Re: Recovery inconsistencies, standby much larger than primary

On 2014-02-14 20:46:01 +0000, Greg Stark wrote:
> Going over this I think this is still a potential issue:
>
> On 31 Jan 2014 15:56, "Andres Freund" <andres(at)2ndquadrant(dot)com> wrote:
>
> >
> > I am not sure that explains the issue, but I think the redo action for
> > truncation is not safe across crashes. A XLOG_SMGR_TRUNCATE will just
> > do a smgrtruncate() (and then mdtruncate) which will iterate over the
> > segments starting at 0 till mdnblocks()/segment_size and *truncate* but
> > not delete individual segment files that are not needed anymore, right?
> > If we crash in the midst of that a new mdtruncate() will be issued, but
> > it will get a shorter value back from mdnblocks().
> >
> > Am I missing something?
> >
>
> I'm not too familiar with md.c but my reading of the code is that we
> truncate the files in reverse order?

That's what I had assumed as well, but it doesn't look that way:

v = mdopen(reln, forknum, EXTENSION_FAIL);

priorblocks = 0; /* <- initial value */
while (v != NULL)
{
MdfdVec *ov = v;

if (priorblocks > nblocks)
{
/* truncate entire segment */
}
else if (priorblocks + ((BlockNumber) RELSEG_SIZE) > nblocks)
{
/* truncate entire segment */
}
else
{
/* nothing to do, all needed */
}
priorblocks += RELSEG_SIZE;
}

So, according to that we start at segment 0, right?

Greetings,

Andres Freund

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