$ man zfsconcepts
Deduplication
Deduplication is the process for removing redundant data at the block level, reducing the total amount of data stored. If a file system has the dedup property enabled, duplicate
data blocks are removed synchronously. The result is that only unique data is stored and common components are shared among files.
Deduplicating data is a very resource-intensive operation. It is generally recommended that you have at least 1.25 GiB of RAM per 1 TiB of storage when you enable deduplication.
Calculating the exact requirement depends heavily on the type of data stored in the pool.
Enabling deduplication on an improperly-designed system can result in performance issues (slow IO and administrative operations). It can potentially lead to problems importing a
pool due to memory exhaustion. Deduplication can consume significant processing power (CPU) and memory as well as generate additional disk IO.
Before creating a pool with deduplication enabled, ensure that you have planned your hardware requirements appropriately and implemented appropriate recovery practices, such as
regular backups. Consider using the compression property as a less resource-intensive alternative.
