We use async also, sync is way too slow but we really need sync (reliable). Did you also try without DRBD? We had same issue without DRBD running on another volume.
We are seeing the exact same issue with enterprise/pvetest and no-subscription, all current kernels. At last we changed to using async to get it perform but it's not what we want. Any ideas?
SAN runs on debian squeezewith DRDB