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Re: dd: copy blocks in reverse order

From: Pádraig Brady
Subject: Re: dd: copy blocks in reverse order
Date: Sun, 6 Dec 2015 17:33:41 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/12/15 15:20, Ivan Pozdeev wrote:
> Hello Coreutils,
> The subj is a requirement if I'm copying between overlapping regions, and the
> destination is further in the media than the source.
> The specific task I'm having is to move a partition a number of sectors 
> forward.
> To be completely clear, the algorithm is: copy a block, then seek to the
> previous block (i.e. 2*bs bytes back).
> I don't currently see a use case for setting this for input/output 
> independently.
> An idea for the option name is `d'.
> There is a workaround for my particular case - `dd if=<block_device> 
> bs=<2*shift_bytes> | dd
> of=<block_device> seek=<shift_blocks>' but it requires a few times 
> 2*shift_bytes of memory.

I see dd_rescue (and ddrescue) have "operate in reverse" options.
The reason stated being:

"If you have one spot of bad sectors within the partition,
it might be a good idea, to approach this spot from both sides."

For your use case you could compute the size and offset of the overlap,
and use 2 dd invocations to copy the overlap area, then the rest.
Or perhaps more simply use a loop to iterate in reverse
using an appropriately large blocksize to minimize dd invocations.

So while this is useful it is a bit of an edge case
and there are alternatives using other tools or multiple dd invocations.

I'd be 60:40 against adding it.


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