linux-ext4 - Re: ext4 and dd of emmc

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Date:   Fri, 23 Oct 2020 09:13:24 -0700
From:   Radivoje Jovanovic <radivojejovanovic@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: ext4 and dd of emmc

Thank you for responding Theodore,
My answers inline below.

On Fri, Oct 23, 2020 at 6:54 AM Theodore Y. Ts'o <tytso@....edu> wrote:
>
> 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.

No emmc partitions are mounted ever before binary copy takes place.
The partitions are mounted only after the binary copy is fully completed.
>
> > 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.
>
Again, the partitions were not mounted before dd was performed.

> > 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.
>
Yes this message does look like a HW failure. However, I am able to
reproduce this
on two different emmc models and over 10 different emmc chips across 2
different plarforms,
so I would not think it is really a HW issue.

Thanks.,
Ogi

> Cheers,
>
>                                                 - Ted
Thanks
Ogi