Re: AdvanceXLInsertBuffer vs. WAL segment compressibility

On Tue, Aug 2, 2016 at 2:33 PM, Bruce Momjian <bruce@momjian.us> wrote:
> On Tue, Jul 26, 2016 at 05:42:43PM -0400, Chapman Flack wrote:
>> Even so, I'd be curious whether it would break anything to have
>> xlp_pageaddr simply set to InvalidXLogRecPtr in the dummy zero
>> pages written to fill out a segment. At least until it's felt
>> that archive_timeout has been so decidedly obsoleted by streaming
>> replication that it is removed, and the log-tail zeroing code
>> with it.
>>
>> That at least would eliminate the risk of anyone else repeating
>> my astonishment. :)  I had read that 9.4 added built-in log-zeroing
>> code, and my first reaction was "cool! that may make the compression
>> technique we're using unnecessary, but certainly can't make it worse"
>> only to discover that it did, by ~ 300x, becoming now 3x *worse* than
>> plain gzip, which itself is ~ 100x worse than what we had.
>
> My guess is that the bytes are there to detect problems where a 512-byte
> disk sector is zeroed by a disk failure.  I don't see use changing that
> for the use-case you have described.

Is there actually any code that makes such a check?

I'm inclined to doubt that was the motivation, though admittedly we're
both speculating about the contents of Heikki's brain, a tricky
proposition on a good day.

-- 
Robert Haas
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