Thread

1. max_wal_senders

   Rick Otten <rottenwindfish@gmail.com> — 2023-02-08T23:07:15Z

   I've been thinking about the max_wal_senders parameter lately and wondering
   if there is any harm in setting it too high.  I'm wondering if I should try
   to shave a few senders off, perhaps to match my logical replicas + 1,
   instead of just leaving it at the default of 10.  Or vice-versa, can
   clients use more than one sender if they are available?  Would increasing
   it result in lower latency?  The documentation is a little vague.
   
   The documentation mentions an orphaned connection slot that may take a
   while to time out.  How can I tell if I have any of those?  I was looking
   for a `pg_wal_slots` table similar to the `pg_replication_slots` table, but
   don't see anything obvious in the catalog.
   

2. Re: max_wal_senders

   Laurenz Albe <laurenz.albe@cybertec.at> — 2023-02-09T05:59:53Z

   On Wed, 2023-02-08 at 18:07 -0500, Rick Otten wrote:
   > I've been thinking about the max_wal_senders parameter lately and wondering if there
   > is any harm in setting it too high.
   
   No, there isn't, except that if you end up having too many *actual* WAL senders, it
   will cause load.  A high limit is no problem as such.
   
   > The documentation mentions an orphaned connection slot that may take a while to time out.
   > How can I tell if I have any of those?  I was looking for a `pg_wal_slots` table
   > similar to the `pg_replication_slots` table, but don't see anything obvious in the catalog.
   
   The view is "pg_stat_replication", but you won't see there if an entry is
   abandoned before PostgreSQL does and terminates it.  You can set "tcp_keepalived_idle"
   low enough so that the kernel will detect broken connections early on.
   
   Yours,
   Laurenz Albe
   
   
   
   

3. Re: max_wal_senders

   Andres Freund <andres@anarazel.de> — 2023-02-09T06:40:12Z

   Hi,
   
   On 2023-02-09 06:59:53 +0100, Laurenz Albe wrote:
   > On Wed, 2023-02-08 at 18:07 -0500, Rick Otten wrote:
   > > I've been thinking about the max_wal_senders parameter lately and wondering if there
   > > is any harm in setting it too high.
   > 
   > No, there isn't, except that if you end up having too many *actual* WAL senders, it
   > will cause load.  A high limit is no problem as such.
   
   That's not *quite* true. The downsides are basically the same as for
   max_connections (It's basically treated the same, see
   InitializeMaxBackends()): You need more shared memory. There's a small
   degradation of performance due to the increased size of some shared
   datastructures, most prominently the lock table for heavyweight locks.
   
   Greetings,
   
   Andres Freund