I wrote:
> * log_heap_update decides that it can set XLOG_HEAP_INIT_PAGE instead
> of storing the full destination page, if the destination contains only
> the single tuple being moved. This is fine, except it also resets the
> buffer indicator for the *source* page, which is wrong --- that page
> may still need to be re-generated from the xlog record. This is the
> proximate cause of the bug report that started this thread.
I have to retract that particular bit of analysis: I had misread the
log_heap_update code. It seems to be doing the right thing, and in any
case, given Alex's output
LOG: REDO @ D/19176644; LSN D/191766A4: prev D/19176610; xid 81148979: Heap -
move: rel 1663/16386/16559898; tid 1/1; new 0/10
we can safely conclude that log_heap_update did not set the INIT_PAGE
bit, because the "new" tid doesn't have offset=1. (The fact that the
WAL_DEBUG printout doesn't report the bit's state is an oversight I plan
to fix, but anyway we can be pretty sure it's not set here.)
What we should be seeing, and don't see, is an indication of a backup
block attached to this WAL record. Furthermore, I don't see any
indication of a backup block attached to *any* of the WAL records in
Alex's printout. The only conclusion I can draw is that he had
full_page_writes turned OFF, and as we have just realized that that
setting is completely unsafe, that is the explanation for his failure.
> Clearly, we need to go through the xlog code with a fine tooth comb
> and convince ourselves that all pages touched by any xlog record will
> be properly reconstituted if they've later been truncated off. I have
> not yet examined any of the code except the above.
I've finished going through the xlog code looking for related problems,
and AFAICS this is the score:
* full_page_writes = OFF doesn't work.
* btree_xlog_split and btree_xlog_delete_page should pass TRUE not FALSE
to XLogReadBuffer for all pages that they are going to re-initialize.
* the recently-added gist xlog code is badly broken --- it pays no
attention whatever to preventing torn pages :-(. It's not going to be
easy to fix, either, because the page split code assumes that a single
WAL record can describe changes to any number of pages, which is not
the case.
Everything else seems to be getting it right.
regards, tom lane
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