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

[Steve Dickson <SteveD@redhat.com>]

On 04/23/2010 06:09 PM, Trond Myklebust wrote:
> 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.
> 
Understood... completely.... 

But I wonder if there some vm tweak (via sysctl) that would
give a better chance of having things go out in order...

steved.