Re: Metric to calculate WAL size left to transfer to Standby

On Wed, 2021-04-14 at 17:50 -0400, Viral Shah wrote:
> We have a PostgreSQL 10.12 cluster of servers in two different data centers.
> Off lately, in the case of a large WAL generation, we are seeing replication
> delay between the master and the standby server. These delays have off lately
> been there for an unusually long time. I was wondering if we have any metric
> that can calculate the amount (size) of WAL transfer left between master and
> standby?
>
> PS: We have ensured we have upgraded our firewalls for better speed transfer.
>
> Any help on how to figure out the slowness in the WAL transfer would be much appreciated.

SELECT pg_wal_lsn_diff(pg_current_wal_lsn(), flush_lsn) AS transfer_lag,
pg_wal_lsn_diff(pg_current_wal_lsn(), replay_lsn) AS replay_lag
FROM pg_stat_replication;

If both are delayed, it might be that the network cannot cope.

If only the second number is delayed, you have replication conflicts
with queries on the standby.

Yours,
Laurenz Albe
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