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Update and clarify the FAQ on soft-updates.

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Noted by:	-stable mailing list
Approved by:	bcr (mentor)
This commit is contained in:
Eitan Adler 2012-12-01 17:00:17 +00:00
parent 22db295d7a
commit 728594c450
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=40228
1 changed files with 15 additions and 16 deletions
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en_US.ISO8859-1/books/faq/book.xml
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@ -4489,24 +4489,25 @@ kern.sched.name: ULE</screen>
<question id="safe-softupdates">
<para>Which partitions can safely use Soft Updates? I have
heard that Soft Updates on <filename class="directory">/</filename> can cause
problems.</para>
problems. What about Journaled Soft Updates?</para>
</question>
<answer>
<para>Short answer: you can usually use Soft Updates safely on
all partitions.</para>
<para>Long answer: There used to be some concern over using
Soft Updates on the root partition. Soft Updates has two
characteristics that caused this. First, a Soft Updates
<para>Long answer: Soft Updates has two
characteristics that may be undesirable on certain
paritions. First, a Soft Updates
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, Soft Updates can cause temporary
simply be lost.) Second, Soft Updates can cause temporary
space shortages.</para>
<para>When using Soft Updates, 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
thirty seconds to write changes to the physical
disk. When a large file is deleted 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.
@ -4522,20 +4523,14 @@ kern.sched.name: ULE</screen>
<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 Soft Updates.</para>
actually written out, data could be lost. This
risk is extremely small, but generally manageable.</para>
<para>These issues affect all partitions using Soft Updates.
So, what does this mean for the root partition?</para>
<para>Vital information on the root partition changes very
rarely. Files such as
<filename>/boot/kernel/kernel</filename> and the contents of
<filename class="directory">/etc</filename> only change during system
maintenance, or when users change their passwords. If the
rarely. If the
system crashed during 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
@ -4551,6 +4546,10 @@ kern.sched.name: ULE</screen>
problems. Symlinking <filename class="directory">/tmp</filename> to
<filename class="directory">/var/tmp</filename> will solve this
problem.</para>
<para>Finally, &man.dump.8; does not work in live mode (-L)
on a filesystem, with Journaled Soft Updates
(<acronym>SU+J</acronym>).</para>
</answer>
</qandaentry>