Message-ID: <52FC0EBF.7020300@2ndquadrant.com>
Date: Thu, 13 Feb 2014 08:15:59 +0800
From: Craig Ringer <craig@2ndquadrant.com>
Organization: 2nd Quadrant
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To: Ian Lawrence Barwick <barwick@gmail.com>,
 PostgreSQL Developers <pgsql-docs@postgresql.org>
Subject: Re: PATCH: Warn users about tablespace abuse data loss risk
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On 02/12/2014 02:35 PM, Ian Lawrence Barwick wrote:
> And taking a look at the page in question I see this prominent example:
> 
>   CREATE TABLESPACE fastspace LOCATION '/mnt/sda1/postgresql/data';

Here's an updated patch, adopting your wording for the first par and
tweaking the example.


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

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From a754f61f5228e0d066edc10ae903282b36f9a256 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

With rewording by Ian Barwick
---
 doc/src/sgml/manage-ag.sgml             | 28 +++++++++++++++++++++++++++-
 doc/src/sgml/ref/create_tablespace.sgml | 10 ++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/doc/src/sgml/manage-ag.sgml b/doc/src/sgml/manage-ag.sgml
index b44d521..efd080f 100644
--- a/doc/src/sgml/manage-ag.sgml
+++ b/doc/src/sgml/manage-ag.sgml
@@ -379,12 +379,33 @@ dropdb <replaceable class="parameter">dbname</replaceable>
    expensive, slower disk system.
   </para>
 
+  <warning>
+   <para>
+     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.
+   </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">
    command, for example:<indexterm><primary>CREATE TABLESPACE</></>:
 <programlisting>
-CREATE TABLESPACE fastspace LOCATION '/mnt/sda1/postgresql/data';
+CREATE TABLESPACE fastspace LOCATION '/ssd1/postgresql/data';
 </programlisting>
    The location must be an existing, empty directory that is owned by
    the <productname>PostgreSQL</> operating system user.  All objects subsequently
@@ -392,6 +413,11 @@ CREATE TABLESPACE fastspace LOCATION '/mnt/sda1/postgresql/data';
    directory.
   </para>
 
+  <para>
+   The location must not be on removable or transient storage, as the database
+   will not work if the tablespace is missing or lost.
+  </para>
+
   <note>
    <para>
     There is usually not much point in making more than one
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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