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Old 04-27-2017, 04:11 PM   #1
drmacro
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dd command and unexpected results


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
 
Old 04-27-2017, 04:15 PM   #2
Didier Spaier
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Hi and welcome to LQ.

After dd, you should issue the command:
Code:
sudo sync
to flush the write buffer. Did you do it?
 
Old 04-27-2017, 04:17 PM   #3
drmacro
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yes, I use sync every time.
 
Old 04-27-2017, 04:22 PM   #4
Didier Spaier
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Then, maybe there is an issue with the SD card itself?
 
Old 04-27-2017, 04:36 PM   #5
drmacro
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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.
 
Old 04-27-2017, 04:39 PM   #6
Didier Spaier
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In any case I would disable automount before using dd.
 
Old 04-27-2017, 04:43 PM   #7
drmacro
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Yeah, it's on by default in Ubuntu and I've been too lazy to figure out how to temporarily disable it...
 
Old 04-27-2017, 06:08 PM   #8
syg00
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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.
 
Old 04-27-2017, 07:13 PM   #9
Laserbeak
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I've had dd do some weird things before too, but it usually works fine.
 
Old 04-27-2017, 07:42 PM   #10
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Just for grins.

sudo dd if=2017-04-10-raspbian-jessie.img of=/dev/sdc, sync
 
Old 04-27-2017, 09:20 PM   #11
Ztcoracat
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Quote:
Originally Posted by jefro View Post
Just for grins.

sudo dd if=2017-04-10-raspbian-jessie.img of=/dev/sdc, sync
Grins are generally accompanied by giggles:-
 
Old 04-27-2017, 09:31 PM   #12
Laserbeak
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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.
 
Old 04-27-2017, 11:56 PM   #13
Didier Spaier
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Quote:
Originally Posted by jefro View Post
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
 
  


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