I have read that, a few times. I also understand that the SLOG is NOT a write cache. However, its my understanding for my NFS connections (Linux server's are copying their backup) that at some point, the freenas server will write the data down to either the zil on the pool, or an slog, to verify that yes, the data is not just sitting in ram, to ensure it's there if the system has a problem. I realize it will almost never be read from. In our case, because of the limited iops we have (we have several large raidz3 vdevs, because of the size of disks we had) we think that it is slowing down the writes, also having to copy it to the ZIL on the pool. (20 servers all writing over 10Gb/s NFS).
If that is the case, we would like to move the SLOG to a separate SSD, to both get it off the overloaded pool, and to make it a bit faster in general. What we are not sure of, if is the SLOG is the raw data that comes in (ie, uncompressed) in which case, we can plan on about 6 months of service with this SSD, before we hit the 150TB warranty (we would probably setup a mirror for the SLOG, and every 3 months, replace one of the members). If the SLOG is after compression/dedupe, then I can write to the drives for several years. (really, this just changes the replacement schedule, and planned visits to the datacenter)