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List:       linux-nfs
Subject:    Re: nfs4 write delegation status
From:       "J. Bruce Fields" <bfields () fieldses ! org>
Date:       2009-07-24 18:44:29
Message-ID: 20090724184429.GA16811 () fieldses ! org
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On Thu, Jul 23, 2009 at 02:11:46PM -0400, Andy Adamson wrote:
>
> On Jul 23, 2009, at 11:10 AM, Rick Macklem wrote:
>
>>>> I really don't want to enable write delegations until we figure  
>>>> out how
>>>> to enforce them correctly against local (non-nfs) users of the  
>>>> exported
>>>> filesystem as well.  In addition to breaking delegations on read  
>>>> opens,
>>>> that means breaking delegations or doing a cb_getattr on  
>>>> operations like
>>>> stat.
>>>
>>> do you know whether there are local FS where the maintainers at  
>>> least plan
>>> to incorporate delegations?
>>
>> I'm not a Linux guy, so I'm not familiar with the internal structure, 
>> but...
>> in general, I don't think the problem is with local file systems.  
>> Usually
>> the problem is with local system call access. For example, if a
>> process running locally on the server opens a file, the delegation  
>> should
>> be recalled, so that changes done locally on the client get flushed  
>> back
>> to the server. Also, a write delegation allows a client to do byte  
>> range
>> locking locally in the client, so the write delegation needs to be
>> recalled before anything gets a byte range lock locally in the server.
>
> The delegation implementation on the Linux server uses the vfs lease  
> subsystem, and so is integrated with local access - conflicting opens  
> done locally do recall delegations.  But the last time I looked, the  
> lease subsystem is not complete as it doesn't recall leases (nor  
> delegations) on remove, rename, etc. Another problem is that while write 
> delegations improve performance for certain workloads, they kill  
> performance for others.

Are there any published results yet with real workloads?

--b.
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