Re: ext4 and dd of emmc

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Thu, Oct 22, 2020 at 10:46:18AM -0700, Radivoje Jovanovic wrote:
> Hello,
> I am creating empty  4GB ext4 partition on emmc with parted like this:
> parted -s -a optimal /dev/emmcblk0 mkpart data ext4 1024 5120
> mkfs.ext4 /dev/mmcblk0p7 (this is the partition that was created in
> the previous step)
> 
> I do not mount this partition before I do dd of the emmc.
> dd of the emmc is done like this:
> dd if=/dev/emmcblk0 | gzip -c | dd of=./image.bin
> 
> after this I write back the emmc with the same binary file:
> dd if=./image.bin | gunzip -c | dd of=. /dev/emmcblk0

Is the root file system (or any file system mounted read/write)
located on /dev/emmcblk0?  You seem to imply that /dev/emmcblk0p7 was
mounted read write, so that would appear to be the case.  If so,
that's a bad idea.  Don't do that.   It's not safe.

> at the boot the kernel reports:
> EXT4-fs (mmcblk0p7): warning: mounting fs with errors, running e2fsck
> is recommended

That's probably because of the fact that mmcblk0p7 was moounted
read/write at the time when you tried to save and restore img.bin.
*Never* mess with a block device containing a mounted file system like
this.

> Buffer I/O error on dev mmcblk0p7, logical block 0, lost sync page write
> EXT4-fs (mmcblk0p7): I/O error while writing superblock

That implies that an I/O error from the eMMC device.  That's a
hardware issue, *probably* not related to the fact that partition was
not mounted, but rather by lousy hardware Quality Assurance along the
way.  If the hardware device is throwing I/O errors, you need to root
cause that issue first before worrying about any file system
complaints.

Cheers,

						- Ted