Re: [GSoC2014] Patch ALTER TABLE ... SET LOGGED

On 2014-07-16 20:53:06 +0200, Andres Freund wrote:
> On 2014-07-16 20:25:42 +0200, Andres Freund wrote:
> > Hi,
> >
> > I quickly looked at this patch and I think there's major missing pieces
> > around buffer management and wal logging.
> >
> > a) Currently buffers that are in memory marked as
> > permanent/non-permanent aren't forced out to disk/pruned from cache,
> > not even when they're dirty.
> > b) When converting from a unlogged to a logged table the relation needs
> > to be fsynced.
> > c) Currently a unlogged table changed into a logged one will be
> > corrupted on a standby because its contents won't ever be WAL logged.
>
> Forget that, didn't notice that you're setting tab->rewrite = true.

So, while that danger luckily isn't there I think there's something
similar. Consider:

CREATE TABLE blub(...);
INSERT INTO blub ...;

BEGIN;
ALTER TABLE blub SET UNLOGGED;
ROLLBACK;

The rewrite will read in the 'old' contents - but because it's done
after the pg_class.relpersistence is changed they'll all not be marked
as BM_PERMANENT in memory. Then the ALTER TABLE is rolled back,
including the relpersistence setting. Which will unfortunately leave
pages with the wrong persistency setting in memory, right?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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