Re: [Bug-ddrescue] direct disk access will not read partial sector

[Scott Dwyer]

I have found a workaround for me by reading the full allocated file and 
then using truncate to cut the file to proper length, although this 
reads a few extra sectors. But I would think that ddrescue should try to 
read a whole sector and only write what is asked of, as opposed to not 
try at all. I am not sure what you mean by domain fragmentation though.


On 11/28/2012 2:54 PM, Antonio Diaz Diaz wrote:
> Hello Scott,
>
> Scott Dwyer wrote:
> > I do not know if this would be considered a bug, or just the nature 
> > of the direct disk access. When using the -d option, ddrescue will 
> > not attempt to read data that does not start or end on a sector 
> > boundary. 
>
> I thinh it is the nature of direct disk access. The kernel does not 
> accept such partial sector reads.
>
> Of course partial reads could be faked by reading the whole sector and 
> writing only the requested part. AFAIK neither linux nor glibc 
> implement this with direct disk access. I am not sure if it would be a 
> good idea for ddrescue to implement it or not. It may mean reading the 
> same sector more than once in extreme cases of domain fragmentation. 
> But I guess such domains are even less normal that what you are doing. 
> :-)
>
>
> Regards,
> Antonio.