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Re: strange nbtree corruption report

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: strange nbtree corruption report
Date: 2011-11-22 04:14:33
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Lists: pgsql-hackers
Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> We got a very strange nbtree corruption report some time ago.  This was
> a btree index on a vey high churn table -- entries are updated and
> deleted very quickly, so the index grows very large and also shrinks
> quickly (AFAICT this is a work queue of sorts).

> The most strange thing of all is that there was this error:

> ERROR:  left link changed unexpectedly in block 3378 of index "index_name" 
> CONTEXT:  automatic vacuum of table "table_name"

> This was reported not once, but several dozens of times, by each new
> autovacuum worker that tried to vacuum the table.

> As far as I can see, there is just no way for this to happen ... much
> less happen repeatedly.

It's not hard to believe that that would happen repeatedly given a
corrupted set of sibling links, eg deletable page A links left to page
B, which links right to C, which links right to A.  The question is how
the index got into such a state.  A dropped update during a page split
would explain it (ie, B used to be A's left sibling, then at some point
B got split into B and C, but A's left-link never got updated on disk).
I wonder how reliable their disk+filesystem is ...

			regards, tom lane

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