SMB3 Unix extensions

David Disseldorp ddiss at
Tue May 2 10:58:41 UTC 2017

On Mon, 1 May 2017 08:05:45 -0700, Christoph Hellwig via samba-technical wrote:

> > and this is plumbed down into source3/lib/system.c:sys_fallocate().
> > 
> > You get to this via SMB2 ioctl FSCTL_SET_ZERO_DATA.  
> Note that in Linux FALLOC_FL_PUNCH_HOLE will release space allocated
> in the range as much as possible, thus creating a sparse file.
> If you just want to zero the range the flag to use is
> The SMB2 ioctl sounds closer to the latter at least by name, I haven't
> found a definition of the semantics.

FSCTL_SET_ZERO_DATA is defined in MS-FSCC 2.3.67. Deallocation is
permitted via the clause:
  How an implementation zeros data within a file is
  implementation-dependent. A file system MAY choose to deallocate
  regions of disk space that have been zeroed.

If a file is marked non-sparse (and strict-allocate is configured),
then we reallocate the punched region. I left a todo to use
FL_ZERO_RANGE for non-sparse files in future:
 * File marked non-sparse and "strict allocate" is enabled -
 * allocate the range that we just punched out.
 * In future FALLOC_FL_ZERO_RANGE could be used exclusively for
 * this, but it's currently only supported on XFS and ext4.
...looks like it's still missing from Btrfs.

Cheers, David

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