hesela.devData-reduction ratio
A data-reduction ratio compares the logical data represented with its corresponding stored footprint under a stated accounting convention.
Calculation: Divide logical bytes by corresponding stored bytes using the same data population, time and units. A 2:1 ratio means 50% savings against the logical baseline. A zero-footprint sample provides no finite ratio for planning.
Scope: Reports differ in whether they count retained copies, metadata and allocation avoidance. Unwritten thin address space is not evidence of compressed data; identify what each counter includes. [1]
Aggregation: For non-overlapping populations with matching accounting, sum logical and stored bytes separately. Two hypothetical 100 TB logical populations occupying 100 TB and 25 TB give 200 / 125 = 1.6:1, not the mean 2.5:1. Shared deduplicated blocks need a pool total or an allocation that counts each block once. This is arithmetic, not an array measurement.
Population: A ratio for reducible data only excludes the unreducible portion. Do not apply it to the whole workload without accounting for that omitted data. [2]
Incoming mix: Let f be the fraction of incoming logical bytes reduced at ratio r, with the rest costing 1:1. For additive footprints, physical bytes per logical byte are (1 - f) + f / r. At f = 0.5 and r = 4, the ratio is 1 / 0.625 = 1.6:1. This hypothetical model fixes old data and overhead; it excludes shared blocks, new metadata, retained-version growth and competing writers. It is not a product counter formula or a capacity guarantee.
Relations
- related
- Data deduplication, Transparent filesystem compression, Usable capacity, Effective capacity, Thin provisioning, All-flash array, Duplication-frequency histogram
Citations
- NetApp ONTAP: capacity reporting and measurements
- Dell PowerStore Monitoring: overall and reducible data-reduction ratios
Sources & caveats
- Terminology follows the cited accounting convention; no storage-array measurement or capacity guarantee is implied.
- NetApp documents that ONTAP 9.13.1 and later exclude snapshots and thin provisioning from its data-reduction ratio; earlier displays differ. [1]