RAIDZ Extension Calculator
3 minute read.
RAIDZ extension allows TrueNAS users to add disks one at a time to an existing RAIDZ VDEV, incrementally expanding capacity. Use this calculator to estimate the storage capacity efficiency of a ZFS RAIDZ VDEV after incremental extension.
Existing data blocks retain their original data-to-parity ratio and block width, but are spread across the larger set of disks. New data blocks adopt the new data-to-parity ratio and width, or overhead. Because of this overhead, an extended RAIDZ VDEV can report a lower total capacity than a newly created VDEV with the same number of disks.
Extended VDEVs recover lost headroom as existing data is read and rewritten to the new parity ratio. This can occur naturally over the lifetime of the pool as you modify or delete data. To manually recover capacity, simply replicate and rewrite the data to the extended pool.
While this process can recover the actual lost capacity, reported capacity continues to rely on the old data-to-parity ratio. An expanded vdev can continue to report a lower than expected capacity, even after rewriting old data to the new parity ratio. This accounting inconsistency does not impact the actual available capacity of the vdev.
See Extending a RAIDZ VDEV for more information, including a detailed overview, considerations, and a tutorial.
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Additional contributions from DayBlur and Louwrentius.