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Date: Wed, 12 Feb 2014 15:16:58 +0900
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Subject: Re: PATCH: Warn users about tablespace abuse data loss risk
From: Ian Lawrence Barwick <barwick@gmail.com>
To: David Johnston <polobo@yahoo.com>
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2014-02-12 14:06 GMT+09:00 David Johnston <polobo@yahoo.com>:
> Ian Lawrence Barwick wrote
>> 2014-02-12 12:52 GMT+09:00 Craig Ringer <
>
>> craig@
>
>> >:
>>> 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.
>>

> While providing additional warnings is good and necessary it may also help
> to be more descriptive as to in what situations tablespaces are appropriate
> and/or necessary so that people leave with a better understanding of why the
> feature exists and not just trying to know what not to use it for.  It goes
> against the more prescriptive tone of the documentation generally but both
> approaches work well together to tackle the knowledge/understanding gap some
> users seem to have.

The warning would appear on this page:

  http://www.postgresql.org/docs/current/static/manage-ag-tablespaces.html

which describes what tablespaces *can* do, but unless you're familiar with the
structure of the PostgreSQL data directories, it's not obvious what you *can't*
do. I recall reading a blog post a while back about tablespaces being "archived"
to the cloud with disastrous results, and a quick search pulls up
stuff like this:

  http://stackoverflow.com/questions/3534415/moving-postgres-tablespaces-and-tables-across-ec2-instance

so it's definitely not a niche issue. Something "official" to link to
would be very
useful in this kind of situation. That doesn't preclude the general
documentation
being improved of course.

Regards

Ian Barwick


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