svn commit: r42810 - projects/zfsupdate-201307/en_US.ISO8859-1/books/handbook/zfs

[Benjamin Kaduk]

On Wed, 2 Oct 2013, Benedict Reuschling wrote:

> Author: bcr
> Date: Wed Oct  2 21:00:07 2013
> New Revision: 42810
> URL: http://svnweb.freebsd.org/changeset/doc/42810
>
> Log:
>  Add a section on deduplication.  This needs some more work and warnings about
>  huge memory requirements for the DDT. But the basic instructions are there on
>  how to activate it, along with an example that shows the dedup ratio and
>  a simulation run with zdb.
>
> Modified: projects/zfsupdate-201307/en_US.ISO8859-1/books/handbook/zfs/chapter.xml
> ==============================================================================
> --- projects/zfsupdate-201307/en_US.ISO8859-1/books/handbook/zfs/chapter.xml	Wed Oct  2 20:02:02 2013	(r42809)
> +++ projects/zfsupdate-201307/en_US.ISO8859-1/books/handbook/zfs/chapter.xml	Wed Oct  2 21:00:07 2013	(r42810)
> +
> +      <para>The <literal>DEDUP</literal> column shows the actual rate
> +	of deduplication for that pool.  A value of
> +	<literal>1.00x</literal> that no data has been deduplicated
> +	due to insufficient duplicate data.  In the following example,
> +	the ports tree is copied three times into different
> +	directories on the deduplicated pool above to provide
> +	redundancy.</para>

I'm not sure that this is the best way to talk about an example use-case 
where deduplication would come into play.  If the user actually wants 
redundancy (that is, to ensure that there is a reliable/correct copy of 
the ports tree around in the event of hardware issues), deduplication is 
not the right tool, 'zfs set -o copies=3' would be more appropriate.  The 
whole point of the deduplication is that there is only one actual copy on 
disk.
It would probably be a better example to talk about copying the ports tree 
three different times for testing different configurations or something 
like that.

-Ben

> +      <screen>&prompt.root; <userinput>zpool list</userinput>
> +for d in dir1 dir2 dir3; do
> +for> mkdir $d && cp -R /usr/ports $d &
> +for> done</screen>
> +
> +      <para>Now that redundant data has been created, ZFS detects that
> +	and makes sure that the data is not taking up additional
> +	space.</para>