leases and F_NOTIFY in samba 3 & 4

[tridge at samba.org]

Abhi,

 > I understand that Samba uses F_SETLEASE, F_GETLEASE fcntls to implement 
 > oplocks in linux. Atleast in Samba3.

yes. These are used to supplement a user-space oplock implementation. 

 > Does Samba4 also use linux kernel leases?

Funny you should ask :-)

Samba4 does not have oplock support yet, but it just so happens that I
am working on it right now. I expect to have it working this week
(assuming other things don't get in the way).

 > How much of a performance impact is it to run Samba with oplocks
 > disabled?

that very much depends on the application. For some applications the
performance loss without oplocks is quite severe. For example, I
recently ran across an application used in the film industry that
opens each frame of a film while playing it anywhere between 8 and 12
times. With a batch oplock these extra opens are never sent on the
network so it performs quite well. Without an oplock the client will
do all those extra opens, which has a severe impact.

 > I'm about to implement lease support for GFS, but at the outset, there 
 > seems to be a significant performance impact for the fs.

yes, for a clustered filesystem it will be quite expensive, but
hopefully it will be a lot less expensive than the extra round trips
from badly behaved applications.

 > Also, can somebody comment on how the F_NOTIFY fcntl is used in Samba?

F_NOTIFY is part of dnotify, which is used in Samba3. In Samba4 we use
inotify instead of dnotify. The dnotify interface is not a good match
for the windows "change notify" functionality as it doesn't tell the
application what file has been changed/added/removed, so when using
dnotify Samba cannot get the change notify semantics right. I expect
that Samba3 will probably add support for inotify soon, as it is a
much better interface.

I'd suggest you work on inotify support rather than dnotify support
for GFS.

Cheers, Tridge