Treating share modes as advisory when set by Mac clients

Volker Lendecke Volker.Lendecke at SerNet.DE
Mon Mar 12 13:59:19 MDT 2012

On Mon, Mar 12, 2012 at 03:27:01PM -0400, Brian Campbell wrote:
> I work on network attached storage for video editing. Our customers use
> both Mac and Windows workstations, and on the Mac, they generally use
> Thursby Software's DAVE as a client, as it works much better than the
> built-in smbfs on Mac OS X.
> On Mac OS X, when opening a file, you can request a shared or exclusive
> lock on the file using the O_SHLOCK or O_EXLOCK flags in the open() call.
> On local filesystems, these are implemented as locks with flock() semantics.
> Thursby recently added support in DAVE to implement these flags using share
> modes (DENY_WRITE/DENY_ALL). They apparently did this because Microsoft
> Word depends on these locks working, and was getting confused when two
> people could edit the same file at the same time. This also appears to be
> how smbfs on Mac OS X implements these open() flags.
> This has caused a problem for our customers using Final Cut (and possibly
> some other video editing software). When Final Cut writes files out in
> certain situations (renders, and log and transfer), it opens the file with
> the O_EXLOCK flag, and holds the file open with that exclusive lock even
> after it's done writing, until you close out the entire project. Locally,
> that doesn't cause any problems, since the lock is advisory and most
> software that needs to read the file doesn't try to obtain any locks on the
> file.

My initial reaction is that it is a bug in Final Cut, but
that does not help you much now. Does that behave in any way
differently if you store it on Windows servers? I can't
imagine that Final Cut can't be used when storing files
there. If Samba behaves differently, then we need
comparative network traces.

As an immediate relief, you might want to take a look at
"share modes = no". This is a per-share parameter which
makes Samba not use and respect share modes at all. You can
play similar tricks as mentioned in to
restrict the dangerous use to specific clients and specific

With best regards,


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