$ mcr sysgen help param acp_extcache
ACP_EXTCACHE sets the number of entries in the extent cache.
Each entry points to one contiguous area of free space on disk.
A specification of 0 means no cache. Too small a value causes
excessive XQP I/O operations, while too large a value causes
excessive physical memory to be consumed by the extent cache.
Also see Kirby McCoy's File System Internals book.