--- description: What is RAID? Levels 0, 1, 5, 6, and 10 explained with honest trade-offs, why rebuild time matters at 30TB+, and how to choose. title: What is RAID? image: https://www.storagereview.com/wp-content/uploads/2025/05/StorageReviewSquare.webp --- [![Storage Review]()](https://storagereview.com/) --- [Facebook](https://www.facebook.com/@storagereview) [X (Twitter)](https://twitter.twitter.com/storagereview) [Instagram](https://www.instagram.com/storagereview/) [LinkedIn](https://www.linkedin.com/company/storagereview-com) [YouTube](https://www.youtube.com/user/storagereview?sub_confirmation=1) [Email](mailto:info@storagereview.com) [Spotify](https://open.spotify.com/show/1y6VnznABhHeOSMOmbDTz0) [Reddit](https://www.reddit.com/r/StorageReview/) [Discord](https://discord.gg/TwMHb4azdC) [RSS Feed](https://www.storagereview.com/rss.xml) [TikTok](https://www.tiktok.com/@storagereview) [Facebook](https://www.facebook.com/@storagereview) [X (Twitter)](https://twitter.com/storagereview) [Instagram](https://www.instagram.com/storagereview/) [LinkedIn](https://www.linkedin.com/company/storagereview-com) [YouTube](https://www.youtube.com/user/storagereview?sub_confirmation=1) [Email](mailto:info@storagereview.com) [Spotify](https://open.spotify.com/show/1y6VnznABhHeOSMOmbDTz0) [Reddit](https://www.reddit.com/r/StorageReview/) [Discord](https://discord.gg/TwMHb4azdC) [RSS Feed](https://www.storagereview.com/rss.xml) [TikTok](https://www.tiktok.com/@storagereview) [![StorageReview.com](https://www.storagereview.com/wp-content/uploads/2026/07/Storage-Review-2x.png)](https://www.storagereview.com) ≡ Menu - [Home](https://www.storagereview.com/) - [Storage Reviews](https://www.storagereview.com/review) - [End User Reviews](https://www.storagereview.com/consumer) - [Enterprise Reviews](https://www.storagereview.com/enterprise) - [Ubiquiti Reviews](https://www.storagereview.com/best/ubiquiti-reviews) - [SR Merch](https://store.storagereview.com) - [Leaderboards](https://www.storagereview.com/best) - [Best Storage Arrays](https://www.storagereview.com/best/storage-arrays) - [Best Enterprise SSDs](https://www.storagereview.com/best/enterprise-ssds) - [Best Servers](https://www.storagereview.com/best/servers) - [Best Enterprise GPUs](https://www.storagereview.com/best/enterprise-gpus) - [Best Desktops for Local AI](https://www.storagereview.com/best/desktops-local-ai) - [Best Laptops for Local AI](https://www.storagereview.com/best/laptops-local-ai) - [Best Local LLM Tools](https://www.storagereview.com/best/local-llm-tools) - [Agentic AI Hardware](https://www.storagereview.com/best/agentic-ai-hardware) - [Best SSDs & Hard Drives](https://www.storagereview.com/best_drives) - [Best Portable SSDs](https://www.storagereview.com/best/portable-ssds) - [Best Business Laptops](https://www.storagereview.com/best/business-laptops) - [Best Mobile Workstations](https://www.storagereview.com/best/mobile-workstations) - [Best Desktop Workstations](https://www.storagereview.com/best/desktop-workstations) - [Best Mini PCs](https://www.storagereview.com/best/mini-pcs) - [Best Laptop Battery Life](https://www.storagereview.com/best/laptop-battery-life) - [Storage Reference Guide](https://www.storagereview.com/storage-reference-guide) - [About SR](https://www.storagereview.com/about-storagereview) - [StorageReview.com Sweepstakes Rules and Regulations](https://www.storagereview.com/storagereview-com-sweepstakes-rules-and-regulations) Search [Home](https://www.storagereview.com/) » What is RAID? # What is RAID? RAID stands for Redundant Array of Independent (originally Inexpensive) Disks: combining multiple drives into one logical volume for redundancy, performance, or both. The concept dates to the late 1980s, when disks meant hard drives; today RAID applies equally to arrays of SSDs, and the trade-offs have shifted as drives have grown to sizes the original designers never imagined. Use our [RAID Calculator](https://www.storagereview.com/raid-calculator) to see exactly how much usable capacity each level yields from your drives. ## The RAID levels that matter ### RAID 0 (striping) Data split across all drives with no redundancy. Full capacity, best speed, and any single drive failure loses everything. Appropriate for scratch space and nothing you cannot re-create. ### RAID 1 (mirroring) Every write goes to two or more drives, so the array survives a drive failure with no data loss and reads can be served from either copy. You pay half your raw capacity for that safety. Two-bay NAS units are the classic RAID 1 home, and it remains the simplest, most predictable redundancy there is. ### RAID 5 (striping with parity) Data plus one drive’s worth of parity spread across the set; survives one drive failure while giving up only one drive of capacity. The catch in 2026: rebuilding after a failure means reading every remaining drive end to end, and with 30TB-class drives that window stretches to a day or more of degraded, vulnerable operation. ### RAID 6 (dual parity) Like RAID 5 but survives two simultaneous failures, at the cost of two drives of capacity. As rebuild windows have grown with drive capacity, RAID 6 (or a second parity drive generally) has become the sane default for large arrays built on big disks. ### RAID 10 (mirrored stripes) Stripes across mirrored pairs: strong performance, fast rebuilds (only the failed mirror is copied), half your raw capacity. The performance-and-safety pick when capacity efficiency matters less. ## What changed: capacity outgrew the rebuild RAID math was designed around drives measured in gigabytes. With NAS drives now at [32TB](https://www.storagereview.com/review/seagate-ironwolf-pro-32tb-review-top-of-stack-capacity-for-multi-bay-nas) and enterprise HAMR models like the [Exos M 30TB](https://www.storagereview.com/review/seagate-exos-m-30tb-review-hamr-ing-storage-density) sustaining roughly 275MB/s, a full rebuild of one drive takes on the order of a day under ideal conditions, longer under load. That window is when a second failure hurts, which is why large home and SMB arrays increasingly run RAID 6, why enterprise systems moved to declustered and erasure-coded schemes, and why hyperscale storage abandoned classic RAID for object storage entirely. RAID is not obsolete; it just needs to be chosen with rebuild time in mind, not only usable capacity. ## RAID is not a backup RAID protects against drive failure, not deletion, ransomware, fire, or theft. An array and a backup are different layers; you need both. This is also where recertified drives earn their place: in a redundant array with backups behind it, [recertified enterprise drives](https://www.storagereview.com/review/maximizing-value-with-recertified-hard-drives) cut the cost of capacity substantially at low practical risk. ## Choosing in practice For a two-bay NAS: RAID 1. Four to eight bays of large drives: RAID 6 or the vendor’s dual-redundancy equivalent, with drives from our [Leaderboard’s](https://www.storagereview.com/best_drives) NAS picks. Performance arrays of SSDs: RAID 10 or the platform’s native equivalent. A practical example of multi-bay RAID done well is the [Ubiquiti UNAS Pro 8](https://www.storagereview.com/review/ubiquiti-unas-pro-8-review-2u-10gbe-nas-with-redundant-power-nvme-cache) we reviewed. Then run the numbers in the [RAID Calculator](https://www.storagereview.com/raid-calculator) before you buy anything. 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