What is ZPL and DMU ?
The ZPL (ZFS POSIX Layer) runs at a high level, taking instruction from the OS on I/O requests. Below that is the DMU (Data Management Unit) that takes those instructions and translates them into transaction batches. Rather than requesting data blocks and sending single write requests, ZFS batches these into object-based transactions that can be optimized before any disk activity occurs. Once this is done, the batches are handed off to the SPA (Storage Pool Allocator) to schedule and aggregate the raw I/O. The copy-on-write basis of I/O transactions, coupled with checksums performed on a per block basis, precludes the need for journaling. An abrupt power loss will be recoverable at any point.
