Re: [REVIEW] Re: Compression of full-page-writes

On Fri, Dec 12, 2014 at 8:27 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2014-12-12 09:24:27 -0500, Bruce Momjian wrote:
>> On Fri, Dec 12, 2014 at 03:22:24PM +0100, Andres Freund wrote:
>> > > Well, the larger question is why wouldn't we just have the user compress
>> > > the entire WAL file before archiving --- why have each backend do it?
>> > > Is it the write volume we are saving? I though this WAL compression
>> > > gave better performance in some cases.
>> >
>> > Err. Streaming?
>>
>> Well, you can already set up SSL for compression while streaming. In
>> fact, I assume many are already using SSL for streaming as the majority
>> of SSL overhead is from connection start.
>
> That's not really true. The overhead of SSL during streaming is
> *significant*. Both the kind of compression it does (which is far more
> expensive than pglz or lz4) and the encyrption itself. In many cases
> it's prohibitively expensive - there's even a fair number on-list
> reports about this.

(late to the party)
That may be true, but there are a number of ways to work around SSL
performance issues such as hardware acceleration (perhaps deferring
encryption to another point in the network), weakening the protocol,
or not using it at all.

OTOH, Our built in compressor as we all know is a complete dog in
terms of cpu when stacked up against some more modern implementations.
All that said, as long as there is a clean path to migrating to
another compression alg should one materialize, that problem can be
nicely decoupled from this patch as Robert pointed out.

merlin