Untangling Block Allocation Policies of ZFS

The ZFS file system from Sun is the latest buzz word in the file system community. The creators of ZFS claim to have re-designed the file system from scratch, providing new features and levels of reliability, performance and efficiency uncommon in traditional file systems. This includes dynamic block allocation that changes the blocks sizes based on workloads. In this paper we have primarily focused on the Block allocation policy of ZFS under varied workloads. We have built our infrastructure based on semantic block analysis and found that ZFS allocates block based on the file offset that are being written and not based on the workload. Block allocation policy works poorly for random writes. We also found that ZFS merges smaller blocks to one big block and as a result a single block write gets converted to read-modify-write of a bigger block. ZFS Intent log also has a poor block allocation policy and for small block writes it wastes a significant amount of storage space in the file system. Overall we find that ZFS tells more that it actually does.