[Samba] Setting Samba Write Cache Size Can Cause File Corruption

[Jeremy Allison]

On Tue, Sep 28, 2010 at 05:34:37PM -0400, Andy Liebman wrote:

> I can tell you that setting write cache size to 131072 (half the
> size) makes the corruption go away, and so does turning off the
> "write cache size" setting altogether.  However, we are now
> wondering why the "write cache size" can have this effect on file
> corruption and whether setting it to 131072 will cause a corruption
> problem under some other circumstance we just haven't hit yet.

Well it's because this code must have a bug :-). Volker is
right, you should check it first with the v3-6-test git tree
code, as he has done some changes there that might have
already fixed it (if so we will back-port to 3.5.next of
course). If not, we'll need voluminous traces to track
down the exact set of writes that cause the problem.
Using the vfs_full_audit might initially help.

> Any ideas?  By the way, we have seen and documented this problem
> with both Samba 3.4.2 and Samba 3.5.3. We also noticed that "write
> cache size" was listed as "deprecated" in 3.4.2 and that in 3.5.3 it
> is no longer listed as "deprecated".  Somebody besides us must have
> thought keeping "write cache size" was still a good idea??

I wrote the original "write cache" code but thought
it wasn't needed (so marked it deprecated) but as
Volker found, sometimes it's really useful in certain
workloads.

Jeremy.