Re: Uninitialized extent races

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Fri, Dec 21, 2012 at 01:02:43PM -0500, Theodore Ts'o wrote:
> On Fri, Dec 21, 2012 at 05:19:29PM +0100, Jan Kara wrote:
> >   No, I'm speaking about merging currently uninitialized extents. I.e.
> > suppose someone does the following on a filesystem with dioread_nolock so
> > that writeback happens via unwritten extents:
> >   fd = open("file", O_RDWR);
> >   pwrite(fd, buf, 4096, 0);
> > 					flusher thread starts writing
> > 					we create uninitialized extent for
> > 					  range 0-4096
> >   fallocate(fd, 0, 4096, 4096);
> >     - we merge extents and now have just 1 uninitialized extent for range
> >       0-8192
> > 					ext4_convert_unwritten_extents() now
> > 					  has to split the extent to finish
> > 					  the IO.
> 
> Ah, I see.  Disabling the the merging that might take place as a
> result of the fallocate.  Yes, I agree that's a completely sane thing
> to do.
> 
> The alternate approach would be to add a flag in the extent status
> tree indicating that an unwritten conversion is pending, but that
> would add more complexity.

Hi Ted and Jan,

Sorry for delay reply.  Indeed we could add a flag in extent status tree
to indicate an pending unwritten extent, and I believe that it can bring
us some benefits.  But I wonder whether this case often happens.  Do we
have some real workloads?

Regards,
                                        - Zheng