linux-ext4 - Re: flashing large eMMC partitions with ext4

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Date:	Wed, 27 Jul 2011 11:16:39 -0600
From:	Andreas Dilger <aedilger@...il.com>
To:	Round Robinjp <roundrobinjp@...oo.co.jp>
Cc:	Ted Ts'o <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: flashing large eMMC partitions with ext4

I still think that using resize2fs and removing/adding the journal provides the clearest way to create a small image. The only drawback is that this needs some post-processing steps after the image is written, so it may not be suitable for some workflows. 

Cheers, Andreas

On 2011-07-27, at 10:40 AM, Round Robinjp <roundrobinjp@...oo.co.jp> wrote:

>>> The flash will then contain _random_ data in the non-used blocks.
>>> That is not a problem, right?
>> 
>> Nope.  So long as the previously written (random) data on the card
>> doesn't contain anything security sensitive.
> 
> I understood.
> 
>>> Although I have very small amount of files in my 4G image,
>>> I see that the image has almost no zero-filled blocks.
>>> Is that normal for ext4?
>> 
>> It depends on how you created the image.
> 
> I create the image like this:
> 
> dd if=/dev/zero of=a.img bs=4096 count=1048576
> mkfs.ext4 a.img
> mount -t ext4 -o loop a.img /mnt
> cp -a /foo/* /mnt/
> umount /mnt
> 
>>> Can zerofree.c recognize them as non-used blocks?
>> 
>> Yes, it uses the block allocation bitmaps to understand what is used
>> and non-used.
> 
> Great.
> 
> Thanks
> Round
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