On 11/13/23 00:53, Laurenz Albe wrote:
> On Sun, 2023-11-12 at 21:55 -0500, Tom Lane wrote:
>> yuansong <yyuansong@126.com> writes:
>> > In PostgreSQL, when a backend process crashes, it can cause other backend
>> > processes to also require a restart, primarily to ensure data consistency.
>> > I understand that the correct approach is to analyze and identify the
>> > cause of the crash and resolve it. However, it is also important to be
>> > able to handle a backend process crash without affecting the operation of
>> > other processes, thus minimizing the scope of negative impact and
>> > improving availability. To achieve this goal, could we mimic the Oracle
>> > process by introducing a "pmon" process dedicated to rolling back crashed
>> > process transactions and performing resource cleanup? I wonder if anyone
>> > has attempted such a strategy or if there have been previous discussions
>> > on this topic.
>>
>> The reason we force a database-wide restart is that there's no way to
>> be certain that the crashed process didn't corrupt anything in shared
>> memory. (Even with the forced restart, there's a window where bad
>> data could reach disk before we kill off the other processes that
>> might write it. But at least it's a short window.) "Corruption"
>> here doesn't just involve bad data placed into disk buffers; more
>> often it's things like unreleased locks, which would block other
>> processes indefinitely.
>>
>> I seriously doubt that anything like what you're describing
>> could be made reliable enough to be acceptable. "Oracle does
>> it like this" isn't a counter-argument: they have a much different
>> (and non-extensible) architecture, and they also have an army of
>> programmers to deal with minutiae like undoing resource acquisition.
>> Even with that, you'd have to wonder about the number of bugs
>> existing in such necessarily-poorly-tested code paths.
>
> Yes.
> I think that PostgreSQL's approach is superior: rather than investing in
> code to mitigate the impact of data corruption caused by a crash, invest
> in quality code that doesn't crash in the first place.
While true, this does nothing to prevent OOM kills, which are becoming
more prevalent as, for example, running Postgres in a container (or
otherwise) with a cgroup memory limit becomes more popular.
And in any case, there are enterprise use cases that necessarily avoid
Postgres due to this behavior, which is a shame.
--
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com