Attachment 17868 Details for Bug 32319 – file.diff

[patch] file.diff

file.diff (text/plain), 3.99 KB, created by Michael W Lucas on 2001-11-27 01:00:01 UTC

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Creator: Michael W Lucas

Created: 2001-11-27 01:00:01 UTC

Size: 3.99 KB

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obsolete

>*** en_US.ISO8859-1/books/faq/book.sgml-dist	Mon Nov 26 13:03:29 2001
>--- en_US.ISO8859-1/books/faq/book.sgml	Mon Nov 26 19:55:02 2001
>***************
>*** 6119,6124 ****
>--- 6119,6192 ----
>        </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>
>          </question>

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