Re: AdvanceXLInsertBuffer vs. WAL segment compressibility

From: Chapman Flack <chap(at)anastigmatix(dot)net>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: AdvanceXLInsertBuffer vs. WAL segment compressibility
Date: 2016-07-26 21:42:43
Message-ID: 5797D953.6040806@anastigmatix.net
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On 07/26/2016 04:21 PM, Robert Haas wrote:

> I'm kind of curious WHY you are using archiving and forcing regular
> segment switches rather than just using streaming replication.
> ... AFAIK, streaming replication
> essentially obsoleted that use case. You can just dribble the
> individual bytes over the wire a few at a time to the standby or, with
> pg_receivexlog, to an archive location. If it takes 6 months to fill
> up a WAL segment, you don't care: you'll always have all the bytes

Part of it is just the legacy situation: at the moment, the offsite
host is of a different architecture and hasn't got PostgreSQL
installed (but it's easily ssh'd to for delivering compressed WAL
segments). We could change that down the road, and pg_receivexlog
would work for getting the bytes over there.

My focus for the moment was just on migrating a cluster to 9.5
without changing the surrounding arrangements all at once.
Seeing how much worse our compression ratio will be, though,
maybe I need to revisit that plan.

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.

-Chap

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