Re: Support for pg_receivexlog --format=plain|tar

On Sat, Jan 7, 2017 at 12:31 AM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:

> On Fri, Jan 6, 2017 at 11:07 PM, Magnus Hagander <magnus(at)hagander(dot)net>
> wrote:
> > A few further notes:
>
> Thanks for the review.
>
> > You are using the filemode to gzopen and the mode_compression variable to
> > set the compression level. The pre-existing code in pg_basebackup uses
> > gzsetparams(). Is there a particular reason you didn't do it the same
> way?
> >
> > Small comment:
> > - if (pad_to_size)
> > + if (pad_to_size && dir_data->compression == 0)
> > {
> > /* Always pre-pad on regular files */
> >
> >
> > That "always" is not true anymore. Commit-time cleanup can be done of
> that.
> >
> > The rest of this looks good to me, but please comment on the gzopen part
> > before we proceed to commit :)
>
> Yes using gzsetparams() looks cleaner. I actually thought about using
> the same logic as pg_dump. Attached is an updated patch.
>
> There is something I forgot. With this patch,
> FindStreamingStart()@pg_receivexlog.c is actually broken. In short it
> forgets to consider files that have been compressed at the last run of
> pg_receivexlog and will try to stream changes from the beginning. I
> can see that gzip -l provides this information... But I have yet to
> find something in zlib that allows a cheap lookup as startup of
> streaming should be fast. Looking at how gzip -l does it may be faster
> than looking at the docs.
>

Do we really care though? As in, does statup of streaming have to be *that*
fast? Even gunziping 16Mb (worst case) doesn't exactly take a long time. If
your pg_receivexlog is restarting so often that it becomes a problem, I
think you already have another and much bigger problem on your hands.

I found another problem with it -- it is completely broken in sync mode.
You need to either forbid sync mode together with compression, or teach
dir_sync() about it. The later would sound better, but I wonder how much
that's going to kill compression due to the small blocks? Is it a
reasonable use-case?

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/