Re: Should vacuum process config file reload more often

On Thu, Apr 27, 2023 at 8:55 AM Daniel Gustafsson <daniel@yesql.se> wrote:
>
> > On 27 Apr 2023, at 14:10, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Thu, Apr 27, 2023 at 6:30 PM John Naylor
> > <john.naylor@enterprisedb.com> wrote:
> >>
> >>
> >> On Fri, Apr 7, 2023 at 6:08 AM Daniel Gustafsson <daniel@yesql.se> wrote:
> >>>
> >>> I had another read-through and test-through of this version, and have applied
> >>> it with some minor changes to comments and whitespace.  Thanks for the quick
> >>> turnaround times on reviews in this thread!
> >>
> >> -       VacuumFailsafeActive = false;
> >> +       Assert(!VacuumFailsafeActive);
> >>
> >> I can trigger this assert added in commit 7d71d3dd08.
> >>
> >> First build with the patch in [1], then:
> >>
> >> session 1:
> >>
> >> CREATE EXTENSION xid_wraparound ;
> >>
> >> CREATE TABLE autovacuum_disabled(id serial primary key, data text) WITH (autovacuum_enabled=false);
> >> INSERT INTO autovacuum_disabled(data) SELECT generate_series(1,1000);
> >>
> >> -- I can trigger without this, but just make sure it doesn't get vacuumed
> >> BEGIN;
> >> DELETE FROM autovacuum_disabled WHERE id % 2 = 0;
> >>
> >> session 2:
> >>
> >> -- get to failsafe limit
> >> SELECT consume_xids(1*1000*1000*1000);
> >> INSERT INTO autovacuum_disabled(data) SELECT 1;
> >> SELECT consume_xids(1*1000*1000*1000);
> >> INSERT INTO autovacuum_disabled(data) SELECT 1;
> >>
> >> VACUUM autovacuum_disabled;
> >>
> >> WARNING:  cutoff for removing and freezing tuples is far in the past
> >> HINT:  Close open transactions soon to avoid wraparound problems.
> >> You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
> >> WARNING:  bypassing nonessential maintenance of table "john.public.autovacuum_disabled" as a failsafe after 0 index scans
> >> DETAIL:  The table's relfrozenxid or relminmxid is too far in the past.
> >> HINT:  Consider increasing configuration parameter "maintenance_work_mem" or "autovacuum_work_mem".
> >> You might also need to consider other ways for VACUUM to keep up with the allocation of transaction IDs.
> >> server closed the connection unexpectedly
> >>
> >> #0  0x00007ff31f68ebec in __pthread_kill_implementation ()
> >>   from /lib64/libc.so.6
> >> #1  0x00007ff31f63e956 in raise () from /lib64/libc.so.6
> >> #2  0x00007ff31f6287f4 in abort () from /lib64/libc.so.6
> >> #3  0x0000000000978032 in ExceptionalCondition (
> >>    conditionName=conditionName@entry=0xa4e970 "!VacuumFailsafeActive",
> >>    fileName=fileName@entry=0xa4da38 "../src/backend/access/heap/vacuumlazy.c", lineNumber=lineNumber@entry=392) at ../src/backend/utils/error/assert.c:66
> >> #4  0x000000000058c598 in heap_vacuum_rel (rel=0x7ff31d8a97d0,
> >>    params=<optimized out>, bstrategy=<optimized out>)
> >>    at ../src/backend/access/heap/vacuumlazy.c:392
> >> #5  0x000000000069af1f in table_relation_vacuum (bstrategy=0x14ddca8,
> >>    params=0x7ffec28585f0, rel=0x7ff31d8a97d0)
> >>    at ../src/include/access/tableam.h:1705
> >> #6  vacuum_rel (relid=relid@entry=16402, relation=relation@entry=0x0,
> >>    params=params@entry=0x7ffec28585f0, skip_privs=skip_privs@entry=true,
> >>    bstrategy=bstrategy@entry=0x14ddca8)
> >>    at ../src/backend/commands/vacuum.c:2202
> >> #7  0x000000000069b0e4 in vacuum_rel (relid=16398, relation=<optimized out>,
> >>    params=params@entry=0x7ffec2858850, skip_privs=skip_privs@entry=false,
> >>    bstrategy=bstrategy@entry=0x14ddca8)
> >>    at ../src/backend/commands/vacuum.c:2236
> >
> > Good catch. I think the problem is that vacuum_rel() is called
> > recursively and we don't reset VacuumFailsafeActive before vacuuming
> > the toast table. I think we should reset it in heap_vacuum_rel()
> > instead of Assert(). It's possible that we trigger the failsafe mode
> > only for either one.Please find the attached patch.
>
> Agreed, that matches my research and testing, I have the same diff here and it
> passes testing and works as intended.  This was briefly discussed in [0] and
> slightly upthread from there but then missed.  I will do some more looking and
> testing but I'm fairly sure this is the right fix, so unless I find something
> else I will go ahead with this.
>
> xid_wraparound is a really nifty testing tool. Very cool.Makes sense to me too.

Fix LGTM.
Though we previously set it to false before this series of patches,
perhaps it is
worth adding a comment about why VacuumFailsafeActive must be reset here
even though we reset it before vacuuming each table?

- Melanie