Re: [nfsv4] [PATCH 16/22] pnfs-submit: rewrite of layout state handling and cb_layoutrecall

[Fred Isaman <iisaman@netapp.com>]

On Mon, Nov 15, 2010 at 11:17 AM, Benny Halevy <bhalevy@panasas.com> wrote:
> On 2010-11-15 16:51, Fred Isaman wrote:
>> On Sun, Nov 14, 2010 at 10:43 AM, Benny Halevy <bhalevy@panasas.com> wrote:
>>>
>>> Using the open stateid after forgetting the layout could be a protocol bug,
>>> or at least it falls into undefined territories.
>>>
>>> The RFC says:
>>>
>>>   The loga_stateid field specifies a valid stateid.  If a layout is not
>>>   currently held by the client, the loga_stateid field represents a
>>>   stateid reflecting the correspondingly valid open, byte-range lock,
>>>   or delegation stateid.  Once a layout is held on the file by the
>>>   client, the loga_stateid field MUST be a stateid as returned from a
>>>   previous LAYOUTGET or LAYOUTRETURN operation or provided by a
>>>   CB_LAYOUTRECALL operation (see Section 12.5.3).
>>>
>>> So the question is does the text above refer to the client view of the state or to
>>> the server's view.
>>> In other words, with the forgetful client model, when the client unilaterally forgets
>>> the layout without letting the server know about it (no LAYOUTRETURN was sent),
>>> does it mean "a layout is not currently held by the client"?
>>>
>>
>> I would argue that yes, this is in fact what it means.
>>
>> It seems the server has two options when confronted with an
>> openstateid.  Either interpret this as a declaration by the client
>> that it has forgotten all previous layouts and behave appropriately
>> (wipe any layout state assigned to the file and create a new
>> layoutstateid), or assume this is part of parallel spew of
>> LAYOUTGET(openstateid) and try to use an existing layout state with
>> the appropriate (possibly not one) seqid.  I argue that, as the spec
>> stands, the second option is not really a choice, because the first
>> option exists.  If a client using the second option encounters a
>> server using the first, bad things happen.  The client will issue
>> multiple LAYOUTGET(openstateids), the server will, upon seeing each,
>> discard any previous state and return a new state with segid=1, with
>
> Is this the specified behavior?
>
>> the final valid state being that of whichever one was processed last.
>> The client will see all the OK returns, and not have any easy method
>> of determining which is the one that the server considers valid.
>>
>> Thus I claim that, because of the forgetful model, the client must
>> serialize its LAYOUTGET(openstateid) calls.
>>
>
> I disagree. LAYOUTGET(openstateid) should be no different than
> any other layout stateid and the client should be able to send multiple
> such LAYOUTGETs *initially* (and only initially).  The server can process
> these as any other LAYOUTGET with the sequenceid rules assuming seqid==0
> (which is disallowed otherwise)
>
>>> The server will see a LAYOUTGET with an open/lock/deleg stateid in this case
>>> while it still thinks that the client is holding a layout.
>>> Since this could normally happen if the client sends multiple LAYOUTGETs in
>>> parallel before it received any layout stateid the server should allow it
>>> within the VALID_SEQID_RANGE constraints (see 12.5.5.2.1.4, although it is
>>> not explicitly called out there), otherwise, it seems like the server is supposed
>>> to return NFS4ERR_OLD_STATEID.
>>>
>>> Strictly reading the spec, the client should use the most recent layout stateid
>>> even in the forgetful model, until it gets a LAYOUTRETURN reply with lrs_present==false
>>> or until it replies NFS4ERR_NOMATCHING_LAYOUT to CB_LAYOUTRECALL with
>>> clora_iomode==LAYOUTIOMODE4_ANY or other values where the client never dropped
>>> a layout (did I say recently how much I hate the forgetful model which introduces
>>> more corner cases rather than simplifying the protocol as it was supposed to do? ;-)
>>>
>>
>> Strict reading again depends on whose point of view, client or server...
>>
>> "Once a client has no more layouts on a file, the layout stateid is no
>> longer valid and MUST NOT be used.  Any attempt to use such a layout
>> stateid will result in NFS4ERR_BAD_STATEID."
>
> In NFSv4.1 the server decides about stateids. It's not up to the client
> to throw away the stateid and revert to the initial stateid.
> It must send an appropriate LAYOUTRETURN and get lrs_present==false
> to do that and then it can be sure its layout state for the file is synchronized
> with the server's.
>
> Benny
>

I actually agree that your method is better.  I merely disagree that
the spec as is allows it.  Another quote:

"When a client has no layout on a file, it MUST present an open stateid...".

The problem is that the spec is currently not clear about how the
forgetful model interacts with sending openstateids, particularly with
multiple parallel LAYOUTGETs.  If a server implementor assumes the
client can silently forget its layouts, then later send a
LAYOUTGET(openstateid), which seems to be what the spec currently
says, then we get potential problems that can only be avoided if the
client serializes the LAYOUTGET(openstate) calls.

If you want your behavior, where the client is expected to remember
the layout stateid even after forgetting the layouts, I think an
errata is needed.

Fred


>>
>>
>> Fred
>>
>>> Benny
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