Re: pg_basebackup creates a corrupt file for pg_stat_tmp and pg_replslot on a backup location

On 2016-04-14 14:55:37 -0400, Robert Haas wrote:
> On Thu, Apr 14, 2016 at 11:12 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2016-04-14 13:43:34 +0530, Ashutosh Sharma wrote:
> >> I tried performing pg_basebackup after creating a symbolic link for
> >> pg_replslot, pg_stat_tmp, pg_log and pg_clog in the source directory
> >
> > That's not supported, and I strongly suspect that you're goint to hit
> > more than just this issue. The only directory you're allowed to symlink
> > away is pg_xlog.
>
> I think various tools might choke on such a configuration, but I'm not
> entirely sure why we haven't made them all work. Is there some more
> fundamental problem?

I don't think there's a fundamental problem, just that it'd require
adding code to numerous places, and that it'd make it harder to reason
about things. For example you have to be a lot more careful about
iterating over the data directory, because loops suddenly are a major
concern. Fsyncing files & directories suddenly needs special care to
fsync the correct directory for a file, lest it's symlinked somewhere.
It's harder to perform checks like the "not at the top of the
filesystem" when every file and directory can be somewhere else.

- Andres