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

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Message-ID: <CAOQ4uxiR7eVbb8_i9ZvM7X1KqaBNs1kx+nU75pTgFjbTjH+S-Q@mail.gmail.com>
Date:	Tue, 2 Aug 2011 20:44:17 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Round Robinjp <roundrobinjp@...oo.co.jp>
Cc:	Andreas Dilger <aedilger@...il.com>, "Ted Ts'o" <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: flashing large eMMC partitions with ext4

On Tue, Aug 2, 2011 at 7:07 PM, Round Robinjp <roundrobinjp@...oo.co.jp> wrote:
> Amir
>
>> > But after extending to 4G, e2fsck makes some complain.
>> > I guess this is not expected behaviour, is it?
>> >
>>
>> it is expected when you use the current resize2fs,
>> which does not respect the flex_bg layout, so new group block bitmaps
>> are allocated beyond the 1G border and initialized.
>
> So that means I have thrown away some important part of
> the filesystem when I did truncate -s 1G, isn't it?

It's not "so important" because it contains information that can be
recovered, so fsck -fp can easily fix that.
Do you have e2fsck on your embedded system?
Can you run it on first boot?

> Will things go wrong if I flash this 1G image to my eMMC
> partition (without using Yongqiang's new 64bit resize patches)?

I think the kernel will shout at you, mark the fs with error_fs flag
and recommend that you run fsck.
This will happen when you write files to the extended 3G.

> I need to understand whether Yongqiang's patch is absolutely
> necessary for this purpose or just a good thing to have.

if you don't use it, you need to use the other method I mentioned using debugfs
or run fsck on the MMC after you flashed the short image.

>
>> if you use Yongqiang's new 64bit resize patches, the final fsck won't complain.
>> unfortunately for you, those patches have not been merged to the kernel yet,
>> so you will have to either build your own ext4 module or wait at least until
>> kernel 3.2 is released to have it in mainline.
>
> As said above.
>
>> It is actually quite simple to fix the 1G image, so it will pass fsck
>> after truncate -s 4G.
>> All it takes it setting the BLOCK_UNINIT flag in groups 8-31
>> this should be possible to do with debugfs (or write a small tool to do it).
>> if I have time, it will try it myself and post the instructions.
>
> OK, thanks in advance.
>
> Best Regards
> Round
>
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