Re: [GENERAL] pg_upgrade --link on Windows

On Fri, Jun 9, 2017 at 07:55:55AM -0700, Adrian Klaver wrote:
> On 06/09/2017 07:39 AM, Arnaud L. wrote:
> >See this page for more details :
> >http://cects.com/overview-to-understanding-hard-links-junction-points-and-symbolic-links-in-windows/
> >
> >
> >Under "Hard Link (Linking for individual files)" :
> >"If the target is deleted, its content is still available through the hard
> >link"
> >
> >Junction Point (Directory Hard Link):
> >"If the target is moved, renamed or deleted, the Junction Point still
> >exists, but points to a non-existing directory"
> >
> >BUT, when I try to "pg_upgrade --link --check" with old-data-dir and
> >new-data-dir on different volumes, I get an error saying that both
> >directories must be on the same volume if --link is used.
> >So maybe pg_upgrade uses hard-links (i.e. to files), and only the
> >documentation is wrong by calling them junctions (i.e. soft links to
> >files) ?
>
> Looks that way. In file.c in ~/src/bin/pg_upgrade I see:
>
> #ifdef WIN32
> 300 /* implementation of pg_link_file() on Windows */
> 301 static int
> 302 win32_pghardlink(const char *src, const char *dst)
> 303 {
> 304 /*
> 305 * CreateHardLinkA returns zero for failure
> 306 * http://msdn.microsoft.com/en-us/library/aa363860(VS.85).aspx
> 307 */
> 308 if (CreateHardLinkA(dst, src, NULL) == 0)
> 309 {
> 310 _dosmaperr(GetLastError());
> 311 return -1;
> 312 }
> 313 else
> 314 return 0;
> 315 }
> 316 #endif

[docs list added]

I apologize for not being smarter on this thread. When I helped with
the Windows port, I was told Windows didn't have hard links for use by
tablespace directories, so I got it into my head that Windows didn't
have hard links. Therefore, when I was writing the docs, I called them
junction points.

Looking back to Postgres 9.0 where pg_upgrade was added to the tree, I
see that the code even at that time used hard links on Windows. I have
created the attached patch which I will apply to all current Postgres
versions to fix this error.

Thanks for the report and the research. :-)

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
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