2.6.32-xen-r1:

-- (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, 550,039 s, 74,5 kB/s

real	9m10.046s
user	0m0.005s
sys	0m0.133s

$ 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, 41,5227 s, 12,6 MB/s

real	0m41.544s
user	0m0.177s
sys	0m2.993s

$ 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, 12,1582 s, 43,1 MB/s

real	0m12.165s
user	0m0.024s
sys	0m0.241s

**************************************************************************
-- (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, 302,612 s, 135 kB/s

real	5m2.619s
user	0m0.003s
sys	0m0.117s

$ 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, 30,9351 s, 16,9 MB/s

real	0m30.973s
user	0m0.159s
sys	0m2.274s

$ 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, 4,80787 s, 109 MB/s

real	0m4.993s
user	0m0.042s
sys	0m0.509s