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@2ndquadrant.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