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<title>JFFS2 & erase block size</title>
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<h1>JFFS2 & erase block size</h1>
<P>Unlike the original JFFS, JFFS2 treats each erase block of the
flash separately. It will never write nodes which cross from one
erase block to another, and will not cope if it encounters a file
system with such nodes. Any node which crosses an erase block
boundary will be ignored by JFFS2, and the data it contains will
be lost, causing the file system to be corrupted.</P>
<P>If there's not enough space at the end of the current erase
block to write a whole node, JFFS2 will leave it blank and
continue writing to a new erase block.</P>
<P>Also, JFFS2 always starts writing at the beginning of an erase
block, and does not expect to find free space in the middle. Any
free space in an erase block should go all the way to the end of
the erase block. JFFS2 will print a complaint such as:
<PRE>
jffs2_scan_empty(): Empty block at 0x0012fffc ends at 0x00130000 (with 0xe0021985)! Marking dirty
</PRE>
if it finds free space in between the nodes in an erase
block. This situation is harmless though - it's just a little bit
of wasted space, that's all.</P>
<P>Both of these situations can occur if you create a JFFS2 image
with <TT>mkfs.jffs2</TT> using the wrong value for the erase
block size (the <tt>-e</TT> option). The default is 64KiB,
because that's the smallest erase block size you're likely to
encounter often, and creating an image with smaller eraseblock
size than the actual hardware is harmless -- it just gives
annoying messages. If you see the messages mentioned above,
check the erase block size of your device (look in
<tt>/proc/mtd</tt> if you don't know), and create your JFFS2
image for it with the correct <tt>-e</tt> option.</P>
<hr>
<address><a href="mailto:dwmw2@infradead.org">David Woodhouse</a></address>
$Id: jffs2-eraseblocks.html,v 1.1 2002/02/24 14:35:40 dwmw2 Exp $
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