Re: AdvanceXLInsertBuffer vs. WAL segment compressibility

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.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com

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