Comment 54 for bug 1023755

I tried this on real hardware and it appears to work although it is abysmally slow:

vishvananda@nebstack006:~$ echo "0 2097152 zero" | sudo dmsetup create zero1
vishvananda@nebstack006:~$ echo "0 2097152 snapshot /dev/mapper/zero1 /dev/sde n 128" | sudo dmsetup create clean1
vishvananda@nebstack006:~$ sudo dd if=/dev/zero of=/dev/mapper/clean1 bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 15.1229 s, 71.0 MB/s
vishvananda@nebstack006:~$ time sudo dd if=/dev/zero of=/dev/mapper/clean1 bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 8.03134 s, 134 MB/s

real 0m8.041s
user 0m0.004s
sys 0m1.208s
vishvananda@nebstack006:~$ time sudo dd if=/dev/zero of=/dev/sde bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.666776 s, 1.6 GB/s

real 0m0.677s
user 0m0.000s
sys 0m0.668s
vishvananda@nebstack006:~$ cat /sys/block/sde/queue/scheduler
noop deadline [cfq]
vishvananda@nebstack006:~$ echo deadline | sudo tee /sys/block/sde/queue/scheduler
deadline
vishvananda@nebstack006:~$ cat /sys/block/sde/queue/scheduler
noop [deadline] cfq
vishvananda@nebstack006:~$ time sudo dd if=/dev/zero of=/dev/sde bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.503988 s, 2.1 GB/s

real 0m0.513s
user 0m0.004s
sys 0m0.504s
vishvananda@nebstack006:~$ time sudo dd if=/dev/zero of=/dev/mapper/clean1 bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 11.316 s, 94.9 MB/s

real 0m11.327s
user 0m0.000s
sys 0m1.184s

going to try the same on quantal to see how it looks.