In his post on How to Size Main Memory for ZFS Deduplication Dominic Kay explains that, prior to using deduplication, one must first determine whether that operation is worthwhile by finding out the deduplication ratio and the memory requirements. Fortunately, ZFS itself provides a means to assist with this determination: the ZFS debugging tool, which will tell users whether deduplication will result in storage space savings. Establishing the memory available for deduplication can be accomplished by multiplying the number of allocated blocks by 320 bytes (the size of each in-core deduplication table). These two operations will let users know whether deduplication will result in the desired storage savings.
Sun ZFS Storage Appliance Deduplication Design and Implementation Guidelines
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