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CONCEPT Cited by 1 source

Storage media tiering

Definition

Storage media tiering is the architectural practice of deploying multiple distinct storage media types in a coordinated hierarchy, so that each data class lands on the media whose cost / performance / endurance profile best matches that data's workload characteristics.

Orthogonal distinctions on the wiki:

  • Storage media tiering — this page, physical-media axis (HDD vs flash vs tape vs DRAM).
  • patterns/tiered-storage-to-object-store — Kafka-era software pattern of offloading cold segment files from a broker's local disk to object storage. Different axis (local vs remote).
  • Hot-cold tiering — data-access-frequency axis, can be implemented via media tiering or not.

These can compose.

The hyperscale media tier ladder (2025)

Tier Media Role Density (drive) BW/TB Endurance Cost/byte
0 DRAM Hot cache <1 TB >1000 MB/s/TB n/a highest
1 SLC / cache flash Write-buffer / index 1-4 TB >500 MB/s/TB highest very high
2 TLC Primary serving 4-30 TB 50-100 MB/s/TB high high
3 QLC (new) Batch IO / read-BW-dense 32-600 TB 10-20 MB/s/TB moderate middle
4 HDD Bulk / cold 20-30 TB 5-10 MB/s/TB high low
5 Tape Archive 15-50 TB <1 MB/s/TB very high very low

Meta's 2025 QLC post is canonical: it adds tier 3 to the ladder, explicitly positioned between TLC (tier 2) and HDD (tier 4).

Why tiers get added

Tiers get added when the gap between existing tiers exceeds what workload-placement-at-the-edges can paper over. Three forcing functions:

  1. Upper-tier cost too high for a workload band that doesn't need its full capability.
  2. Lower-tier capability too low for a workload band whose requirements grew (or whose lower-tier BW/TB shrank).
  3. New media becomes economical for the gap band.

Meta's 2025 argument hits all three: (1) TLC too expensive for batch IO; (2) HDD BW/TB falling as drive densities climb; (3) QLC 2 Tb dies + 32-die stacks mainstream, write endurance sufficient for read-dominant workloads.

Architectural consequences

  • Each tier has its own form factor optimization. See systems/u2-15mm-form-factor (QLC) vs systems/e1s-form-factor (TLC) at Meta — density / thermal / slot-count trade-offs differ per media.
  • Each tier may have its own software stack. See patterns/userspace-ftl-via-io-uring for QLC — different arbitration needs mean different stacks.
  • Workload migration tooling becomes load-bearing — when a tier is added, large amounts of existing data must move to the new tier, which takes years at hyperscale.
  • Tier-boundary analysis is a recurring capacity-planning exercise — which workloads are stranded on the wrong tier? The answer drives the next tier's business case.

Relationship to heat

concepts/heat-management is about distributing load within a tier (placement). Storage media tiering is about choosing which tier at all. The two compose: you tier first, then distribute heat within the chosen tier.

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