setting allocation size in cifs/smb3

Jeremy Allison jra at
Fri Jul 18 11:09:52 MDT 2014

On Fri, Jul 18, 2014 at 01:15:07AM -0500, Steve French wrote:
> I created a patch to cifs.ko to add fallocate FALLOC_FL_KEEP_SIZE
> support and did some tests against a Samba server (Samba 4.1.6 on ext4
> on Ubuntu) and also a Windows 8.1 server (NTFS file system) and ran
> into an obvious problem.  How to tell if the allocation size is
> changed?  Neither stat on the Samba server local file system nor the
> Windows 8.1 properties show the size on disk changing.
> I am sending SMB3 SetInfo level 0x13 (set allocation size) and getting
> success back from the Windows server and from Samba server.
> If I set the length past the end of file I get no error, but the
> properties of the file in Windows Explorer still show the same (e.g.
> zero) as the file size.  If I try setting it to a size smaller than
> the size of the file, the file shrinks (by the way this is an awkward
> behavior to work around since on Linux fallocate can't shrink the file
> - so if you have any ideas short of revalidating the file size before
> deciding whether to send the fallocate request to Windows ...).
> Any ideas why Samba shows success but doesn't actually call ext4 to
> update the allocation size?

smbd does use VFS_FALLOCATE_KEEP_SIZE, which maps
into fallocate FALLOC_FL_KEEP_SIZE if the underlying
storage supports it.

You need to set "strict allocate = true" on the
share in order to get this behavior though.

If you're setting allocation size smaller
than file size we just use ftruncate, I
don't see another possibility here. What
behavior do you expect in the shrink case ?


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