The number of stat calls caused by a single CREATE request

[Jeremy Allison]

On Wed, Aug 09, 2017 at 03:15:16PM +0200, Ralph Böhme via samba-technical wrote:
> Howdy Richard,
> 
> On Thu, Jul 27, 2017 at 05:37:45PM -0700, Richard Sharpe via samba-technical wrote:
> > I have been looking at the performance of CREATE and am seeing as many
> > as 15 stat-like requests for a single create.
> > 
> > Now, some of those are the result of our VFS module trying to get
> > CREATE TIME, but I have the stack-trace info for 11 of them and it is
> > not pretty:
> > 
> > 1. Four of them result from calls to smbd_check_access_rights (a
> > couple it seems are for calculating the MxAc Extra Create Parameter.)
> > 2. Two of them at least result from vfs_GetWd
> > 3. The majority of them are for "." while two are for the actual path
> > that is ".".
> > 
> > It is amazing to me that we do not use VALID_STAT to handle that, and
> > I have a VALID_STAT use in the VFS module  but it seems Samba is
> > creating fresh struct smb_filenames all over the place for ".".
> 
> yes, but can you check with perf or similar tools whether this still shows up in
> profiles? Once the metadata is cached after the first access subsequent stat
> calls shouldn't hurt too badly. When I worked on this last year I was able to
> get rid of a few redundant stat calls, but I left some for you. ;)

My guess is that this hurts VFS vendors who have custom
VFS modules that don't cache the stat values, but do some
RPC-calls on each one. We should certainly tidy up these
in our code if we can.