By default, zFS does not fix pages in any of the caches except when an I/O is pending to or from the cache buffers. The administrator can permanently page fix the user file cache, the metadata cache, metaback_cache_size, and the log file cache by choosing the fixed option for the cache. This option ensures that the cache experiences no paging and avoids page fixing for each I/O. This option does come at the expense of using real storage for the cache, which means the real storage is not available for other applications.

If your file system performance is critical and you have enough real memory to support it, the fixed option can be useful. Otherwise, do not set it.