Dynamic Levelling Cannot Reach Data That Never Moves
Wear levelling runs on two tracks. Dynamic levelling chooses only from the free block pool, picking the block with the lower erase count on each allocation. It is simple and its overhead is close to zero, but its reach is limited to blocks currently participating in allocation. Static levelling addresses the other half of the problem: data written once and left unmodified holds physical blocks that never return to the free pool and therefore never enter dynamic levelling's field of view.
How the Skew Accumulates
A drive running dynamic levelling alone develops, after enough runtime, a clearly bimodal erase count distribution — some blocks sit at very low cycle counts because they hold cold data, while the rest absorb all the writes and climb quickly. The problem is that lifetime is determined by the upper tail of that distribution, not by its mean. A drive whose average cycle count is only thirty percent consumed while a handful of blocks approach the media specification limit is, for lifetime management purposes, near end of life. The higher the proportion of cold data and the larger the drive, the worse the skew, and high-capacity QLC products need particular protection because QLC has a tighter cycle budget to begin with.
Block Swaps Must Be Costed In
Static levelling moves cold data from a low-wear block to a high-wear one, returning the freed low-wear block to the allocation pool. That relocation is itself a write: it counts toward write amplification and consumes wear budget. The trigger policy usually keys on the gap between the highest and lowest erase counts. Set the threshold too low and the drive shuffles cold data repeatedly in pursuit of a tidy distribution, spending lifetime on levelling activity. Set it too high and the skew is already entrenched before it is detected. The reasonable approach tightens the threshold as remaining life declines — loose early to avoid pointless relocation, tight later so the distribution does not run away near end of life — while scheduling static levelling into idle windows so it does not compete with foreground writes for bandwidth.
Convergence Comes Before Lifetime Prediction
Whether an endurance figure can be honoured depends on whether the wear distribution converges. The J750 is planned at 1 DWPD and the J770 at 3 DWPD; the total write volume those imply across a five-year warranty holds only if the gap between the most worn block and the mean stays small. The more dispersed the distribution, the more safety margin must be reserved and the lower the DWPD that can be committed. Endurance across the portfolio is planned up to 120 DWPD, and the higher the endurance target, the more it depends on a tight distribution. Practically, erase count distribution statistics should be reported continuously through Telemetry and judged on maximum, mean and dispersion together rather than on a single percentage-used figure, and watched alongside spare block consumption rate so that a skew trend is caught during validation.