Re: [Bug-tar] seek_hole proposal

[Joerg Schilling]

Pavel Raiskup <address@hidden> wrote:

> Hello all,
>
> I am trying to prepare patch which would reuse lseek's
> SEEK_HOLE/SEEK_DATA, the [v1] is attached.  Some info:
>
> Note that after discussion [1] I still think that existing ST_IS_SPARSE
> macro is better for file-sparseness detection than using SEEK_HOLE (not
> worth having additional syscalls open~>seek~>close).

Your code is not compatible to the SEEK_HOLE interface. A file is sparse in 
case that pathconf()/fpathconf(f, _PC_MIN_HOLE_SIZE) return a positive number 
and lseek(f, (off_t)0, SEEK_HOLE) returns a number < stat.st_size.

see "man pathconf":

         11.  If a filesystem supports  the  reporting  of  holes
              (see  lseek(2), pathconf() and fpathconf() return a
              positive number that represents  the  minimum  hole
              size  returned  in  bytes.  The  offsets  of  holes
              returned will be aligned to this same value. A spe-
              cial  value of 1 is returned if the filesystem does
              not specify the minimum hole size but still reports
              holes.

In other cases, the file stil may be sparse, but the filesystem does not 
support SEEK_HOLE. I e.g. doubt that Linux correctly implements SEEK_HOLE
for NFS.

Jörg

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