Replication: complete copies
A replicated pool stores multiple complete copies. With three copies, 120 TB of raw capacity gives a theoretical 40 TB before reserves and other overhead. Placement across separate hosts avoids keeping all copies within a single host failure domain.
Erasure coding: data plus parity
A 4+2 profile divides data into four data chunks and adds two coding chunks. Its theoretical efficiency is 4/6, or about 66.7%. Capacity savings come with additional computation and recovery work. Suitability depends on the workload, pool features and deployment.
Capacity is not an availability guarantee
For host-level placement, a 4+2 layout requires at least six suitable host failure domains. The minimum placement count does not provide extra hosts for rebuilding after a host loss. Plan additional resources and verify min_size and recovery behaviour for the selected profile.
Keep a separate recovery strategy
A reserve percentage in a calculator cannot guarantee enough recovery space for every failure. Distribution and the largest failure domain matter. Retained backups outside the affected failure domain are needed for historical recovery and protection against deletion or corruption.
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