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Date: Wed, 12 Feb 2014 13:44:03 +0900
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Subject: Re: PATCH: Warn users about tablespace abuse data loss risk
From: Ian Lawrence Barwick <barwick@gmail.com>
To: Craig Ringer <craig@2ndquadrant.com>
Cc: PostgreSQL Developers <pgsql-docs@postgresql.org>
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2014-02-12 12:52 GMT+09:00 Craig Ringer <craig@2ndquadrant.com>:
> Hi all
>
> I've just seen another case of data loss due to misuse of /
> misunderstanding of tablespaces:
>
> http://dba.stackexchange.com/questions/58704/how-do-i-access-a-old-saved-tablespace-after-reinstalling-postgres
>
> and it's prompted me to write some docs amendments to make it more
> obvious that *you shouldn't do that*.
>
> Not that it'll stop people, but it'll at least mean they can't say we
> didn't warn them.
>
> This is actually quite important, because many users are used to MySQL's
> MyISAM, where each table contains its own metadata and is readable by
> simply copying the table into a different MySQL install's data
> directory. It doesn't even have to be the same version! Users are
> clearly surprised that PostgreSQL tablespaces don't have the same
> properties.
>
> Thoughts?

People still use MyISAM!?

I had a similar issue pop up at work a while back, having something
explicit to point to is definitely a good idea.

Suggestion for the first paragraph of the patch (sorry I can't provide it in
patch form right now):

  Even if they are located outside the main PostgreSQL data directory,
tablespaces
  are an integral part of the database cluster and
<emphasis>cannot</emphasis> be
  treated as an autonomous collection of data files. They rely on
metadata contained
  in the main data directory, without which they are useless. In
particular, tablespaces
  cannot be reattached to a different database cluster, and backing up
individual
  tablespaces makes no sense as a backup/redundancy method. Similarly,
if you lose a
  tablespace (file deletion, disk failure, etc) the main database may
become unreadable
  or fail to start.

Regards


Ian Barwick


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