[pnfs] [PATCH 1/1] pnfsd: DLM file layout only support read iomode layouts

[J. Bruce Fields]

On Wed, Dec 16, 2009 at 09:02:52AM +0100, Tigran Mkrtchyan wrote:
> On 12/15/2009 11:41 PM, J. Bruce Fields wrote:
>> On Tue, Dec 15, 2009 at 03:03:54PM -0500, andros at netapp.com wrote:
>>    
>>> From: Andy Adamson<andros at netapp.com>
>>>
>>> In a DLM cluster, writing to a node other than the node where the open call
>>> occurred will have performance implications due to meta data updates.
>>>
>>> Return only LAYOUTIOMODE4_READ layouts so that writes go through the MDS
>>> LAYOUTGET with LAYOUTIOMODE4_RW iomodes will receive the NFS4ERR_BADIOMODE
>>> error.
>>>      
>> Hm.  The 4.1 draft does refer (in 13.8) to using LAYOUTIOMODE4_READ
>> layouts for the read-only data-server case, but doesn't really give a
>> clear error recommendation for it.  BADIOMODE says "An invalid or
>> inappropriate layout iomode was specified".  The client definitely isn't
>> making an *invalid* request.  I don't know what inappropriate means.
>>
>> LAYOUTUNAVAILABLE might be better, though it doesn't mention this
>> particular case: "Returned when layouts are not available for the
>> current file system or the particular specified file."  So a client
>> might reasonably take that to mean that it shouldn't ever ask for that
>> file again.
>>
>>    
> By reading this thread I surprised why not simply return MDS as yet  
> another DS and
> recall layout on close. This is at least what I do.

That sounds doable to.

Simplest would be if there was a way for ths erver to just say that it's
not worth asking for RW layouts on a given filesystem, but I don't see
one.

--b.