ddrescue: new option, ""--compare-before-write", "check destination block content before write"

[Dr. Hinner EDV]

Dear Antonio Diaz Diaz

First of all Thank you very much for your work and your contribution!

I'm about to write a backup system, it should be energy-efficient and fast.

There shall also be a function "create mirror of HDD".

That's where "ddrescue" could step in, mirroring an HDD to an external 
connected USB HDD.

Because of current HDD sizes, it takes very long to completely write 
let's say an 8 TB HDD.

So I'd like to suggest to introduce a new option "--compare-before-write".

If a block of data is about to be written to the destination, "ddrescue" 
first reads this block and checks, if it is the same which is about to 
be written.

a) if it is the same, skip this block and read the next block

b) if it is not the same, write this block

What do you think about this function?

I'm very sorry if this option already exists and skipped my research.

I also wonder if SSDs do this check by themselves, in my opinion it 
would be smart to do because of wear, but it also takes some time.

All the best and Happy New Year to you and your family,

Kajetan Hinner
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