Re: RE: Finding hardlinks

[Benny Halevy <bhalevy@panasas.com>]

How about discussing this topic in the upcoming Connectathon?

Benny

Noveck, Dave wrote:
> For now, I'm not going to address the controversial issues here,
> mainly because I haven't decided how I feel about them yet.
> 
>      Whether allowing multiple filehandles per object is a good
>      or even reasonably acceptable idea.
> 
>      What the fact that RFC3530 talks about implies about what
>      clients should do about the issue.
> 
> One thing that I hope is not controversial is that the v4.1 spec
> should either get rid of this or make it clear and implementable.
> I expect plenty of controversy about which of those to choose, but
> hope that there isn't any about the proposition that we have to 
> choose one of those two.
> 
>> SECINFO information is, for instance, given
>> out on a per-filehandle basis, does that mean that the server will
> have
>> different security policies? 
> 
> Well yes, RFC3530 does say "The new SECINFO operation will allow the 
> client to determine, on a per filehandle basis", but I think that
> just has to be considered as an error rather than indicating that if
> you have two different filehandles for the same object, they can have 
> different security policies.  SECINFO in RFC3530 takes a directory fh
> and a name, so if there are multiple filehandles for the object with
> that name, there is no way for SECINFO to associate different policies
> with different filehandles.  All it has is the name to go by.  I think
> this should be corrected to "on a per-object basis" in the new spec no 
> matter what we do on other issues.
> 
> I think the principle here has to be that if we do allow multiple 
> fh's to map to the same object, we require that they designate the 
> same object, and thus it is not allowed for the server to act as if 
> you have multiple different object with different characteristics.
> 
> Similarly as to:
> 
>> In some places, people haven't even started
>> to think about the consequences: 
>>
>>     If GETATTR directed to the two filehandles does not return the
>>     fileid attribute for both of the handles, then it cannot be
>>     determined whether the two objects are the same.  Therefore,
>>     operations which depend on that knowledge (e.g., client side data
>>     caching) cannot be done reliably.
> 
> I think they (and maybe "they" includes me, I haven't checked the
> history
> here) started to think about them, but went in a bad direction.
> 
> The implication here that you can have a different set of attributes
> supported for the same object based on which filehandle is used to 
> access the attributes is totally bogus.
> 
> The definition of supp_attr says "The bit vector which would retrieve
> all mandatory and recommended attributes that are supported for this 
> object.  The scope of this attribute applies to all objects with a
> matching fsid."  So having the same object have different attributes
> supported based on the filehandle used or even two objects in the same
> fs having different attributes supported, in particular having fileid
> supported for one and not the other just isn't valid.
> 
>> The fact is that RFC3530 contains masses of rope with which
>> to allow server and client vendors to hang themselves. 
> 
> If that means simply making poor choices, then OK.  But if there are 
> other cases where you feel that the specification of a feature is simply
> 
> incoherent and the consequences not really thought out, then I think 
> we need to discuss them and not propagate that state of affairs to v4.1.
> 
> -----Original Message-----
> From: Trond Myklebust [mailto:trond.myklebust@fys.uio.no] 
> Sent: Friday, January 05, 2007 5:29 AM
> To: Benny Halevy
> Cc: Jan Harkes; Miklos Szeredi; nfsv4@ietf.org;
> linux-kernel@vger.kernel.org; Mikulas Patocka;
> linux-fsdevel@vger.kernel.org; Jeff Layton; Arjan van de Ven
> Subject: Re: [nfsv4] RE: Finding hardlinks
> 
> 
> On Fri, 2007-01-05 at 10:28 +0200, Benny Halevy wrote:
>> Trond Myklebust wrote:
>>> Exactly where do you see us violating the close-to-open cache
>>> consistency guarantees?
>>>
>> I haven't seen that. What I did see is cache inconsistency when
> opening
>> the same file with different file descriptors when the filehandle
> changes.
>> My testing shows that at least fsync and close fail with EIO when the
> filehandle
>> changed while there was dirty data in the cache and that's good.
> Still,
>> not sharing the cache while the file is opened (even on a different
> file
>> descriptors by the same process) seems impractical.
> 
> Tough. I'm not going to commit to adding support for multiple
> filehandles. The fact is that RFC3530 contains masses of rope with which
> to allow server and client vendors to hang themselves. The fact that the
> protocol claims support for servers that use multiple filehandles per
> inode does not mean it is necessarily a good idea. It adds unnecessary
> code complexity, it screws with server scalability (extra GETATTR calls
> just in order to probe existing filehandles), and it is insufficiently
> well documented in the RFC: SECINFO information is, for instance, given
> out on a per-filehandle basis, does that mean that the server will have
> different security policies? In some places, people haven't even started
> to think about the consequences:
> 
>       If GETATTR directed to the two filehandles does not return the
>       fileid attribute for both of the handles, then it cannot be
>       determined whether the two objects are the same.  Therefore,
>       operations which depend on that knowledge (e.g., client side data
>       caching) cannot be done reliably.
> 
> This implies the combination is legal, but offers no indication as to
> how you would match OPEN/CLOSE requests via different paths. AFAICS you
> would have to do non-cached I/O with no share modes (i.e. NFSv3-style
> "special" stateids). There is no way in hell we will ever support
> non-cached I/O in NFS other than the special case of O_DIRECT.
> 
> 
> ...and no, I'm certainly not interested in "fixing" the RFC on this
> point in any way other than getting this crap dropped from the spec. I
> see no use for it at all.
> 
> Trond
> 
> 
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