ARC basically is a read cache. ZIL improves performance of writes, but by itself is almost never read (only written to) except on failures. It will be read for

ARC basically is a read cache. ZIL improves performance of writes, but by itself is almost never read (only written to) except on failures. It will be read for example on power failure to finish writing the data to the disk. It is used to speed up synchronous writes.