Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 19, 2011 at 09:58:15AM +0800, Yongqiang Yang wrote:
> On Mon, Apr 18, 2011 at 10:45 AM, Andreas Dilger <adilger@dilger.ca> wrote:
> > On 2011-04-17, at 6:40 PM, Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Sat, Apr 16, 2011 at 08:21:28AM -0400, Theodore Tso wrote:
> >
> > On Apr 16, 2011, at 1:11 AM, Andreas Dilger wrote:
> >
> > In that case, it means cp should just always use FIEMAP_FLAG_SYNC, which is
> > fine.
> >
> > Except that if someone is copying a large delay allocated file, it will
> > cause
> >
> > the file to immediately snapped to disk, which might not be the greatest
> >
> > thing in the world.
> >
> > Obvious workaround - if the initial fiemap call shows unwritten
> > extents, redo it with the sync flag set. Though that assumeѕ that
> > you can trust things like delalloc extents to only cover the range
> > that valid data exists in. Which, of course, you can't assume,
> > either. :/
> >
> > Always passing FIEMAP_FLAG_SYNC is fine in this case. It should only do
> > anything if there is unwritten data, which is the only case we are concerned
> > with at this point.  In any case, this is a simple solution for coreutils
> > until such a time that a more complex solution is added in the kernel (if
> > ever).
> >
> > Christoph is write, SEEK_HOLE and SEEK_DATA are
> >
> > a much better API for what cp woulld lke to do.  Unfortunately it hasn't
> >
> > been implemented yet in the VFS...
> >
> > Agreed, SEEK_HOLE/SEEK_DATA is the right way to solve this problem.
> >
> > I don't see how this will change the problem in any meaningful way. There
> > will still need to be code that is traversing the on-disk mapping, and also
> > keeping it coherent with unwritten data in the page cache.
> 
> It seems that we are being messed up by page cache and disk.
> Unwritten flag returned from FIEMAP indicates blocks on disk are not
> written, but it does not say if there is data in page cache.  So
> FIEMAP itself just tells user the map on disk.  However there is an
> exception for delayed allocation,  FIEMAP tells users the data is in
> page cache.

No, FIEMAP does not tell the user there is data in the page cache.
It tells there user there is a delayed allocation extent. For XFS, a
delayed allocation extent can cover a range _greater_ than there is
data in the page cache - we do allocation allignment, speculative
allocation and other tricks to avoid fragmentation via
delayed allocation. When XFSs says there is a delalloc extent, it is
simply showing the in-memory representation of the extent. if you
want to know where the data in the page cache actually is, you need
to sync the file to disk to get those ranges converted to real
extents. This is how xfs_bmap has worked for 15 years....

> Maybe FIEMAP should return all known messages for unwritten extent, if
> unwritten data exists in page cache, FIEMAP should let users know that
> data is in page cache and space on disk has been preallocated, but
> data has not been flushed into disk.  Actually, delayed allocation has
> done like this. Then user-space applications can determine how to do.
> Taking cp as an example, it will copy from page cache rather ignore
> it.

Once again, FIEMAP is for showing the filesystem's current extent
state, not the page cache state. Ext4 may implement FIEMAP by doing
page cache walks, but that is a filesystem specific implementation
detail.

> We need a definite definition for FIEMAP, in other words, it tells
> users map on disk or both disk and page cache.

We already have a definition - and it has nothing to do with the
page cache state.

> If the former one is taken, then FIEMAP should not consider
> delayed allocation.

Not at all. the delayed allocation extent is a first class extent
type in XFS and it is reported directly from the extent list. Your
viewpoint is very ext4-specific and ignores the fact that other
filesystems were doing this sort of mapping long before even ext3
(let alone ext4) was a glint in the designer's eye....

> otherwise, FIEMAP should return all known messages for unwritten case
> like delayed allocation.

See my previous comments about extents being unwritten until data is
physically written to them.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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