Subject: Re: pg_upgrade --link on Windows
To: Bruce Momjian <bruce@momjian.us>, "Arnaud L." <arnaud.listes@codata.eu>
Cc: PG-General Mailing List <pgsql-general@postgresql.org>
References: <8da3dbd9-7daf-5589-0cc8-0eede930a492@codata.eu>
 <20170609140724.GB21215@momjian.us>
Reply-To: arnaud.listes@codata.eu
From: "Arnaud L." <arnaud.listes@codata.eu>
Message-ID: <a892509c-0eb4-42a4-1370-6cb0ac3bc1c3@codata.eu>
Date: Fri, 9 Jun 2017 16:39:05 +0200
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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) ?

Regards
--
Arnaud


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