dd command and unexpected results

drmacro · 04-27-2017, 04:11 PM

This is puzzle ...at least to me.

I dd a rasbian image to a new sd card like so:

sudo dd bs=4M if=2017-04-10-raspbian-jessie.img of=/dev/sdc

First time the image is no good (gets the root partition, but no boot partition, or what's supposed to be the boot partition is full of files with garbled names).

But if I re-do the same command, it works the second time.

Regards,
Mac

Didier Spaier · 04-27-2017, 04:15 PM

Hi and welcome to LQ.

After dd, you should issue the command:

Code:

sudo sync

to flush the write buffer. Did you do it?

drmacro · 04-27-2017, 04:17 PM

yes, I use sync every time.

Didier Spaier · 04-27-2017, 04:22 PM

Then, maybe there is an issue with the SD card itself?

drmacro · 04-27-2017, 04:36 PM

Happened with multiple cards. Latest just yesterday with new card, fresh out of packaging.

Has happened on different writers and different hardware.

I'm wondering if automount is somehow trying to mount before it's done. But, then why does it work the second time...

Or maybe whatever the existing partitioning is messes it up first time around, but since after the first go the partition is screwy and the second time it works.

Didier Spaier · 04-27-2017, 04:39 PM

In any case I would disable automount before using dd.

drmacro · 04-27-2017, 04:43 PM

Yeah, it's on by default in Ubuntu and I've been too lazy to figure out how to temporarily disable it...

syg00 · 04-27-2017, 06:08 PM

Looks like a race - the second execution will read the image from page cache. Use "oflag=sync" on the dd command rather than sync afterwards - slows it down (probably what you want in this case), but always works for me.

Laserbeak · 04-27-2017, 07:13 PM

I've had dd do some weird things before too, but it usually works fine.

jefro · 04-27-2017, 07:42 PM

Just for grins.

sudo dd if=2017-04-10-raspbian-jessie.img of=/dev/sdc, sync

Ztcoracat · 04-27-2017, 09:20 PM

Quote:

Originally Posted by jefro

Just for grins.

sudo dd if=2017-04-10-raspbian-jessie.img of=/dev/sdc, sync

Grins are generally accompanied by giggles:-

Laserbeak · 04-27-2017, 09:31 PM

I didn't even know people still had to use sync manually. I don't think I've used it since ancient versions of SunOS (the BSD version, not Solaris SVR4 versions). I thought those problems were fixed.

Didier Spaier · 04-27-2017, 11:56 PM

Quote:

Originally Posted by jefro

Just for grins.

sudo dd if=2017-04-10-raspbian-jessie.img of=/dev/sdc, sync

A semicolon instead of a comma should separate the commands, see POSIX