Re: BUG #12779: pg_dump -Fd doesn't care about -Z

Hi Cristopher.

On Tue, Feb 17, 2015 at 4:54 PM, Christoph Berg <cb(at)df7cb(dot)de> wrote:

> Re: To pgsql-bugs(at)postgresql(dot)org 2015-02-17 <
> 20150217153446(dot)2590(dot)24945(at)wrigleys(dot)postgresql(dot)org>
> > 1164 1.fd
> > 1164 9.fd
> >
> > 1488 1.fc
> > 1160 9.fc
>
> Re: Francisco Olarte 2015-02-17 <CA+bJJbza-+UOuofSVCoHdn6kMa3u4zWzgKZhyU=
> Qab46R4vY-w(at)mail(dot)gmail(dot)com>
> > > pg_dump -Fd doesn't seem to care about -Z for Z > 0:
> > >
> >
> > With such a small dump, are you sure du's block granularity is not
> masking
> > the differences in -Fd ? ( Have you tried du -b, or just comparing every
> > size ?) ( In -Fc it is a single file, harder to mask ).
>
> I noticed when trying to compress an assumed-to-be-Z4 file a bit more
> using -Z7. The directory was 241GB in both cases.
>

​I understand, but how did you measure it? I mean, from this asertion one
could be 240.01 and the other 241, and the differences you are quoting is
between a -Z4 and -Z7, not -Z1 and -Z9 as in the above quoted text. Because
if you used du -h it may well be masking it again. ​It seems rather strange
that it is processing -Z0 and -Z1 and masking the rest of them ( also, it
seems they use the same routines for both modes, but compressor, specially
in the hi-compression modes which use larger windows, are much better when
fed a large file ).

> And if you compare with -Fc, the difference is a lot more than a few
> FS blocks.
>

​That is what I was pointing. With -Fc you have a single file, so anything
bigger than a block is going to show ( this seem maybe be extents, rather
than blocks ). With -Fd you need a difference bigger than a block in a
single file. With du -b you just need a byte of diff, so just testing it
with it ( or asserting you've done it, I'm not gonna doubt it ) will make
your point stand out more.

​​Regards.
Francisco Olarte.