Every change to zFS file system metadata is bounded by a transaction that describes its changes by using records written to the log file. The transaction cache is a cache of data structures representing transactions.

The transaction cache is stored in the zFS primary address space with a default of 2000 transactions. zFS dynamically increases the size of this cache based on the number of concurrent pending transactions (transactions that have not been fully committed to disk) in the zFS file system. Therefore, the administrator does not need to tailor the transaction cache size. However, the modify zfs,query,all output shows the transaction count at any given time.