Re: PATCH: Warn users about tablespace abuse data loss risk

2014-02-12 12:52 GMT+09:00 Craig Ringer <craig(at)2ndquadrant(dot)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