Re: BUG #10675: alter database set tablespace and unlogged table

From: "Andres Freund" <andres@2ndquadrant.com>
> That'd mean that the next pointrelease will incur *significantly* higher
> IO in many setups. If you currently have a workload where all dirty
> buffers of unlogged tables fit in s_b you'll never have any OS
> writes. That'd completely change.

Yes, that's the only headache...  But I'm not worried so much, because
bgwriter and/or backends may write out dirty buffers for unlogged tables, so
the total I/O is not low anyway.


> A compromise would be to couple it to CHECKPOINT_IMMEDIATE and/or _FORCE
> instead of creating a new flag - I think this is actually what Tom
> proposed.

That rescues the CREATE DATABASE case, and would probably prevent potential
similar problems.


>> * There's a greater danger of losing data during operating system
>> restart.
>> For example, IIRC, Windows gives only 20 seconds to terminate all
>> services
>> during OS shutdown.  If many dirty buffers for unlogged tables linger in
>> the
>> shared buffers, PostgreSQL service may fail to complete database
>> shutdown.
>> Even if the online checkpoint writes out all dirty buffers, the
>> possibility
>> of there being many dirty buffers at shutdown is not zero, but the
>> probability would be lower.
>
> Meh. That won't lead to data loss, just recovery on restart. And 20s
> isn't sufficient for any halfway busy database anyway.

The unlogged tables are emptied (= data loss) at recovery restart.

Regards
MauMau