Re: [NFS] blocks of zeros (NULLs) in NFS files in kernels >= 2.6.20

["Chuck Lever" <chuck.lever@oracle.com>]

On Mon, Sep 22, 2008 at 1:04 PM, Aaron Straus <aaron@merfinllc.com> wrote:
> Hi,
>
> On Sep 22 12:35 PM, Trond Myklebust wrote:
>> Revert _what_ exactly?
>
> Yep.  I narrowed the problem down to an offending hunk in a particular
> patch.  Removing that hunk did eliminate the problem.   However,
> reverting that hunk is likely wrong and the code has changed _a lot_
> since that commit.
>
>> My understanding was that this is a consequence of unordered writes
>> causing the file to be extended while some other task is reading.
>
> Yes.  I added some debugging statements to look at the writeout path.
>
> I think the following happens:
>
>  - page 0 gets partially written to by app
>  - VM writes out partial dirty page 0
>  - page 0 gets fully written by app
>  - page 1 gets partially written by app
>  - VM writes out dirty page 1
>
>  At this point there is a hole in the file.  The tail end of page 0 is
> still not written to server.
>
>  - VM writes out dirty page 0
>  ...
>
>> AFAICS, this sort of behaviour has _always_ been possible. I can't see
>> how reverting anything will fix it.
>
> Here is the crux.  It was possible previously but unlikely e.g. our app
> never saw this behavior.  The new writeout semantics causes visible
> holes in files often.
>
> Anyway, I agree the new writeout semantics are allowed and possibly
> saner than the previous writeout path.  The problem is that it is
> __annoying__ for this use case (log files).
>
> I'm not sure if there is an easy solution.  We want the VM to writeout
> the address space in order.   Maybe we can start the scan for dirty
> pages at the last page we wrote out i.e. page 0 in the example above?

Why can't you use O_SYNC | O_APPEND ?

--
Chuck Lever