What SMART Data Means for SSDs
Every NVMe and SATA SSD reports health telemetry through SMART (Self-Monitoring, Analysis and Reporting Technology). Unlike HDDs where SMART tracks mechanical metrics like head seek errors, SSD SMART data tracks NAND flash wear. The critical attributes: Percentage Used (cumulative wear indicator, 0% new to 100% rated endurance consumed), Available Spare (reserve NAND blocks, alarm below 10%), Media and Data Integrity Errors (uncorrectable read/write errors — any non-zero value demands attention), Data Units Written (total writes for TBW calculation), and Critical Warning (composite health flag).
Write Endurance: TBW and NAND Types
TBW (Terabytes Written) is the manufacturer's rated write endurance. A 1TB Samsung 990 Pro is rated at 600 TBW. At typical consumer workloads of 10-30 GB/day, that translates to 54-164 years of writes. Enterprise workloads differ dramatically: a database server writing 200 GB/day reaches 600 TBW in 8 years. NAND type determines raw endurance: SLC (100,000 P/E cycles), MLC (10,000), TLC (3,000), QLC (1,000). Consumer SSDs use TLC or QLC with wear leveling algorithms that distribute writes evenly, extending practical lifespan well beyond raw P/E cycle counts.
Performance Degradation Detection
SSDs slow down before they fail. A drive that benchmarked at 7,000 MB/s sequential read when new but now measures 5,500 MB/s has lost 21% of read performance — often from NAND cell degradation requiring more aggressive error correction. Establish a baseline benchmark when drives are new, then test quarterly. Sequential read degradation above 15% combined with Percentage Used above 80% indicates the drive is approaching end of practical life even if SMART reports no critical warnings.
When to Actually Replace
Replace immediately if: Available Spare reaches 0%, Media Errors are non-zero and increasing, or Critical Warning flag is set. Plan replacement within 6 months if: Percentage Used exceeds 90%, sequential performance has degraded more than 20% from baseline, or the drive is beyond its warranty TBW rating. Monitor normally if: Percentage Used is under 80%, no media errors, and performance benchmarks within 10% of baseline. Most consumer SSDs outlive the systems they are installed in — the panic around SSD lifespan is disproportionate to actual failure rates.
