[Toybox] dd tests for transaction size?

[Samuel Holland]

On 07/09/17 18:25, Rob Landley wrote:
> On 07/09/2017 05:39 PM, Samuel Holland 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.
> 
> Does the read failure automatically advance the file pointer by the 
> full amount, or does it have to lseek after the read? (I'm assuming 
> the first, but that's an implementation detail I have so many 
> questions about.

I don't know the implementation details.

> I'm assuming your read block size is the device physical block size, 
> and thus the short read isn't skipping data after the missing data 
> which you otherwise could have read but gets turned into a short read
> because the read would otherwise have holes in it?

Everything within ibs past the point of failure is replaced with zeroes.
So if the ibs=16k and your third 512b sector is bad, you'll have 1k of
good data followed by 15k of zeroes, even if the rest of the sectors are
okay. If you want to maximize the amount of data retrieved, then yes,
you want ibs=sectorsize. If you want to quickly find good/bad areas of
the disk, then ibs can be larger.

> How do you distinguish older 512 byte blocks from modern 4k blocks 
> from whatever the heck flash block sizes are? I assume there's an 
> ioctl() or something to find this out? Or do you just always use 512 
> bytes and hammering away at different parts of the same bad block 
> eventually makes its way out the far side?)

I take the disk out of the machine and look at the label :) If it has
the "AF" logo, it's 4k sectors, otherwise it's 512b. There's also an
ioctl: `blockdev --getbsz /dev/sda`. In my experience, flash storage
doesn't get bad sectors; it works until it doesn't.

> Rob

Samuel