Cache workload planner
NAS SSD Cache Endurance Planner
Estimate per-drive cache writes without treating mirror writes as stripe writes or an estimate as a measurement.
Per-drive cache result
Decision guide
Plan cache endurance from the writes each drive actually receives
What this tool helps you decide
A NAS cache can accelerate a workload while concentrating writes on a small set of SSDs. This planner estimates whether each cache drive has enough TBW for a defined horizon. It distinguishes measured cache host writes from an ingest-based estimate and makes the layout rule explicit. In a mirror, every mirrored drive receives the full logical write stream; in a stripe, the model divides that stream across drives. That distinction matters more than a generic pool capacity number.
Before you start
Prefer a measured cache host-write counter over a percentage estimate whenever the NAS exposes one. Otherwise collect a representative daily ingest figure and state what percentage is written through the cache. Identify the cache layout, drive count, per-drive TBW, current writes, and whether the appliance uses write-back or read-only behavior. Enter custom shares only when they are known and make them total 100 percent. Choose WAF and reserve values as documented scenarios.
How the calculation works
The pool logical rate is either the measured rate or daily ingest multiplied by the cache percentage. Single and mirror layouts assign that full rate to each participating drive. Stripe layouts divide it by the drive count; custom layouts apply supplied percentages. Per-drive effective GB per day multiplies the assigned logical rate by WAF. Required TBW converts effective daily GB across the horizon and adds reserve. The result compares that requirement with remaining per-drive TBW.
How to interpret the results
Read the result per drive, not as an aggregate pool allowance. A strong pool TBW total can conceal a weak individual result when mirrored writes are replicated. The status is a planning signal: Pass offers margin, Marginal needs monitoring and a replacement plan, and Insufficient calls for a different drive class, lower write demand, shorter horizon, or revised cache design. Measured writes should take priority over the estimate when they disagree.
Worked example
A two-drive mirrored cache receiving 200 GB per day assigns 200 logical GB per day to each SSD, not 100 GB. With WAF 1.5, each drive sees 300 effective GB per day. Over five years with a 20 percent reserve, required endurance is 657 TBW. A 600 TBW drive therefore falls short before existing writes are considered. In a two-drive stripe under this simplified model, the per-drive logical rate becomes 100 GB per day.
Assumptions and limitations
The planner cannot infer NAS cache algorithms, metadata churn, cache hit ratio, flush behavior, TRIM, write-back policy, firmware, or vendor-specific mirroring semantics. Read-only cache may not carry the write share assumed here. It does not promise that an endurance rating maps to a physical failure date. Cache activity can be bursty, so a short sample may miss maintenance, sync, rebuild, or backup periods. Treat estimates as a reason to measure rather than a substitute for measurement.
Next steps
Record cache counters at two dates and run the Remaining Endurance Planner for a measured trend. Revisit the underlying HDD vs SSD Storage Planner to decide whether caching is justified by the workload. Keep a replacement interval and a tested backup plan; cache endurance does not replace data recovery planning.