Michael Dexter returns with another piece on ZFS, this one titled The ZFS ZIL and SLOG Demystified on the FreeNAS website. The article clarifies the usage of ZIL (ZFS Intent Log) and SLOG (Separate Intent Log) on your FreeNAS or FreeBSD pools.
The ZIL and SLOG are two of the most misunderstood concepts in ZFS and hopefully this will clear things up
As you surely know by now, ZFS is taking extensive measures to safeguard your data and it should be no surprise that these two buzzwords represent key data safeguards. What is not obvious however is that they only come into play under very specific circumstances.
The first thing to understand is that ZFS behaves like any other file system with regard to asynchronous and synchronous writes…
Check out his full article here: http://www.freenas.org/whats-new/2015/11/zfs-zil-and-slog-demystified.html