within a transaction.

Greetings,

Andres Freund