Re: [Bug-ddrescue] Preallocating space, if possible

[Antonio Diaz Diaz]

Hello Matthew,

Matthew Ruffalo wrote:
> First, thank you for the wonderful tool. I've used ddrescue many times 
> to recover data from failing hard drives, as well as for manually 
> imaging working drives (the progress/speed display is very nice).

:-)


> I'm curious whether it's possible for ddrescue to preallocate space for 
> the file that it writes using posix_fallocate.

I think this is a very useful idea. Even on systems where size 
information is not available at all, user can tell ddrescue the size to 
copy, and this size can be preallocated. I expect I'll be able to 
implement it in 1.13.


> I did a quick grep through the source code and didn't find any instance 
> of 'allocate'. My understanding (which is not all that likely to be 
> correct) is that this behavior is distinct from the --sparse option. A 
> sparse file's size is reported to be much larger than the allocated size 
> on disk, whereas the new behavior would tell the filesystem in advance 
> that e.g. the image file will eventually contain 250GB. The filesystem 
> will then allocate this space as contiguously as possible.

I suspect preallocation may prevent the creation of sparse files on some 
sytems, so I plan to add a --preallocate option. See for example 
http://www.linux-mag.com/id/7272/2/

"The fallocate() system call is not in most glibc's as of this writing, 
but posix_fallocate() is; the problem with posix_fallocate is that if 
you use it on ext3, it will attempt to emulate fallocate() by writing 
all zeros to the file".


> Semi-related question: is there a publicly accessible source code 
> repository for ddrescue? I created a repository on GitHub ( 
> http://github.com/mruffalo/ddrescue ) that I will use to publish my 
> changes in the event that I implement this feature myself.

I work on a computer without internet connection, so I have not created 
a public repository for ddrescue. Thanks for creating one.


Best regards,
Antonio.