2.6.29-xen-r4:

-- (guest's root partition, ext3, another hard-drive) --

$ time dd oflag=dsync if=/dev/zero of=TESTFILE bs=4k count=10000

10000+0 records in
10000+0 records out
40960000 bytes (41 MB) copied, 477,071 s, 85,9 kB/s

real	7m57.077s
user	0m0.004s
sys	0m0.130s

$ time dd conv=fdatasync if=/dev/zero of=TESTFILE bs=4k count=128000 
128000+0 records in
128000+0 records out
524288000 bytes (524 MB) copied, 28,7759 s, 18,2 MB/s

real	0m28.782s
user	0m0.190s
sys	0m4.361s

$ time dd if=/dev/xvda1 of=/dev/null bs=4k count=128000
128000+0 records in
128000+0 records out
524288000 bytes (524 MB) copied, 11,943 s, 43,9 MB/s

real	0m11.951s
user	0m0.014s
sys	0m0.329s

**************************************************************************
-- (storage hard drive, the one fully formatted with XFS) --

$ time dd oflag=dsync if=/dev/zero of=TESTFILE bs=4k count=10000
10000+0 records in
10000+0 records out
40960000 bytes (41 MB) copied, 431,506 s, 94,9 kB/s

real	7m11.548s
user	0m0.002s
sys	0m0.274s

$ time dd conv=fdatasync if=/dev/zero of=TESTFILE bs=4k count=128000
128000+0 records in
128000+0 records out
524288000 bytes (524 MB) copied, 21,6901 s, 24,2 MB/s

real	0m21.697s
user	0m0.186s
sys	0m3.140s

$ time dd if=/dev/xvdb of=/dev/null bs=4k count=128000
128000+0 records in
128000+0 records out
524288000 bytes (524 MB) copied, 4,884 s, 107 MB/s

real	0m4.934s
user	0m0.059s
sys	0m0.650s