Message-ID: <52FAEFEA.4060609@2ndquadrant.com>
Date: Wed, 12 Feb 2014 11:52:10 +0800
From: Craig Ringer <craig@2ndquadrant.com>
Organization: 2nd Quadrant
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To: PostgreSQL Developers <pgsql-docs@postgresql.org>
Subject: PATCH: Warn users about tablespace abuse data loss risk
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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?

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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From 6d0abd7bcaca596ac2298d8f7ac408e465ec1aef Mon Sep 17 00:00:00 2001
From: Craig Ringer <craig@2ndquadrant.com>
Date: Wed, 12 Feb 2014 11:48:23 +0800
Subject: [PATCH] Warn about risks of tablespace misuse

---
 doc/src/sgml/manage-ag.sgml             | 17 +++++++++++++++++
 doc/src/sgml/ref/create_tablespace.sgml | 10 ++++++++++
 2 files changed, 27 insertions(+)

diff --git a/doc/src/sgml/manage-ag.sgml b/doc/src/sgml/manage-ag.sgml
index b44d521..dd43e19 100644
--- a/doc/src/sgml/manage-ag.sgml
+++ b/doc/src/sgml/manage-ag.sgml
@@ -379,6 +379,23 @@ dropdb <replaceable class="parameter">dbname</replaceable>
    expensive, slower disk system.
   </para>
 
+  <warning>
+   <para>
+    Tablespaces are <emphasis>not</emphasis> suitable for backup/redundancy.
+    If the main database cluster is lost, tablespaces cannot simply be reattached
+    to a different database cluster, the metadata from the original is required
+    to read the tablespace. Similarly, if you lose a tablespace (deletion,
+    disk failure, etc) the main database may become unreadable or fail to start.
+   </para>
+
+   <para>
+    Do not attempt to use tablespaces to transfer tables between database
+    clusters, for backup and restore, or to partition your data based on
+    the reliability of the storage it is on. <emphasis>Never put a tablespace
+    on a ramdisk or temporary file system</emphasis>.
+   </para>
+  </warning>
+
   <para>
    To define a tablespace, use the <xref
    linkend="sql-createtablespace">
diff --git a/doc/src/sgml/ref/create_tablespace.sgml b/doc/src/sgml/ref/create_tablespace.sgml
index 04c5fb8..83edd88 100644
--- a/doc/src/sgml/ref/create_tablespace.sgml
+++ b/doc/src/sgml/ref/create_tablespace.sgml
@@ -50,6 +50,16 @@ CREATE TABLESPACE <replaceable class="parameter">tablespace_name</replaceable>
    <command>CREATE INDEX</> or <command>ADD CONSTRAINT</> to have the data
    files for these objects stored within the specified tablespace.
   </para>
+
+  <warning>
+   <para>
+    You can't use the contents of a tablespace without the database it's part of,
+    nor can you use the main database if a tablespace is missing.  Don't try to
+    use tablespaces for backup or redundancy.
+    See <xref linkend="manage-ag-tablespaces" />.
+   </para>
+  </warning>
+
  </refsect1>
 
  <refsect1>
-- 
1.8.3.1


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