The ZFS file system has been heavily tuned for workloads where file rewrite activity is minimal or is aligned and sized to match ZFS's native record size. Exporting ZFS storage to block consumers, however, presents a situation where every write is rewriting an existing block, and unaligned writes incur a performance killing synchronous read. This paper and talk presents Spectra Logic's optimizations to ZFS's data management layer (DMU) to convert the majority of these synchronous reads to be asynchronous and, for sequential access patterns, to avoid them entirely. We also describe a new scheme that allows concurrent reads to be issued through the DMU without the need to allocate a thread context for each I/O. The result, as implemented and tested using the FreeBSD operating system, is up to a five fold performance increase for unaligned write workloads and a three fold improvement for random read workloads.