Re: flashing large eMMC partitions with ext4

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On Tue, Jul 26, 2011 at 2:36 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Mon, Jul 25, 2011 at 9:10 PM, Ted Ts'o <tytso@xxxxxxx> wrote:
>> On Sat, Jul 23, 2011 at 12:49:28AM +0900, Round Robinjp wrote:
>>> Hi
>>>
>>> I have a question regarding making ext4 image for
>>> large eMMC partition.
>>>
>>> We have a 4G partition in our embedded device
>>> in which we want to use ext4 filesystem.
>>> But for that we have to create a 4G image.
>>> flashing this 4G image to the eMMC takes a long
>>> time. Is there any way to reduce this time?
>>>
>>> For vfat, you can truncate the image leaving only
>>> non zero-filled blocks which makes the image very
>>> short and the time for flashing is reduced.
>>> Is something similar to that possible for ext4?
>>
>> OK, so it's not obvious what problem you are trying to ask here.
>>
>> It sounds like Andreas was trying to help you solve the problem of
>> minimizing the number of blocks written by mke2fs.
>>
>> I'm guessing the problem is you've already created a file system image
>> which is 4G, and for which a large number of the blocks are not used,
>> and you're trying to optimize the amount of time it takes to flash the
>> image.  Is that right?
>>
>
> I am going to make a 3rd guess and we will see who was closest ;-)
> My guess is that Round wan't a "short" image, just like he wrote.
> vfat image can simply be truncated, because all initial blocks are at the
> beginning of the partition.
>
> One could truncate the result of an ext4 image to quite less than 4G,
> for example, if flex_bg is set to 32, all bitmaps are at the block group 0.
> copies of the super block can be discarded, since one has the copy in
> the image file.
>
> The only important piece of information, not in the beginning of the partition
> is the super block of the journal, which is usually in the middle of the fs,
> so one needs do some tricks to allocate the journal from group 0.
> Is there a mke2fs option to do that?
Now the fs is clean, so we can delete journal.  After the image is
flashed to the eMMC, we can add journal again.  I think tune2fs can
add and delete journal.  Am I rgiht?

Yongqiang.
>
> All-in-all, I think it should be easy to prepare a 4G image that could
> be truncated
> to one group (128M).
>
> Amir.
>
>> The way to do that is to use a program like zerofree.c (google it, or
>> see attached) to make sure the non-used blocks are zero-filled, and
>> then use a program like make-sparse.c (see the e2fsprogs sources, in
>> the contrib directory) to only write the non-zero blocks to the flash
>> device.
>>
>> Regards,
>>
>>                                                - Ted
>>
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-- 
Best Wishes
Yongqiang Yang
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