From ab52f82026bd5939d09faf21bdec205989c304f8 Mon Sep 17 00:00:00 2001
From: Giorgos Keramidas <keramida@FreeBSD.org>
Date: Tue, 27 Nov 2001 02:21:16 +0000
Subject: [PATCH] Add an entry that describes why softupdates is dangerous to
 use on the root filesystem.

PR:		docs/32319
Submitted by:	Michael Lucas <mwlucas@blackhelicopters.org>
---
 en_US.ISO8859-1/books/faq/book.sgml | 68 +++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)

diff --git a/en_US.ISO8859-1/books/faq/book.sgml b/en_US.ISO8859-1/books/faq/book.sgml
index 46ae1bc60a..7ef9e5cc7e 100644
--- a/en_US.ISO8859-1/books/faq/book.sgml
+++ b/en_US.ISO8859-1/books/faq/book.sgml
@@ -6118,6 +6118,74 @@ C:\="DOS"</programlisting>
         </answer>
       </qandaentry>
 
+      <qandaentry>
+        <question id="safe-softupdates">
+          <para>Which partitions can safely use softupdates?  I've
+            heard that softupdates on <filename>/</filename> can cause
+            problems.</para>
+        </question>
+
+        <answer>
+          <para>Short answer: you can usually use softupdates safely
+            on all partitions.</para>
+
+          <para>Long answer: There used to be some concern over using
+            softupdates on the root partition.  Softupdates has two
+            characteristics that caused this.  First, a softupdates
+            partition has a small chance of losing data during a
+            system crash.  (The partition will not be corrupted; the
+            data will simply be lost.)  Also, softupdates can cause
+            temporary space shortages.</para>
+
+          <para>When using softupdates, the kernel can take up to
+            thirty seconds to actually write changes to the physical
+            disk.  If you delete a large file, the file still resides
+            on disk until the kernel actually performs the deletion.
+            This can cause a very simple race condition.  Suppose you
+            delete one large file and immediately create another large
+            file.  The first large file is not yet actually removed
+            from the physical disk, so the disk might not have enough
+            room for the second large file.  You get an error that the
+            partition don't have enough space, although you know
+            perfectly well that you just released a large chunk of
+            space!  When you try again mere seconds later, the file
+            creation works as you expect.  This has left more than one
+            user scratching his head and doubting his sanity, the
+            FreeBSD filesystem, or both.</para>
+
+          <para>If a system should crash after the kernel accepts a
+            chunk of data for writing to disk, but before that data is
+            actually written out, data could be lost or corrupted.
+            This risk is extremely small, but generally manageable.
+            Use of IDE write caching greatly increases this risk; it
+            is strongly recommended that you disable IDE write caching
+            when using softupdates.</para>
+
+          <para>These issues affect all partitions using softupdates.
+            So, what does this mean for the root partition?</para>
+
+          <para>Vital information on the root partition changes very
+            rarely.  Files such as <filename>/kernel</filename> and
+            the contents of <filename>/etc</filename> only change
+            during system maintenance, or when users change their
+            passwords.  If the system crashed during the the
+            thirty-second window after such a change is made, it is
+            possible that data could be lost.  This risk is negligible
+            for most applications, but you should be aware that it
+            exists.  If your system cannot tolerate this much risk,
+            don't use softupdates on the root filesystem!</para>
+
+          <para><filename>/</filename> is traditionally one of the
+            smallest partitions.  By default, FreeBSD puts the
+            <filename>/tmp</filename> directory on
+            <filename>/</filename>.  If you have a busy
+            <filename>/tmp</filename>, you might see intermittent
+            space problems.  Symlinking <filename>/tmp</filename> to
+            <filename>/var/tmp</filename> will solve this
+            problem.</para>
+        </answer>
+      </qandaentry>
+
       <qandaentry>
         <question id="add-swap-space">
           <para>How can I add more swap space?</para>