SSD recommendations for ZFS cache/log

[Eitan Adler]

On 13 November 2012 23:25, Bryan Drewery <bryan at shatow.net> wrote:
> IMHO this whole post should be enshrined into an FAQ or manpage or wiki.
> It's very informative and compelling.

Sounds good.   Can people here please tell me what is wrong in the
following content?
Is there additional data or questions to add?

Please note that I've never used ZFS so being whacked by a cluebat
would be helpful.

commit 1675274e40464fb37f822a176b9eed28ea729947
Author: Eitan Adler <lists at eitanadler.com>
Date:   Thu Nov 15 22:52:33 2012 -0500

    Add a section in the FAQ about ZFS

    Reviewed by:	???
    Approved by:	???

diff --git a/en_US.ISO8859-1/books/faq/book.xml
b/en_US.ISO8859-1/books/faq/book.xml
index 7ad4974..caffad1 100644
--- a/en_US.ISO8859-1/books/faq/book.xml
+++ b/en_US.ISO8859-1/books/faq/book.xml
@@ -5367,6 +5367,62 @@ C:\="DOS"</programlisting>
 	</answer>
       </qandaentry>
     </qandaset>
+
+    <sect1 id="all-about-zfs">
+      <title>ZFS</title>
+
+      <qandaset>
+	<qandaentry>
+	  <question id="what-is-zil">
+	    <para>What is the ZIL and when does it get used?</para>
+	  </question>
+
+	  <answer>
+	    <para>The <acronym>ZIL</acronym> (<acronym>ZFS</acronym>
+	      intent log) is a write cache for ZFS.  All writes get
+	      recorded in the ZIL.  Eventually ZFS will perform a
+	      <quote>Transaction Group Commit</quote> in which it
+	      flushes out the data in the ZIL to disk.</para>
+	  </answer>
+	</qandaentry>
+
+	<qandaentry>
+	  <question id="what-is-l2arc">
+	    <para>What is the L2ARC?</para>
+	  </question>
+
+	  <answer>
+	    <para>The <acronym>L2ARC</acronym> is a read cache stored
+	      on a fast device such as an <acronym>SSD</acronym>.  It is
+	      used to speed up operations such as deduplication or
+	      encryption.  This cache is not persisent across
+	      reboots.  Note that RAM is used as the first layer
+	      of cache and the L2ARC is only needed if there is
+	      insufficient RAM.</para>
+	  </answer>
+	</qandaentry>
+
+	<qandaentry>
+	  <question id="should-enable-dedup">
+	    <para>Is enabling deduplication advisable?</para>
+	  </question>
+
+	  <answer>
+	    <para>The answer very much depends on the expected workload.
+	      Deduplication takes up a signifigent amount of RAM and CPU
+	      time and may slow down read and write disk access times.
+	      Unless one is storing data that is very heavily
+	      duplicated (such as virtual machine images, or user
+	      backups) it is likely that deduplication will do more harm
+	      than good.  Another consideration is the inability to
+	      revert deduplication status.  If deduplication is enabled,
+	      data written, and then dedup is disabled, those blocks
+	      which were deduplicated will not be duplicated until
+	      they are next modified.</para>
+	  </answer>
+	</qandaentry>
+      </qandaset>
+    </sect1>
   </chapter>

   <chapter id="admin">



-- 
Eitan Adler
Source, Ports, Doc committer
Bugmeister, Ports Security teams