Re: visibilitymap_set and checksums

On 2013-05-24 19:09:57 +0100, Simon Riggs wrote:
> On 24 May 2013 18:40, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>
> > That pattern looks dangerous. Setting the lsn of the heap page will
> > prevent the next action from doing a FPI even if it would be required.
>
> Can you be more specific about the danger you see?

CHECKPOINT at lsn 0/10;
vacuum starts
vacuum finds page which is all visible
vacuum sets all_visible
PageSetAllVisible(page);
MarkBufferDirty(buf);
visibilitymap_set(onerel, blkno, buf, InvalidXLogRecPtr,
vmbuffer, visibility_cutoff_xid);
recptr = log_heap_visible(rel->rd_node, heapBuf, vmBuf,
cutoff_xid);
if (DataChecksumsEnabled())
PageSetLSN(heapPage, recptr);

So at this point the *heap* page will have the lsn of the
xl_heap_visible record. Which I thought to be rather dangerous because I
somewow missed the fact that log_heap_visible does:
if (DataChecksumsEnabled())
{
rdata[1].next = &(rdata[2]);

rdata[2].data = NULL;
rdata[2].len = 0;
rdata[2].buffer = heap_buffer;
rdata[2].buffer_std = true;
rdata[2].next = NULL;
}

So. Forget what I said, I just was confused.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services