fiemap issues with XFS

[Pádraig Brady]

Hi Markus,

I noticed your fiemap issues here:
http://oss.sgi.com/pipermail/xfs/2011-April/050102.html

FAIL: cp/fiemap-empty (exit: 1)
===============================
...
+ fallocate -l 10MiB -n unwritten.withdata
+ dd count=10 if=/dev/urandom conv=notrunc iflag=fullblock
of=unwritten.withdata
10+0 records in
10+0 records out
5120 bytes (5.1 kB) copied, 0.00219578 s, 2.3 MB/s
+ cp unwritten.withdata cp.test
++ stat -c %s unwritten.withdata
++ stat -c %s cp.test
+ test 5120 = 5120
+ cmp unwritten.withdata cp.test
unwritten.withdata cp.test differ: char 1, line 1
+ fail=1

cp was changed in 8.11 to not bother reading
an extent if it is marked as UNWRITTEN.
The comment in fiemap.h says that this means that the
space is allocated, but zero.

We tested on XFS, on F15 x86_64, which is earlier
than your 2.6.39-rc3 and didn't notice this issue.

I'm guessing so that XFS is reporting the extent
as UNWRITTEN, even though there is data in it now,
and that it might sort itself out after a while,
or after a sync I suppose (note we also stopped
using sync before fiemap for 2.6.39).

It would help a lot if you could insert this command
into the test above (just before the failing cp)
and show the test output again:

  filefrag -v unwritten.withdata

thanks,
Pádraig.