Re: Introduce XID age and inactive timeout based replication slot invalidation

On Mon, Mar 11, 2024 at 4:09 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> I don't see how it will be easier for the user to choose the default
> value of 'max_slot_xid_age' compared to 'max_slot_wal_keep_size'. But,
> I agree similar to 'max_slot_wal_keep_size', 'max_slot_xid_age' can be
> another parameter to allow vacuum to proceed removing the rows which
> otherwise it wouldn't have been as those would be required by some
> slot. Now, if this understanding is correct, we should probably make
> this invalidation happen by (auto)vacuum after computing the age based
> on this new parameter.

Currently, the patch computes the XID age in the checkpointer using
the next XID (gets from ReadNextFullTransactionId()) and slot's xmin
and catalog_xmin. I think the checkpointer is best because it already
invalidates slots for wal_removed cause, and flushes all replication
slots to disk. Moving this new invalidation functionality into some
other background process such as autovacuum will not only burden that
process' work but also mix up the unique functionality of that
background process.

Having said above, I'm open to ideas from others as I'm not so sure if
there's any issue with checkpointer invalidating the slots for new
reasons.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com