On 6/15/21 3:31 PM, Andrew Dunstan wrote:
>
> On 6/15/21 8:04 AM, Heikki Linnakangas wrote:
>>
>> Yeah, WAL-logging the contents of the source database would certainly
>> be less weird than the current system. As Julien also pointed out, the
>> question is, are there people using on "CREATE DATABASE foo TEMPLATE
>> bar" to copy a large source database, on the premise that it's fast
>> because it skips WAL-logging?
>
>
> I'm 100% certain there are. It's not even a niche case.
>
>
>>
>> In principle, we could have both mechanisms, and use the new
>> WAL-logged system if the database is small, and the old system with
>> checkpoints if it's large. But I don't like idea of having to maintain
>> both.
>>
>>
>
> Rather than use size, I'd be inclined to say use this if the source
> database is marked as a template, and use the copydir approach for
> anything that isn't.
>
I think we should be asking what is the benefit of that use case, and
perhaps try addressing that without having to maintain two entirely
different ways to do CREATE DATABASE. It's not like we're sure the
current code is 100% reliable in various corner cases, I doubt having
two separate approaches will improve the situation :-/
I can see three reasons why people want to skip the WAL logging:
1) it's faster, because there's no CPU and I/O for building the WAL
I wonder if some optimization / batching could help with (1), as
suggested by Andres elsewhere in this thread.
2) it saves the amount of WAL (could matter with large template
databases and WAL archiving, etc.)
We can't really do much about this - we need to log all the data. But
the batching from (1) might help a bit too, I guess.
3) saves the amount of WAL that needs to be copied to standby, so that
there's no increase of replication lag, etc. particularly when the
network link has limited bandwidth
I think this is a more general issue - some operations that may
generate a lot of WAL, and we generally assume it's better to do
that rather than hold exclusive locks for long time. But maybe we
could have some throttling, to limit the amount of WAL per second,
similarly to what we have to plain vacuum.
regards
--
Tomas Vondra
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