What if you have ZFS deployed on SAN in a clustered environment and you require a dedicated SLOG? It would be really helpful if you could create a small LUN (2-4GB) directly out of disk array's cache. This would be perfect for SLOG. All reads and writes to such LUN would be serviced entirely from array's cache - meaning low latency and no double-writes to backend disks/SSDs for synchronous I/O. Some disk arrays actually do provide such feature, for example see Hitachi's Cache Residency Manager.