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Add a section about using the ZFS reservation feature.

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This work was done as part of Google Code-In (GCIN) 2011.

GCIN student:		Isabell Long (isabell121 gmail com)
GCIN task mentor:	theraven, pgj
Patch reapplied by:	Niclas Zeising (zeising daemonic se)
Minor cleanups by:	me
This commit is contained in:
Benedict Reuschling 2012-01-30 20:01:24 +00:00
parent f1485856ea
commit 5e41f5f784
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=38349
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en_US.ISO8859-1/books/handbook/filesystems/chapter.sgml
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@ -688,6 +688,62 @@ errors: No known data errors</screen>
<screen>&prompt.root; <userinput>zfs get quota storage/home/bob</userinput></screen>
</sect3>
<sect3>
<title>ZFS Reservations</title>
<para>ZFS supports two types of space reservations. This
section will explain the basics of each one, and include
some usage instructions.</para>
<para>The <literal>reservation</literal> property makes it
possible to reserve a minimum amount of space guaranteed for a
dataset and its descendants. This means that if a 10&nbsp;GB
reservation is set on <filename>storage/home/bob</filename>,
if disk space gets low, at least 10&nbsp;GB of space is
reserved for this dataset. The
<literal>refreservation</literal> property sets or indicates
the minimum amount of space guaranteed to a dataset excluding
descendants, such as snapshots. As an example, if a snapshot
was taken of <filename>storage/home/bob</filename>, enough
disk space would have to exist outside of the
<literal>refreservation</literal> amount for the operation to
succeed because descendants of the main data set are not
counted by the <literal>refreservation</literal> amount and
so do not encroach on the space set.</para>
<para>Reservations of any sort are useful in many situations,
for example planning and testing the suitability of disk space
allocation in a new system, or ensuring that enough space is
available on file systems for system recovery procedures and
files.</para>
<para>The general format of the <literal>reservation</literal>
property is
<literal>reservation=<replaceable>size</replaceable></literal>,
so to set a reservation of 10&nbsp;GB on
<filename>storage/home/bob</filename>the below command is
used:</para>
<screen>&prompt.root; <userinput>zfs set reservation=10G storage/home/bob</userinput></screen>
<para>To make sure that no reservation is set, or to remove a
reservation, instead use:</para>
<screen>&prompt.root; <userinput>zfs set reservation=none storage/home/bob</userinput></screen>
<para>The same principle can be applied to the
<literal>refreservation</literal> property for setting a
refreservation, with the general format
<literal>refreservation=<replaceable>size</replaceable></literal>.</para>
<para>To check if any reservations or refreservations exist on
<filename>storage/home/bob</filename>, execute one of the
following commands:</para>
<screen>&prompt.root; <userinput>zfs get reservation storage/home/bob</userinput>
&prompt.root; <userinput>zfs get refreservation storage/home/bob</userinput></screen>
</sect3>
</sect2>
</sect1>