Re: Should vacuum process config file reload more often

> 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.

--
Daniel Gustafsson

[0] CAAKRu_b1HjGCTsFpUnmwLNS8NeXJ+JnrDLhT1osP+Gq9HCU+Rw@mail.gmail.com