Re: [PATCH] NFSv4: Use exponential backoff delay for NFS4_ERRDELAY

[Chuck Lever]

On Apr 25, 2013, at 2:19 PM, "bfields@xxxxxxxxxxxx" <bfields@xxxxxxxxxxxx> wrote:

> On Thu, Apr 25, 2013 at 02:10:36PM +0000, Myklebust, Trond wrote:
>> On Thu, 2013-04-25 at 09:49 -0400, bfields@xxxxxxxxxxxx wrote:
>>> On Thu, Apr 25, 2013 at 01:30:58PM +0000, Myklebust, Trond wrote:
>>>> On Thu, 2013-04-25 at 09:29 -0400, bfields@xxxxxxxxxxxx wrote:
>>>> 
>>>>> My position is that we simply have no idea what order of magnitude even
>>>>> delay should be.  And that in such a situation exponential backoff such
>>>>> as implemented in the synchronous case seems the reasonable default as
>>>>> it guarantees at worst doubling the delay while still bounding the
>>>>> long-term average frequency of retries.
>>>> 
>>>> So we start with a 15 second delay, and then go to 60 seconds?
>>> 
>>> I agree that a server should normally be doing the wait on its own if
>>> the wait would be on the order of an rpc round trip.
>>> 
>>> So I'd be inclined to start with a delay that was an order of magnitude
>>> or two more than a round trip.
>>> 
>>> And I'd expect NFS isn't common on networks with 1-second latencies.
>>> 
>>> So the 1/10 second we're using in the synchronous case sounds closer to
>>> the right ballpark to me.
>> 
>> OK, then. Now all I need is actual motivation for changing the existing
>> code other than handwaving arguments about "polling is better than flat
>> waits".
>> What actual use cases are impacting us now, other than the AIX design
>> decision to force CLOSE to retry at least once before succeeding?
> 
> Nah, I've got nothing, and I agree that the AIX problem is there bug.
> 
> Just for fun I looked at re-checked the Linux server cases.  As far as I
> can tell they are:
> 
> 	- delegations: returned immediately on detection of any
> 	  conflict.  The current behavior in the sync case looks
> 	  reasonable to me.
> 	- allocation failures: not really sure it's the best error, but
> 	  it seems to be all the protocol offers.  We probably don't
> 	  care much what the client does in this case.
> 	- some rare cases that would probably indicate bugs (e.g.,
> 	  attempting to destroy a client while other rpc's from that
> 	  client are running.)  Again we don't care what the client does
> 	  here.
> 	- the 4.1 slot-inuse case.
> 
> We also by default map four errors (ETIMEDOUT, EAGAIN, EWOULDBLOCK,
> ENOMEM) to delay.  I thought I remembered one of those being used by
> some HFS system, but can't actually find an example now.  A quick grep
> doesn't show anything interesting.

It's worth mentioning that servers that have frozen state (say, in preparation for Transparent State Migration) may use NFS4ERR_DELAY to prevent clients from modifying open or lock state until that state has transitioned to a destination server.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com




--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/