Re: NFS client send out of order offsets....

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Fri, 2010-04-23 at 17:56 -0400, Steve Dickson wrote: 
> Hey Trond (and all),
> 
> It been brought to my attention that Linux NFS clients, with big-ish 
> writes (8GB and above), quickly started sending out writes with
> out-of-order offsets... Meaning in the start of an 8GB write a 7GB offset 
> will be send and then smaller offsets, closer to the the beginning 
> of the file, will follow.
> 
> Now I realize this is perfectly fine from a protocol standpoint and (w/out 
> any on hands investigation) pretty sure it has to do with how pages are
> being kicked out of the cache (i.e. memory pressure), meaning is 
> not an NFS issue at all...  but... 
> 
> It was also point out to me that this type of out of order-ness does 
> not happen with Solaris clients and with an AIX clients there is some 
> mount option fix out of order-ness... 
> 
> So my question is there some way to tweak the client to ensure
> offset are send sequential order... 
> 
> Note this happening on a older kernel, so before I go off  and 
> "re-invent the wheel", I was wonder if other people have seen this
> issue and has it been addressed... 
> 
> tia,
> 
> steved.

The NFS layer will always write out pages in order whenever it initiates
a flush.

The problem here is the VM's use of the 'range_cyclic' writeback control
option in functions like 'balance_dirty_pages()' and in the kflush
daemons.
That neither can nor should be fixed in the NFS layer. It really needs
to be addressed in the VM.

Cheers
  Trond