Big blocks are actually slower

[David]

Just an observation about ddrescue, and one of the settings for ddrescueGUI.

The default setting for ddrescueGUI is to do reads of 128 sectors at a time. The fastest recovery setting uses 256.

My experience with ddrescue working on two USB disks is that 256 actually takes twice as long as 128. I am guessing this might be some limitation of USB and that if they were connected by SATA, 256 might be faster.

Thinking about this, even at 128 it seems to take much longer than I would have expected.

[Hamish McIntyre-Bhatty (hamishmb)]

Hi,

Yeah I've found that USB drives are just generally slow for this, especially if using reverse mode, especially (for some unknown reason) using direct disk access on Mac OS X (are you using OS X?).

I'm not sure why this happens, but if the above cases are your situation you could try lowering to the smallest option. I may add more options or try to be more intelligent about it in a future release.

Hamish

[Hamish McIntyre-Bhatty (hamishmb)]

Not really a bug, so marked as invalid, but I'm happy to discuss it with you here or as a question if you want to.

Hamish

[David (davidmorr)]

I had a try copying 64 blocks at a time and it was going to be slightly
longer than 128. So it seems that 128 is the sweet spot.

On 18/11/16 8:25 , Hamish McIntyre-Bhatty wrote:
> Hi,
>
> Yeah I've found that USB drives are just generally slow for this,
> especially if using reverse mode, especially (for some unknown reason)
> using direct disk access on Mac OS X (are you using OS X?).
>
> I'm not sure why this happens, but if the above cases are your situation
> you could try lowering to the smallest option. I may add more options or
> try to be more intelligent about it in a future release.
>
> Hamish
>
> ** Changed in: ddrescue-gui
> Importance: Undecided => Wishlist
>
> ** Changed in: ddrescue-gui
> Assignee: (unassigned) => Hamish McIntyre-Bhatty (hamishmb)
>
> ** Changed in: ddrescue-gui
> Milestone: None => 1.6.1
>
> ** Changed in: ddrescue-gui
> Status: New => Invalid
>

[Hamish McIntyre-Bhatty (hamishmb)]

Okay. ddrescue apparently defaults to 64 / <block size of device> so that's interesting.