Re: [PATCH 0/6] writeback time order/delay fixes take 3

David Chinner
2007-08-28T16:33:36Z

On Tue, Aug 28, 2007 at 11:08:20AM -0400, Chris Mason wrote:
> On Wed, 29 Aug 2007 00:55:30 +1000
> David Chinner <dgc@sgi.com> wrote:
> > On Fri, Aug 24, 2007 at 09:55:04PM +0800, Fengguang Wu wrote:
> > > On Thu, Aug 23, 2007 at 12:33:06PM +1000, David Chinner wrote:
> > > > On Wed, Aug 22, 2007 at 09:18:41AM +0800, Fengguang Wu wrote:
> > > > > On Tue, Aug 21, 2007 at 08:23:14PM -0400, Chris Mason wrote:
> > > > > Notes:
> > > > > (1) I'm not sure inode number is correlated to disk location in
> > > > > filesystems other than ext2/3/4. Or parent dir?
> > > >
> > > > The correspond to the exact location on disk on XFS. But, XFS has
> > > > it's own inode clustering (see xfs_iflush) and it can't be moved
> > > > up into the generic layers because of locking and integration into
> > > > the transaction subsystem.
> > > >
> > > > > (2) It duplicates some function of elevators. Why is it
> > > > > necessary?
> > > >
> > > > The elevators have no clue as to how the filesystem might treat
> > > > adjacent inodes. In XFS, inode clustering is a fundamental
> > > > feature of the inode reading and writing and that is something no
> > > > elevator can hope to acheive....
> > >
> > > Thank you. That explains the linear write curve(perfect!) in Chris'
> > > graph.
> > >
> > > I wonder if XFS can benefit any more from the general writeback
> > > clustering. How large would be a typical XFS cluster?
> >
> > Depends on inode size. typically they are 8k in size, so anything
> > from 4-32 inodes. The inode writeback clustering is pretty tightly
> > integrated into the transaction subsystem and has some intricate
> > locking, so it's not likely to be easy (or perhaps even possible) to
> > make it more generic.
>
> When I talked to hch about this, he said the order file data pages got
> written in XFS was still dictated by the order the higher layers sent
> things down.

Sure, that's file data. I was talking about the inode writeback, not the
data writeback.

> Shouldn't the clustering still help to have delalloc done
> in inode order instead of in whatever random order pdflush sends things
> down now?

Depends on how things are being allocated. if you've got inode32 allocation
and >1TB filesytsem, then data is nowhere near the inodes. If you've got large
allocation groups, then data is typically nowhere near the inodes, either. If
you've got full AGs, data will be nowehere near the inodes. If you've got
large files and lots of data to write, then clustering multiple files together
for writing is not needed. So in many cases, clustering delalloc writes by
inode number doesn't provide any better I/o patterns than not clustering...

The only difference we may see is that if we flush all the data on inodes
in a single cluster, we can get away with a single inode cluster write
for all of the inodes....

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group