RAID Levels Guide: RAID 1, 5, 6 and 10

Raw capacity is not the capacity available to applications. RAID or erasure coding, filesystems, reserved space, metadata, snapshots and growth headroom are separate deductions. TB and TiB representations also change the displayed number. Write calculations with units, establish protected usable capacity, then subtract operating reserves. Track growth rate as well as current utilization. The projected exhaustion date should leave enough time to procure and deploy additional capacity.

IOPS measures operations per second, whereas MB/s measures transferred data. Identical IOPS values produce very different bandwidth at different block sizes. For example, 10,000 operations/s at 4 KiB is approximately 39.1 MiB/s. Read/write mix, random versus sequential access and protection mechanisms affect the result. Measure actual block distributions and concurrency rather than transferring a benchmark result directly to production. High disk IOPS does not prove good application response; latency must be examined alongside it.

SSD selection requires more than capacity and initial performance. Consider TBW, DWPD, warranty duration, power-loss protection and workload write patterns together. DWPD calculations must use the capacity and conditions defined by the manufacturer. Database logs, small random writes and temporary work files affect wear differently. Monitor SMART or vendor health data, considering temperature, media errors and remaining endurance together. Write amplification means application-written bytes may differ from the amount written to NAND.

Survival of committed data depends on write guarantees across the database, OS, filesystem, controller and disks. Do not disable safety settings for speed without understanding caches and flush behavior. Power-loss-protected storage helps but does not prove correctness of the entire chain. Run failure and recovery tests in a controlled lab, not production. Validate application records and supported consistency checks rather than merely opening a sample file.

High availability aims to keep service running through specified failures with a short interruption; backup recovers lost or corrupted data from an earlier point. A cluster can replicate an accidental deletion to another node. HA therefore does not replace backup. Consider DNS, identity, network, storage and power dependencies together. Successful node failover is insufficient by itself: measure user sessions, application writes and external integrations after the transition as well.

RTO specifies how soon a service must become usable after an incident. Downloading a backup or booting a VM accounts for only part of that duration. Record detection, approval, infrastructure preparation, data transfer, application startup and business validation separately. Define exactly what starts and stops the test clock. The same technical restore duration can produce different business interruptions when dependencies or access approvals introduce additional waiting.