Re: nfsd: supports read buffer from multiples pages

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Feb 02, 2016 at 10:20:54AM +0100, Christoph Hellwig wrote:
> On Mon, Feb 01, 2016 at 01:38:05PM -0500, J. Bruce Fields wrote:
> > This is with the xfs block layout?
> > 
> > Christoph, do we know anything about average or worst-case sizes for
> > that layout update field?
> 
> The average is rather small and fits into a single page, the worst
> case is basically unlimited:
> 
> 	(file size / block size) * sizeof(pnfs_block_extent)
> 
> by the protocol, and about half that for a non-stupid client as
> it would merge consecutive blocks and only trigger something close
> to the worst case for a "block allocated, block hole, block allocated, ..."
> pattern.

OK.  And what's the failure mode if the layoutcommit fails?  I guess the
client returns the layout and redoes IO through the MDS.  For a rare
failure maybe that's not so terrible.

So I guess the right thing to do is take Kinglong's patch for now.

After that, it wouldn't be that hard for nfsd4_block_proc_layoutcommit
to decode from an array of pages.  But does the iomaps array end up
being just as big?

--b.