Re: whither NFS umount?

[Steve Dickson <SteveD@redhat.com>]

On 10/13/2010 02:13 PM, Jeff Layton wrote:
> On Wed, 13 Oct 2010 13:40:37 -0400
> Steve Dickson <SteveD@redhat.com> wrote:
> 
>> Sorry for joining late... 
>>
>> On 10/12/2010 03:44 PM, Trond Myklebust wrote:
>>> On Tue, 2010-10-12 at 15:18 -0400, Jeff Layton wrote:
>>>
>>>> I think the part that causes problems is having userspace do this. In
>>>> theory, if the kernel were in charge of sending the UMNT, then it's not
>>>> really a problem since it knows when to do it. If we have code that
>>>> sends a UMNT already, why not do a best-effort UMNT call from the
>>>> kernel when we tear down the sb?
>>>
>>> Purely for the pleasure of allowing the server to maintain inaccurate
>>> statistics about who is currently mounting what? I think not...
>>>
>>> You can get far more accurate results by replacing the MNT/UMNT state
>>> counter with a purely server-based scheme to track who accessed one or
>>> more files on each exported partition in the past 5 minutes or so. That
>>> would even work with NFSv4...
>>>
>>>> Either way, eliminating umount.nfs would be nice...
>>>
>>> Agreed.
>> I having a hard time understanding this logic... 
>>
>> Why do we think we (the Linux community) can simply 
>> throw way an established part of the protocol just because 
>> we deem it advisory... Now maybe in our implementation UMNT its
>> advisory and it might even be advisory in the spec, but how do we 
>> know with  other NFS implementation is not advisory, its actually needed.
>> We don't known and we can't known....
>>
>> Now when our implementation becomes an NFSv4 only implementation, 
>> I say fine; Eliminate all the protocols that go along
>> with both v2 and v3. But until then lets just have leave
>> the legacy protocols along and move forward in more meaningful 
>> efforts... 
>>
> 
> It's not clear to me what you're advocating here...
I'm advocating do remove the umount.nfs command which
in turn would not remove the UMNT calls. Its just not 
worth the pain... because there is no gain! 

> 
> umount.nfs is clearly broken today -- it sends UMNT calls even when it
> shouldn't and there are problems with mtab handling.
Well that's a bug, so lets fix it... But the fix should not
be to completely remove call. 

> 
> The latter part is the main problem. The question is though -- should
> we fix umount.nfs or just do away with it?
Fix it... IMHO..

> 
> My position is that it really serves no good purpose these days. Only
> the kernel knows when a UMNT call should be sent, so that really has no
> place in umount.nfs. Once that piece is out of userspace, we might as
> well just let /bin/umount do everything and not bother with umount.nfs.
Just curious... how would we maintain backwards completable with old
kernels? 

Again, why even put effort in things like this? All the testing
something this could cause... all the (potential) breakage of 
third party applicants.... There is absolution no reason to
do this... because, again, there is no real gain to it... IMHO... 

> 
> A separate question is whether we should sent a UMNT request at all.
> Trond has basically said "don't bother" to that one. I don't feel
> strongly either way, but wouldn't be opposed to sending a best-effort
> UMNT call from the kernel if it makes some servers happy.
> 
I would say send the UMNT, since it does not cause any pain to send it
verses the pain that could be cause by not sending it...

This is a perfect example of fixing something that is not
broken... We can put our energy in better place that worrying
about things like this... IMHO...

steved.