Subject: Re: pg_upgrade --link on Windows
To: arnaud.listes@codata.eu, Bruce Momjian <bruce@momjian.us>
Cc: PG-General Mailing List <pgsql-general@postgresql.org>
References: <8da3dbd9-7daf-5589-0cc8-0eede930a492@codata.eu>
 <20170609140724.GB21215@momjian.us>
 <a892509c-0eb4-42a4-1370-6cb0ac3bc1c3@codata.eu>
From: Adrian Klaver <adrian.klaver@aklaver.com>
Message-ID: <0405d125-929b-ab4a-b7be-d49c09e7c056@aklaver.com>
Date: Fri, 9 Jun 2017 07:55:55 -0700
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On 06/09/2017 07:39 AM, Arnaud L. wrote:
> Le 9/06/2017 à 16:07, Bruce Momjian a écrit :
>> I was told junction points on Windows were hard links and no one has
>> ever complained about not being able to remove them.
> 
> Sorry, I think my explanation was not very clear.
> You can remove the link, but the point is to remove the target (i.e. the 
> old-data-dir).
> You can do this with a hard link (there still exists a hardlink pointing 
> to the inode so it remains), but with a soft link you end up with a link 
> to nothing.
> Deleting a junction target in Windows will work, but you'll have an 
> error trying to access the junction directory (directory not found).
> 
> 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


> 
> Regards
> -- 
> Arnaud
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com


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