[SOLVED] any reason I shouldn't dd two partitions at once?

newbiesforever · 12-31-2021, 10:01 AM

I am going to dd two partitions to their respective targets. I could dd one and then the other; but is there any reason I wouldn't want to dd both at once? I could do it by, of course, simply opening a new tab in the terminal and running the new dd from there.

pan64 · 12-31-2021, 10:22 AM

That can work, but without details hard to say anything.

lvm_ · 12-31-2021, 11:33 AM

In case of magnetic disks if both partitions are written to the same disk or read from the same disk dd processes will compete for i/o and performance will drop to a point when running them sequentially will be much faster.

remmilou · 01-01-2022, 04:06 AM

Speed is the main consideration here (as LVM explains).
I used to "wipe" drives in the old days with dd (dd if=/dev/zero of=/dev/sdX). Did up to 8(!) drives in parallel. Was a mix of internal, USB and ESATA drives. Booted from a live CD. Slow, but started at friday and over the weekend it was ready.
I also found that the stability of the system had its linits, when overdoing it.
Happy dd'ing...

fatmac · 01-01-2022, 04:44 AM

As said above, the processes will be competing for resources, better to do them separately.

syg00 · 01-01-2022, 04:53 AM

Don't use dd. Simple.

remmilou · 01-04-2022, 03:56 AM

Quote:

Originally Posted by syg00

Don't use dd. Simple.

I'm curious for the reasons for not using dd. Any alternatives? Or alternative strategies?
I personally prefer dderescue or dcfldd, but basically the do the same job.

pan64 · 01-04-2022, 05:18 AM

Quote:

Originally Posted by remmilou

I'm curious for the reasons for not using dd. Any alternatives? Or alternative strategies?
I personally prefer dderescue or dcfldd, but basically the do the same job.

dd is a dangerous tool, it will overwrite everything without further notice and usually there is no way to recover (just a backup). So if you are unsure don't use it.
Copying 2 partitions in one can work, but need to check a lot of things before. Otherwise the result will be useless.

lvm_ · 01-04-2022, 05:57 AM

Quote:

Originally Posted by pan64

dd is a dangerous tool, it will overwrite everything without further notice and usually there is no way to recover (just a backup).

The same applies to cp - dangerous and to be banned too?

rknichols · 01-04-2022, 09:15 AM

Quote:

Originally Posted by pan64

dd is a dangerous tool, it will overwrite everything without further notice and usually there is no way to recover (just a backup). So if you are unsure don't use it.
Copying 2 partitions in one can work, but need to check a lot of things before. Otherwise the result will be useless.

Quote:

Originally Posted by lvm_

The same applies to cp - dangerous and to be banned too?

And the same applies to output redirection:

Code:

cat /dev/zero >/dev/sdx

so perhaps output redirection should be avoided, too.

There is nothing inherently dangerous about dd. It's just that dd is typically used to do dangerous things, like using root privileges to write to raw devices. Any method you use for that is going to be "dangerous".

pan64 · 01-04-2022, 09:28 AM

Quote:

Originally Posted by lvm_

The same applies to cp - dangerous and to be banned too?

I did not tell dd is banned. And actually you cannot cp two partitions "in one", or at least it is even harder, and quite unusual (and actually yes, cp should not be used to copy two partitions in one step).

You can read a lot about it here: https://www.linuxquestions.org/quest...ommand-362506/ (why and how dd can be dangerous - and also you can check if the same applies to cp, cat or any other command).

lvm_ · 01-04-2022, 12:17 PM

I was being sarcastic and my question was purely rhetorical. I am just not at all keen on things being called dangerous because they do exactly what you told them to do.

Also, OP was asking not about merging two partitions but about parallel copying n partitions to n destinations.

newbiesforever · 01-04-2022, 01:18 PM

Those who want to persist in heated debate whether dd should be used at all, would you please start your own thread. Otherwise, you're starting to drag my thread off topic.