Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb_nr

[Jan Kara <jack@suse.cz>]

On Wed 29-06-11 16:12:34, Christoph Hellwig wrote:
> On Wed, Jun 29, 2011 at 09:15:18PM +0200, Jan Kara wrote:
> >   True but writeback_single_inode() has in it:
> >         if (wbc->sync_mode == WB_SYNC_ALL) {
> >                 int err = filemap_fdatawait(mapping);
> >                 if (ret == 0)
> >                         ret = err;
> >         }
> >   So we end up waiting much earlier. Probably we should remove this wait
> > but that will need some audit I guess.
> 
> Uhh, indeed.  We'll need this wait for things like sync_inode, though.
  Yes. Actually, specifically for filesystems like XFS which update inode
after IO completion we would need more passes to be efficient and correct:
  for all inodes
    fdatawrite
  for all inodes
    fdatawait
  for all inodes
    write_inode
  for all inodes
    wait for inode IO

  But maybe this could be handled by having new WB_SYNC_ mode indicating
that writeback_single_inode() should not bother waiting (thus we'd really
end up waiting in sync_inodes_sb()) and then XFS and other filesystems that
need it would writeout inodes in their sync_fs() implementation (possibly
using a generic helper)?
								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR