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

[Trond Myklebust]

On Mon, 2008-09-22 at 10:04 -0700, Aaron Straus wrote:
> 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).

There is always the option of using syslog.

> 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?

You can never guarantee that in a multi-threaded environment.

Two threads may, for instance, force 2 competing fsync() calls: that
again may cause out-of-order writes.
...and even if the client doesn't reorder the writes, the _server_ may
do it, since multiple nfsd threads may race when processing writes to
the same file.

Anyway, the patch to force a single threaded nfs client to write out the
data in order is trivial. See attachment...

Trond
> --- Begin Message ---
> Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
> ---
>
>  fs/nfs/write.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index 3229e21..eb6b211 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -1428,7 +1428,8 @@ static int nfs_write_mapping(struct address_space *mapping, int how)
>  		.sync_mode = WB_SYNC_NONE,
>  		.nr_to_write = LONG_MAX,
>  		.for_writepages = 1,
> -		.range_cyclic = 1,
> +		.range_start = 0,
> +		.range_end = LLONG_MAX,
>  	};
>  	int ret;
>  
>
> --- End Message ---