diff --git a/faq/ubifs.xml b/faq/ubifs.xml
index b6b0b14..a39ef46 100644
--- a/faq/ubifs.xml
+++ b/faq/ubifs.xml
@@ -29,6 +29,7 @@
 	<li><a href="ubifs.html#L_nfs">Does UBIFS support NFS?</a></li>
 	<li><a href="ubifs.html#L_slow_when_full">Does UBIFS become slower when it is full?</a></li>
 	<li><a href="ubifs.html#L_df_report">Why <code>df</code> reports too few free space?</a></li>
+	<li><a href="ubifs.html#L_jffs2_space">Why does my UBIFS volume have significantly lower capacity than my equivalent jffs2 volume?</a></li>
 	<li><a href="ubifs.html#L_comproff">How do I disable compression?</a></li>
 	<li><a href="ubifs.html#L_ubifs_nandsim">How do I use UBIFS with nandsim?</a></li>
 	<li><a href="ubifs.html#L_ubifs_extract">How do I extract files from an UBI/UBIFS image?</a></li>
@@ -871,6 +872,63 @@ accurate is free space reporting.</p>
 
 
 
+<h2><a name="L_jffs2_space">Why does my UBIFS volume have significantly lower
+capacity than my equivalent JFFS2 volume?</a></h2>
+
+<p>When migrating a filesystem image from JFFS2 to UBIFS you may notice some
+or all of the following:</p>
+
+<ul>
+	<li>The size of the image file to be flashed is significantly larger
+	than before.</li>
+	<li><code>df</code> reports significantly less available free space
+	than before (while there may be <em>some</em> truth in this, please be
+	aware of the other considerations that must be applied to
+	<code>df</code> values described elsewhere in this FAQ).</li>
+	<li>When measuring the capacity of the disk by seeing how much data
+	can be written, JFFS2 is able to store significantly more data than
+	UBIFS.</li>
+</ul>
+
+<p>There are several reasons for this.</p>
+
+<p>Firstly, did you create your volumes with default arguments to
+<code>mkfs.jffs2</code> and <code>mkfs.ubifs</code>? This alone introduces
+a significant difference: <code>mkfs.jffs2</code> effectively defaults to zlib
+compression and disables lzo, but <code>mkfs.ubifs</code> defaults to LZO
+compression. zlib compresses significantly better, but is noticeably slower to
+compress/decompress at runtime. Using zlib compression with
+<code>mkfs.ubifs</code> will probably reduce the size of your NAND image file
+by 10-15%, put it on-par with an equivalent JFFS2 image, and increase the available capacity, at the price of performance.</p>
+
+<p>Secondly, did you choose a good value for <code>vol_size</code> or enable
+autoresize in your ubinize configuration? It is possible that your UBI volume
+is not utilising the full space of the available flash.</p>
+
+<p>Beyond that, it is unfortunately true that UBI/UBIFS has higher space
+overhead than its predecessors. OLPC has
+<a href="http://lists.laptop.org/pipermail/devel/2011-September/033274.html">measured</a>
+this overhead to be approximately 50mb per 1GB of storage (using JFFS2 as a
+baseline). While UBIFS cannot be made as good as JFFS2 in this respect, work
+could be undertaken to improve space efficiency for current or future
+UBI/UBIFS versions. Reasons for overhead and opportunities for improvement
+include:</p>
+
+<ul>
+	<li>For MTD controllers (such as CaFe) that do not support sub-page
+	writes, 2 KiB is taken from each eraseblock. If such controller drivers
+	were adapted (or hacked) to use software ECC for the first NAND page of
+	every eraseblock, around 8 MiB could be recovered per 1GB of
+	storage.</li>
+	<li>UBIFS on-flash data structures could be reduced in size. These
+	headers currently include many unused fields that are reserved for
+	future use.</li>
+	<li>UBIFS could be improved to reserve less space for internal needs,
+	but touching this part of the code is risky.</li>
+</ul>
+
+
+
 <h2><a name="L_comproff">How do I disable compression?</a></h2>
 
 <p>UBIFS compression may be disabled for whole file system during the image