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I am trying to backup my system that is running on emmc memory on my embedded device. The problem is that I keep getting input/output errors when trying. I have tried at 4M, 1M, and 64K, but still getting them. is this normal?
dd if=/dev/nbd1 conv=sync,noerror bs=64K |
gzip -c >
/mnt/usbdrive/backup_image.img.gz
dd if=/dev/nbd1 …
/dev/ndbX is a Network Block Device. You're not backing up your EMMC, but something you've mounted via network; I'm almost sure that's not what you want to do.
I presume this means you run an NBD daemon on the embedded device, and mounted the shared volume on your PC. That is a possible approach, but I'd really just go for ncat here; run
ncat -l … > backup_image.img
on your PC, and
ncat … < /dev/mmblk0
on your embedded system (assuming mmblk0 is your EMMC).
Note: Many embedded systems do not run off self-managing flash devices that offer a block device interface such as MMC/SD cards/USB flash drives/SATA SSDs, but off raw flash ICs. These are NOT block devices (but MTDs) and don't offer a contiguous access method. Backing these up as physical layout to a file is hence (logically) not possible. Backing up the contents is.
If you'd try to export your MTD using a NBD daemon, things will go very wrong.
You are almost there, but blocksize is only part of the equation.
The trouble here is that the nbd generally has no idea how big it is, so using dd as the example displays will happily try marching right off the end of the device and produce an I/O error. On Technologic Systems devices using sdctl, a nbd is generally pointed at a local eMMC, SD, or other local storage resource.
One thing you can do to confirm this is what happened on TS hardware is to run a couple diagnostic commands with sdctl. You will probably see something like this:
# sdctl --stats
...
nbd_seek_past_eof_errs=3
...
# sdctl --dmesg
...
Jan 1 01:38:57 Access beyond EOF, sec=3776488, len=256, req=0
Jan 1 01:38:57 Access beyond EOF, sec=3776744, len=256, req=0
Jan 1 01:38:57 Access beyond EOF, sec=3776504, len=8, req=0
The way to avoid the I/O (ran past EOF) error is to always give your dd a constraint when dealing with nbd-using storage drivers, such as:
dd if=/dev/nbd1 of=/my/backup.dd bs=1M count=1024 conv=fsync
By providing a count of the blocks you want to read, you can avoid falling off the edge of your media. Blocksize and count can be almost anything, so long as bs*count <= actual media size.
