[Toybox] dd tests for transaction size?

[Robert Thompson]

It's pretty heavily used in combination with noerror. I can personally
attest to its usefulness when working with both damaged optical media and
spinning rust (with the correct blocksize in each case).

Each block read either contains blocksize data bytes or blocksize null
bytes, so that the source and destination still have matching offsets.

Incidentally, I've seen this used to progressively recover all readable
sectors on a damaged hard disk. Use a large
multiple-of-hardware-sector-size blocksize to start. The destination file
will retain (large) holes wherever there are bad blocks. searching for
aligned blocks of nulls will give you lists of offsets to seek and skip to
to retry with the smaller blocksize.

>From what I've seen, if a blocksized read overlaps with a bad sector, once
the kernel gives up on the sector, it fails the read without attempting any
further sectors. Since (before the days of widespread block-remapping)
sector failures often come in clusters, and avoiding reading bad sectors
can improve the odds of getting all the not-yet-bad sectors, you can play
iterative blocksize games to delay repeated risky reads until after most of
the good data is recovered.

I *have* seen this used with non-blockdevices, although at the moment I
can't recall the details (it wasn't recent).

Oddly enough, sync does what i expect. It's fsync and fdatasync that I was
surprised by. Guess I'm showing my age ;)




On Sun, Jul 9, 2017 at 5:39 PM, Samuel Holland <samuel at sholland.org> wrote:

> On 07/09/17 17:07, Rob Landley wrote:
>
>> Has anybody actually used the conv=sync option in the past 20 years? It
>> doesn't do what you think (that's conv=fsync), instead it pads short reads
>> with zeroes so the input block size is always the same.
>>
>> How is that useful?
>>
>
> It's very useful when trying to image dying hard disks, so bad sectors
> (that cause short reads or read failure) do not affect the alignment of
> the rest of the data in the image file.
>
> Rob
>>
>
> Samuel
>
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