On Sat, Apr 13, 2013 at 12:38 AM, Jeff Davis <pg...@j-davis.com> wrote:
> On Fri, 2013-04-12 at 23:03 +0300, Heikki Linnakangas wrote:
>> I think this is a bad idea. It complicates the WAL format significantly.
>> Simon's patch didn't include the changes to recovery to validate the
>> checksum, but I suspect it would be complicated. And it reduces the
>> error-detection capability of WAL recovery. Keep in mind that unlike
>> page checksums, which are never expected to fail, so even if we miss a
>> few errors it's still better than nothing, the WAL checkum is used to
>> detect end-of-WAL. There is expected to be a failure every time we do
>> crash recovery. This far, we've considered the probability of one in
>> 1^32 small enough for that purpose, but IMHO one in 1^16 is much too weak.
> One thing that just occurred to me is that we could make the SIMD
> checksum a 32-bit checksum, and reduce it down to 16 bits for the data
> pages. That might give us more flexibility to later use it for WAL
> without compromising on the error detection nearly as much (though
> obviously that wouldn't work with Simon's current proposal which uses
> the same data page checksum in a WAL backup block).

The simple 32bit version of the algorithm would need CPU capability
checks for the fast version and would work only on CPUs produced in
the last few years. Not a show stopper but but more complex code and
less applicability for sure.

An alternative would be to calculate 2 16 bit checksums, concat them
for the 32bit checksum and add them for the 16 bit one. In this case
we wouldn't need to change the current algorithm. A future change
could just factor out everything until the last add as the common
function. But keep in mind that we are talking about sharing about 400
bytes of machine code here.

> In general, we have more flexibility with WAL because there is no
> upgrade issue. It would be nice to share code with the data page
> checksum algorithm; but really we should just use whatever offers the
> best trade-off in terms of complexity, performance, and error detection
> rate.
> I don't think we need to decide all of this right now. Personally, I'm
> satisfied having SIMD checksums on data pages now and leaving WAL
> optimization until later.


I feel quite uneasy about reducing the effectiveness of WAL end
detection. There are many ways to improve WAL performance and I have
no idea what would be the best one. At the very least some performance
tests are in order. As this is not an essential part of having usable
checksums, but a general performance optimization I feel that it is
not fair to others to postpone the release to resolve this now. I'd be
more than happy to research this for 9.4.

Ants Aasma
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de

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