⭐ Starlight Tools / RAID Calculator

Understanding Results

This calculator estimates the usable capacity and redundancy overhead for common RAID levels.

• Usable Capacity – Space available after parity/mirroring.
• Overhead – Space reserved for redundancy.
• Fault Tolerance – How many drives can fail without data loss (worst-case).
• Efficiency – Usable / Raw capacity.

Note: Real-world results depend on filesystem formatting, controller behavior, and manufacturer units.

RAID Best Practices & Practical Standards

RAID improves availability and/or performance, but it is not a backup. Use the guidance below to plan safer, faster arrays.

1) Data Protection Basics

• Follow 3-2-1 backups: 3 copies of data, on 2 different media, with 1 off-site/immutable copy.
• Use a UPS to protect against write-hole corruption and sudden power loss. Prefer controllers or NVMe devices with power-loss protection.
• Test restores, not just backups: Verify integrity and practice a restore workflow periodically.

2) Picking a RAID Level

• Large SATA/NL-SAS drives (≥ 8 TB): prefer RAID 6 / RAIDZ2 or better; rebuilds are long and URE risk increases with size.
• Performance + safety: RAID 10 (striped mirrors) gives excellent IOPS and simple failure domains.
• Capacity with resilience: RAID 6 (or RAID 60) balances usable space and fault tolerance for big pools.
• Avoid RAID 0 for important data—zero fault tolerance.

3) Drive & Layout Considerations

• Match technologies: Don’t mix SMR with CMR in the same set; SMR can degrade rebuilds and sustained writes.
• Use similar models/firmware to minimize performance variance and weird edge cases during rebuilds.
• Sector size alignment: Keep 4Kn/512e consistent across members; align partitions to 1 MiB boundaries to avoid RMW penalties.
• Hot spares: Keep at least one global hot spare for big enclosures to shorten time-to-recovery.
• Cooling & vibration: High temps and chassis vibration increase failure rates—ensure good airflow and isolation.

4) Controller / Filesystem Settings

• Write cache safety: If using a RAID controller write cache, pair it with BBU/flash-backed cache; otherwise disable volatile write-back.
• TLER/ERC: For HDDs behind hardware RAID, enable short error recovery (e.g., TLER/ERC) to prevent long drive timeouts.
• TRIM/Discard for SSD arrays: Enable periodic TRIM and leave 10–20% over-provisioning for endurance.
• Checksums & scrubs: Use filesystems with end-to-end checksums (e.g., ZFS, Btrfs) and schedule regular scrubs/patrol-reads.

5) Scrubbing, Patrol Reads & Monitoring

• Monthly or quarterly scrubs (large pools) help surface latent errors before a failure forces a rebuild.
• SMART monitoring: Watch Reallocated/Pending sectors, UDMA CRC errors, temperature, and sudden changes in these metrics.
• Alerts on failures/degrades: Email/Slack/Syslog notifications reduce Mean-Time-to-Repair (MTTR).

6) Rebuild Strategy

• Prioritize resiliency during rebuilds: Temporarily throttle background jobs, increase rebuild priority/IO if possible, and avoid heavy workloads.
• Keep cold spares on site if hot spares aren’t feasible—hours matter when degraded.
• Plan capacity headroom: Leave 10–20% free to maintain performance and reduce write amplification during rebuilds.

7) Practical “Rules of Thumb”

8) Light-Touch Standards & References (Practical)

There isn’t a single universal RAID “law,” but common practice aligns with:

• Controller vendor guidance (e.g., minimum drives, cache/BBU requirements, patrol read/scrub cadence).
• Filesystem vendor guidance (ZFS/Btrfs docs for scrub frequency, checksum/repair behavior, recommended redundancy levels).
• Enterprise norms such as using dual-parity for large nearline HDD pools, verified backups (3-2-1), and UPS-backed write caches.

Remember: RAID increases availability—but only backups protect against deletion, ransomware, or site loss.