=======================================================
128GB, 3.0 Gbps no-name SATA SSD, x86-64, ext3, 2.6.29 vanilla

First dd(1) creates the file, others simply rewrite it.
=======================================================
24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 917.599 s, 27.4 MB/s)

real	15m30.928s
user	0m0.016s
sys	1m3.924s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 1056.92 s, 23.8 MB/s)

real	18m1.686s
user	0m0.016s
sys	1m4.816s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 1044.25 s, 24.1 MB/s)

real	17m37.884s
user	0m0.020s
sys	1m4.300s


writing 2800 buffers of size 8m
21.867 GB written in 645.56 (34 MB/s)

real	10m46.502s
user	0m0.044s
sys	0m35.990s


writing 2800 buffers of size 8m
21.867 GB written in 634.55 (35 MB/s)

real	10m35.448s
user	0m0.036s
sys	0m36.466s


writing 2800 buffers of size 8m
21.867 GB written in 642.00 (34 MB/s)

real	10m42.890s
user	0m0.044s
sys	0m34.930s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 639.49 (35 MB/s)

real	10m40.384s
user	0m0.036s
sys	0m38.582s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 636.17 (35 MB/s)

real	10m37.061s
user	0m0.024s
sys	0m39.146s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 636.07 (35 MB/s)

real	10m37.003s
user	0m0.060s
sys	0m39.174s


=======================================================
500GB, 3.0Gbps Seagate SATA drive, x86-64, ext3, 2.6.29 vanilla

First dd(1) creates the file, others simply rewrite it.
=======================================================
24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 494.797 s, 50.9 MB/s)

real	8m42.680s
user	0m0.016s
sys	0m58.176s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 498.295 s, 50.5 MB/s)

real	8m27.505s
user	0m0.016s
sys	0m58.744s


24000+0 records in
24000+0 records out
25165824000 bytes (25 GB) copied, 492.145 s, 51.1 MB/s)

real	8m23.616s
user	0m0.016s
sys	0m59.064s


writing 2800 buffers of size 8m
21.867 GB written in 478.41 (46 MB/s)

real	7m59.690s
user	0m0.032s
sys	0m33.210s


writing 2800 buffers of size 8m
21.867 GB written in 513.54 (43 MB/s)

real	8m34.461s
user	0m0.048s
sys	0m33.342s


writing 2800 buffers of size 8m
21.867 GB written in 471.38 (47 MB/s)

real	7m52.641s
user	0m0.020s
sys	0m33.486s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 467.67 (47 MB/s)

real	7m48.756s
user	0m0.048s
sys	0m36.838s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 462.69 (48 MB/s)

real	7m43.597s
user	0m0.020s
sys	0m37.462s


using fadvise()
writing 2800 buffers of size 8m
21.867 GB written in 463.56 (48 MB/s)

real	7m44.472s
user	0m0.036s
sys	0m37.342s