Re: [PATCH 2/2] nfs41: handle BLK_LAYOUT CB_RECALL_ANY

[Benny Halevy <bhalevy@tonian.com>]

On 2011-10-31 19:42, Trond Myklebust wrote:
> On Mon, 2011-10-31 at 19:08 +0200, Benny Halevy wrote: 
>> On 2011-10-31 18:45, Trond Myklebust wrote:
>>> On Tue, 2011-11-01 at 00:38 +0800, Peng Tao wrote: 
>>>> On Mon, Oct 31, 2011 at 11:49 PM, Trond Myklebust
>>>> <Trond.Myklebust@netapp.com> wrote:
>>>>> On Mon, 2011-10-31 at 08:15 -0700, Peng Tao wrote:
>>>>>> For blocklayout, we need to issue layoutreturn to return layouts when
>>>>>> handling CB_RECALL_ANY.
>>>>>
>>>>> Why?
>>>> Because replying NFS4_OK to CB_RECALL_ANY indicates that client knows
>>>> that server wants client to return layout. And server will be waiting
>>>> for layoutreturn in such case.
>>>
>>> No it doesn't. NFS4_OK means that the client acknowledges that it has
>>> been given a new limit on the number of recallable objects it can keep.
>>> There is no requirement in the text that it should send layoutreturn or
>>> that the server should expect that.
>>
>> The motivation for CB_RECALL_ANY is to reduce the state on the *server* side.
>> Quoting from RFC5661:
>>    The server may decide that it cannot hold all of the state for
>>    recallable objects, such as delegations and layouts, without running
>>    out of resources.  In such a case, while not optimal, the server is
>>    free to recall individual objects to reduce the load.
>> ...
>>    In order to implement an effective reclaim scheme for such objects,
>>    the server's knowledge of available resources must be used to
>>    determine when objects must be recalled with the clients selecting
>>    the actual objects to be returned.
>>                       ^^^^^^^^^^^^^^
>> ...
>>    When a given resource pool is over-utilized, the server can send a
>>    CB_RECALL_ANY to clients holding recallable objects of the types
>>    involved, allowing it to keep a certain number of such objects and
>>    return any excess.
>>    ^^^^^^^^^^^^^^^^^
>> ...
>>    RCA4_TYPE_MASK_FILE_LAYOUT
>>
>>       The client is to return layouts of type LAYOUT4_NFSV4_1_FILES.
>>                     ^^^^^^^^^^^^^^^^^
>>
>> Isn't that explicit enough?
> 
> Leaving aside the fact that the above quotes contain no normative
> language:
> Right now, we do a bulk return of all layouts. Doing a layoutreturn for
> each and every layout in that case is just ridiculous. Either do a

The idea is to return the layouts for files that are the least used,
not each and every layout.

> LAYOUTRETURN4_ALL after freeing all the layouts, or don't do anything at
> all and just wait for the server to revoke the layouts for us (which is
> what we currently do).
> Both options should be faster than doing a LAYOUTRETURN4_FILE on each
> and every file that is currently in use.

Doing LAYOUTRETURN4_ALL might cause a bug hiccup if the client needs to then send
a LAYOUTGET for each and every file that *is* currently in use.
So serving a CB_RECALL_ANY keeping more than 50% of the recallable objects means
the client would be better off returning the excess rather than returning everything
and reclaiming > 50% back.

Waiting for revocation may work well with some servers but would be disastrous in
terms of performance and responsiveness with others.

Benny

> 
> Trond
>