sdb1 is aligned to 1MiB
/tmp is in RAM (tmpfs)

# mkfs.ext4 /dev/sdb1
# mount /dev/sdb1 /mnt/test -o noatime,barrier,discard
# cd /mnt/test
# tar -xf /tmp/linux-3.2-rc5.tar
# sync; time rm -rf linux-3.2-rc5/; time sync
real    0m0.436s
user    0m0.024s
sys     0m0.392s

real    0m0.415s
user    0m0.000s
sys     0m0.004s

The discards happen during sync:

Dec 11 18:15:31 charonlocal kernel: [  258.444950] DISCARD: 4392 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.449151] DISCARD: 1264 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.458871] DISCARD: 52632 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.477563] DISCARD: 4504 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.479179] DISCARD: 97112 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.504913] DISCARD: 262144 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.554922] DISCARD: 128 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.558750] DISCARD: 80696 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.579996] DISCARD: 888 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.588856] DISCARD: 4424 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.599219] DISCARD: 54960 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.618127] DISCARD: 136 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.619417] DISCARD: 65536 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.639901] DISCARD: 4392 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.649227] DISCARD: 1328 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.658912] DISCARD: 336 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.668754] DISCARD: 80 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.678726] DISCARD: 216 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.688739] DISCARD: 65536 sectors
Dec 11 18:15:31 charonlocal kernel: [  258.710083] DISCARD: 262144 sectors
Dec 11 18:15:32 charonlocal kernel: [  258.764941] DISCARD: 344 sectors
Dec 11 18:15:32 charonlocal kernel: [  258.768790] DISCARD: 69336 sectors
Dec 11 18:15:32 charonlocal kernel: [  258.791369] DISCARD: 184 sectors
Dec 11 18:15:32 charonlocal kernel: [  258.798756] DISCARD: 152 sectors