Re: making a file sparse - in-place?

[Pádraig Brady]

On 01/24/2014 02:41 AM, Rodrigo Campos wrote:
> On Fri, Jan 24, 2014 at 01:07:21AM +0000, Pádraig Brady wrote:
>> On 01/24/2014 12:47 AM, Bernhard Voelker wrote:
>>> Inspired by a recent post on util-linux ML [1], talking about turning
>>> a file into a sparse file in-place, i.e. not using a 2-step approach
>>> like `cp --sparse file file2 && mv file2 file`), I thought, hey, don't
>>> we have this in coreutils already?
>>
>>> b)
>>> Then, I tried
>>>   $ dd if=file of=file conv=sparse,notrunc
>>> to avoid truncating the output file. That didn't corrupt the data,
>>> but the file still was not sparse afterward.
>>> What's the reason for conv=sparse not to work in this situation?
>>> BTW: generally, writing to the same file seems to work, e.g.:
>>>   dd if=file of=file conv=ucase,notrunc
>>
>> To deallocate the zeros we'd have to use fallocate(FALLOC_FL_PUNCH_HOLE).
>> Also for efficiency reasons it would be nice to detect holes efficiently.
>> We can do this with the current fiemap code, but really we should try
>> and use the new SEEK_HOLE functionality available in the kernel.
> 
> I looked into this, but I think it won't. I even tried (maybe I did it wrong 
> ?)
> when implementing the tool to make a file sparse in-place, but it didn't 
> report
> the '\0's already allocated.

Right, you need to manually detect those, which dd does with is_nul():
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/system.h;h=39750e82f#l499

Then detected runs of zeros could be sparsified with FALLOC_FL_PUNCH_HOLE ?
I've not tried this myself but would be optimistic is works on some file 
systems.
If this wasn't supported then we'd stop immediately when if=of,
or otherwise revert to seeking.

Note also theh caveats noted for the conv=sparse option:

   Be careful when using
   this option in conjunction with `conv=notrunc' or
   `oflag=append'.  With `conv=notrunc', existing data in the
   output file corresponding to NUL blocks from the input, will
   be untouched.  With `oflag=append' the seeks performed will
   be ineffective.  Similarly, when the output is a device
   rather than a file, NUL input blocks are not copied, and
   therefore this option is most useful with virtual or pre
   zeroed devices.

dd specifying if=of without conv=notrunc couldn't error()
for example when dealing with devices.

thanks,
Pádraig.