Re: [PATCH] Improve buffered streaming write ordering

[Chuck Lever]

On Oct 7, 2008, at Oct 7, 2008, 9:55 AM, Peter Staubach wrote:

> Aneesh Kumar K.V wrote:
> > On Tue, Oct 07, 2008 at 05:05:54AM -0400, Christoph Hellwig wrote:
> >
> > > On Tue, Oct 07, 2008 at 02:15:31PM +0530, Aneesh Kumar K.V wrote:
> > >
> > > > +static int ext4_write_cache_pages(struct address_space *mapping,
> > > > +		      struct writeback_control *wbc, writepage_t writepage,
> > > > +		      void *data)
> > > > +{
> > > Looking at this functions the only difference is killing the
> > > writeback_index and range_start updates.  If they are bad why  
> > > would we
> > > only remove them from ext4?
> >
> > I am also not updating wbc->nr_to_write.
> >
> > ext4 delayed allocation writeback is bit tricky. It does
> >
> > a) Look at the dirty pages and build an in memory extent of  
> > contiguous
> > logical file blocks. If we use writecache_pages to do that it will
> > update nr_to_write, writeback_index etc during this stage.
> >
> > b) Request the block allocator for 'x' blocks. We get the value x  
> > from
> > step a.
> >
> > c) block allocator may return less than 'x' contiguous block. That  
> > would
> > mean the variables updated by write_cache_pages need to corrected.  
> > The
> > old code was doing that. Chris Mason suggested it would make it easy
> > to use a write_cache_pages which doesn't update the variable for  
> > ext4.
> >
> > I don't think other filesystem have this requirement.
>
> The NFS client can benefit from only writing pages in strictly
> ascending offset order.  The benefit comes from helping the
> server to do better allocations by not sending file data to the
> server in random order.

For the record, it would also help prevent the creation of temporary  
holes in O_APPEND files.

If an NFS client writes the front and back ends of a request before it  
writes the middle, other clients will see a temporary hole in that  
file.  Applications (especially simple ones like "tail") are often not  
prepared for the appearance of such holes.

Over a client crash, data integrity would improve if the client was  
less likely to create temporary holes in files.

> There is also an NFS server in the market which requires data
> to be sent in strict ascending offset order.  This sort of
> support would make interoperating with that server much easier.

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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