Date: Tue, 7 Jan 2014 09:56:34 -0500
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Anna Schumaker <Anna.Schumaker@netapp.com>
Cc: Trond.Myklebust@primarydata.com, linux-nfs@vger.kernel.org
Subject: Re: [PATCH 0/3] READ_PLUS rough draft
Message-ID: <20140107145633.GC3342@fieldses.org>
References: <1389045433-22990-1-git-send-email-Anna.Schumaker@netapp.com>
 <20140106223201.GA3342@fieldses.org>
 <52CC123C.7000806@netapp.com>
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On Tue, Jan 07, 2014 at 09:42:04AM -0500, Anna Schumaker wrote:
> On 01/06/2014 05:32 PM, J. Bruce Fields wrote:
> > On Mon, Jan 06, 2014 at 04:57:10PM -0500, Anna Schumaker wrote:
> >> These patches are my initial implementation of READ_PLUS.  I still have a
> >> few issues to work out before they can be applied, but I wanted to submit
> >> them anyway to get feedback before going much further.  These patches were
> >> developed on top of my earlier SEEK and WRITE_PLUS patches, and probably
> >> won't apply cleanly without them (I am willing to reorder things if necessary!).
> >>
> >> On the server side, I handle the cases where a file is 100% hole, 100% data
> >> or hole followed by data.  Any holes after a data segment will be expanded
> >> to zeros on the wire.
> > 
> > I assume that for "a file" I should read "the requested range of the
> > file"?
> 
> Yes.
> 
> > 
> > hole+data+hole should also be doable, shouldn't it?  I'd think the real
> > problem would be multiple data extents.
> 
> It might be, but I haven't tried it yet.  I can soon!
> 
> > 
> >> This is due to a limitation in the the NFSD
> >> encode-to-page function that will adjust pointers to point to the xdr tail
> >> after reading a file to the "pages" section.  Bruce, do you have any
> >> suggestions here?
> > 
> > The server xdr encoding needs a rewrite.  I'll see if I can ignore you
> > all and put my head down and get a version of that posted this week.
> 
> :)
> 
> > 
> > That should make it easier to return all the data, though it will turn
> > off zero-copy in the case of multiple data extents.
> > 
> > If we want READ_PLUS to support zero copy in the case of multiple
> > extents then I think we need a new data structure to represent the
> > resulting rpc reply.  An xdr buf only knows how to insert one array of
> > pages in the middle of the data.  Maybe a list of xdr bufs?
> > 
> > But that's an annoying job and possibly a premature optimization.
> > 
> > It might be useful to first understand the typical distribution of holes
> > in a file and how likely various workloads are to produce reads with
> > multiple holes in the middle.
> 
> I already have a few performance numbers, but nothing that can be trusted due to the number of debugging printk()s I used to make sure the client decoded everything correctly.  My plan is to collect the following information using:  v4.0, v4.1, v4.2 (SEEK), v4.2 (SEEK + WRITE_PLUS), and v4.2 (SEEK + WRITE_PLUS + READ_PLUS).

What's the workload and hardware setup?

--b.