Subject: RE: [nfsv4] pnfs: lingering writes

From: "Noveck, Dave" <[email-masked]>
Date: Thu, 15 Jun 2006 11:40:07 -0400

Now that I look at it, I'm very doubtful about this paragraph.  Rather
than augment it, I think we should just remove it.

First of all it is not clear to me why "this approach", i.e. rewriting
data that you once wrote asynchronously, to where it supposed to go,
after you find out you can't write it to the original place makes
you any more subject to a lingering write problem (if your transport
protocol has one to begin with).  If your protocol has a lingering write
problem then you have a lingering write problem; get a better protocol,
but nowhere do we propose using a protocol with such a problem as a
transport protocol.

Also the first two paragraphs don't talk about situations subject to the
lingering write problem.  They talk about a situation is which you have 
had async writes acknowledged and async write only apply to file
variant.

But even if you had the lingering write problem, i.e. you did a write,
got no response, and then wrote to the MDS or a different DS, then 
sessions isn't needed to save you.  What saves you is that the storage
management protocol should have told the old DS to ignore writes done
under the old layout and if he is not and going to let the old DS
write data now held by the new DS then he belongs in the Broken pNFS
Implementation Hall of Fame (or Shame), but there isn't anything that
sessions can do about it, since the lingering write may be the first 
instance of that write request that he did.

I guess there is a case for somewhere saying "The surgeon-general has
determined that using transport protocols subject to the lingering 
write problem can cause you grief" so that someone inclined to use
v2/v3/v4.0 as a transport protocol is appropriately warned off, but
I don't think it makes sense in this section.

-----Original Message-----
From: Benny Halevy [mailto:[email-masked]] 
Sent: Thursday, June 15, 2006 10:09 AM
To: Goodson, Garth; 'David Black' (E-mail)
Cc: [email-masked]
Subject: [nfsv4] pnfs: lingering writes

Garth, the last paragraph in section 14.7.4 talks about the "lingering
write" 
problem. Its last sentence says "However, this is solved by sessions in
NFSv4.x"
I'd like to add to the paragraph that the problem is also solved by
iSCSI error 
recovery as of RFC3720.  This is applicable to OSD over iSCSI and I
believe also 
to SCSI block commands over iSCSI.

Benny

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From: Garth Goodson <[email-masked]>
Date: Thu, 15 Jun 2006 14:42:48 -0700

Noveck, Dave wrote:
> Implementation Hall of Fame (or Shame)

I think this is the key.  Perhaps we should replace it with something 
that gets this point across.

-Garth

, but there isn't anything that