Re: [Bug-ddrescue] New features patch now available

[Scott D]

On 7/5/2014 8:26 PM, Franc Zabkar wrote:
> A reverse pass effectively disables the drive's read-ahead caching. 
> Data recovery professionals will tell you that this can be (or is?) a 
> good thing.
First, how does that effectively disable it? If a block of data is read, 
the drive is still going to perform read-ahead because it expects you to 
read forward. By going in reverse and reading the previous block you 
just miss out on utilizing the read-ahead data.

Second (sorry for this read-ahead rant), they can tell me, but I might 
not listen. I don't think a drive reads ahead that far, and in reality 
the head likely already passes over the data  (or at least some of it) 
that is read ahead. And as far as I can tell, it only helps with read 
speed. I have yet to see a read affected by an error in the read-ahead 
data. As far as I can tell (from my testing) a drive does not try to do 
any error recovery on read-ahead data. I could be wrong, and there may 
be other reasons they have (bad sector reallocation?). What I don't like 
about it is that slower read speed = less data read in the same amount 
of time = more time the drive has to be spun up. The professionals will 
open the drive up and replace the heads if there is failure, but most 
using ddrescue are not professionals and are unable to do that, so it 
seems best to get the most good data as fast as possible before the 
possibility of total failure.

Like I said, the --no-reverse-pass option is for those that want it, for 
those that want (or need) the control. There may even be a time I may 
want to use it. But in my benchmark testing it is not beneficial.

Scott