Re: GIN data corruption bug(s) in 9.6devel

Hi,

On 01/05/2016 10:38 AM, Tomas Vondra wrote:
> Hi,
>
...
>>
>> There shouldn't be a difference between the two approaches (although I
>> guess there could be if one left a larger pending list than the other,
>> as pending lists is very space inefficient), but since you included
>> 9.5 in your test I thought it would be interesting to see how either
>> patched version under 9.6 compared to 9.5.
>
> Well, turns out there's a quite significant difference, actually. The
> index sizes I get (quite stable after multiple runs):
>
>     9.5 : 2428 MB
>     9.6 + alone cleanup : 730 MB
>     9.6 + pending lock : 488 MB
>
> So that's quite a significant difference, I guess. The load duration for
> each version look like this:
>
>     9.5                 : 1415 seconds
>     9.6 + alone cleanup : 1310 seconds
>     9.6 + pending lock  : 1380 seconds
>
> I'd say I'm happy with sacrificing ~5% of time in exchange for ~35%
> reduction of index size.
>
> The size of the index on 9.5 after VACUUM FULL (so pretty much the
> smallest index possible) is 440MB, which suggests the "pending lock"
> patch does a quite good job.
>
> The gin_metapage_info at the end of one of the runs (pretty much all the
> runs look exactly the same) looks like this:
>
>                    pending lock   alone cleanup      9.5
> --------------------------------------------------------
>   pending_head                2               2   310460
>   pending_tail              338             345   310806
>   tail_free_size            812             812      812
>   n_pending_pages           330             339      347
>   n_pending_tuples         1003            1037     1059
>   n_total_pages               2               2        2
>   n_entry_pages               1               1        1
>   n_data_pages                0               0        0
>   n_entries                   0               0        0
>   version                     2               2        2
>
> So almost no difference, except for the pending_* attributes, and even
> in that case the values are only different for 9.5 branch. Not sure what
> conclusion to draw from this - maybe it's necessary to collect the
> function input while the load is running (but that'd be tricky to
> process, I guess).

Are we going to anything about this? While the bug is present in 9.5 
(and possibly other versions), fixing it before 9.6 gets out seems 
important because reproducing it there is rather trivial (while I've 
been unable to reproduce it on 9.5).

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
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