kqueue of a nfs mounted file not working

[Alfred Perlstein]

On 11/16/15 6:00 AM, Rick Macklem wrote:
> Daniel Braniss wrote:
>>> On 15 Nov 2015, at 17:26, Konstantin Belousov <kostikbel at gmail.com> wrote:
>>>
>>> On Sun, Nov 15, 2015 at 11:22:55AM +0200, Daniel Braniss wrote:
>>>> HI,
>>>> I???m writing a program to monitor a file using kqueue(2), if the file is
>>>> local
>>>> all is OK, but if the file is via a nfs mounted fs, it only works once.
>>>> stat shows the file growing, but kevent is not triggered.
>>> Does file grow due to local changes on the nfs client, or some other
>>> client changes the file, while your client tries to get kevent
>>> notifications ?
>> it gets updated by a host which has the file as local, so yes, it gets
>> updated
>> by another client/host.
>>
> Hmm, I am not surprised that this doesn't work. The only indication to the
> client that the file has changed on the server is a change in the file's
> attributes when they're acquired (via a Getattr RPC or similar) from the server.
>
> There is a vfs operation called VFS_SYSCTL(). This isn't implemented on
> the current NFS client. It was implemented on the old one, but only for
> NFS locking events and I didn't understand what needed to be done, so I
> didn't do it.
> Kostik, do you know if there is a VFS_SYSCTL() call done when the kevent
> stuff is probing for a file size change? (Or does it not probe and events
> get triggered via the write syscall or ???) I took a quick look at the kevent
> stuff, but admit I got lost and couldn't figure out what triggered events
> being logged?
>
> Also, is the event for "file growing" or "file changed"?
> If it is the latter, all the NFS client can do is look for a change in
> the file's modify time and this is often at a resolution of 1sec., which
> implies that a change within the same second as the previous one may not
> be noticed. (NFSv4 has a Change attribute that is always guaranteed to
> change, but that is only NFSv4.) Also, you see metadata changes as well
> as data changes, at least for the NFSv4 attribute.
>
> rick
>
Hello Rick,

I implemented the VFS_SYSCTL work in NFS.  The goal was to allow a path 
to query filesystems via sysctl.

This was used in OS X to provide a way to query the filesystem for "events".

https://github.com/opensource-apple/xnu/blob/10.10/bsd/nfs/nfs_vfsops.c#L5188

For NFS you want to inform the user that an nfs filesystem is down, or 
the locking daemon is down.  That was inside a GUI you can pop up a 
dialog box to allow the user to force-unmount or turn off locking.

Image you're connected to multiple NFS shares inside of X11 or whatever 
windowing system you have.  Then there is a network outage.  You'll want 
to know which filesystems are not responding and why.

-Alfred

-Alfred