From: "Paul Smith" <pausmith@nortelnetworks.com>
Subject: NFS client write performance issue ... thoughts?
Date: Mon, 5 Jan 2004 17:11:11 -0500
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Hi all; we've been doing some examination of NFS client performance, and
have seen this apparently sub-optimal behavior: anyone here have any
comments on this observation or thoughts about it?

Thanks!

> I was looking at the code (which doesn't appear to be fixed in 2.6),
> and here's what I think it's doing.
> 
> The Linux NFS client code is capable of only remembering a write to a
> single contiguous chunk in each 4KB page of a file.  If a second
> non-contiguous write occurs to the page, the first write has to be
> flushed to the server.  So if the view server is seeking around in a
> file writing a few bytes here and a few there, whenever it does a
> write to a page that has already been written, the first write has to
> be flushed to the server.  The code that does most of this is in
> fs/nfs/write.c, function nfs_update_request().  That's the routine
> that, given a new write request, searches for an existing one that is
> contiguous with the new one.  If it finds a contiguous request, the
> new one is coalesced with it and no NFS activitity is required.  If
> instead it finds a pending write to the same page, it returns EBUSY to
> its caller, which tells the caller (nfs_updatepage()) to synchronously
> write the existing request to the server.
> 
> The Solaris NFS client (and probably most other NFS client
> implementations) doesn't work this way.  Whenever a small write is
> made to a block of a file, the block is read from the server, and then
> the write is applied to the cached block.  If the block is already in
> cache, the write is applied to block without any NFS transactions
> being required.  When the file is closed or fsync()ed, the entire
> block is written to the server.  So the client code doesn't need to
> record each individual small write to a block, it just modifies the
> cached block as necessary.
> 
> I don't know why the Linux NFS client doesn't work this way, but I
> think it wouldn't be difficult to make it work that way.  In some
> scenarios, this might be a performance hit (because an entire block of
> the file has to be read from the server just to write a few bytes to
> it), but I think that in most cases, doing it the Solaris way would be
> a performance win.  I'd guess that if any sort of database, such as a
> GDBM database, is accessed over NFS, the Linux method would result in
> much poorer performance than the Solaris method.


-- 
-------------------------------------------------------------------------------
 Paul D. Smith <psmith@nortelnetworks.com>   HASMAT: HA Software Mthds & Tools
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist
-------------------------------------------------------------------------------
   These are my opinions---Nortel Networks takes no responsibility for them.


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