How to create holes in files ?

[Bakul Shah]

On Thu, 28 Sep 2017 16:39:28 +0200 Ben RUBSON <ben.rubson at gmail.com> wrote:
Ben RUBSON writes:
> Hi,
> 
> I'm trying to make holes in files in C.
> Goal is to deallocate huge files on ZFS while (randomly) reading them.
> 
> The only thing I found is on Linux, fallocate(2) with FALLOC_FL_PUNCH_HOLE.
> 
> What about FreeBSD ?

You can copy such a file using rsync with -S or --sparse
option or dd with conv=sparse option. I don't know if these
programs use seek with SEEK_HOLE and SEEK_DATA to avoid
reading existing holes in the source file as zeroes.

Someone mentioned TRIM support but AFAIK this is only used (if
available) when a file is freed.  Replacing a zero block write
with lseek will impact every write (you have to check if the
block is all zeroes) so not worth doing in the general case.