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

Yongqiang Yang
2011-04-19T08:08:13Z

On Tue, Apr 19, 2011 at 3:45 PM, Dave Chinner <david@fromorbit.com> wrote:
> On Tue, Apr 19, 2011 at 02:53:20PM +0800, Yongqiang Yang wrote:
>> On Tue, Apr 19, 2011 at 11:44 AM, Dave Chinner <david@fromorbit.com> wrote:
>> > 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.
>> Understood, thank you for your explanation.
>>
>> Ok.  Let's look at it from a higher view.  What you described about
>> extent state is specific to xfs.
>>
>> I think there are 2 ways to provide a definite definition for FIEMAP
>> for all filesystems:
>>
>> 1. FIEMAP returns extent state on disk.
>> 2. FIEMAP returns extent both in memory and on disk.
>
> You are *not listening*. There is no #2. FIEMAP returns the extent
> state _on disk_ at the time of the call. If you want it to reflect
> the in-memory state at the time of the call (for data or metadata),
> you *must* use the the SYNC flag to convert that in-memory state to
> on-disk state, which FIEMAP then reports just fine.

Sorry for being dense.

I think delayed extent is an exception. because it is not on the disk.

Yongqiang.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
>

--
Best Wishes
Yongqiang Yang