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RAID capacitycalculator
Accurately calculate the available capacity, fault tolerance and usage of RAID 0/1/5/6/10 disk arrays. Intuitively configure multiple hard drives and quickly compare the storage efficiency and protection levels of different RAID types.
RAID disk array knowledge encyclopedia
In-depth understanding of RAID technology principles, differences between types and application scenarios
What is RAID?
RAID (Redundant Array of Independent Disks) is a technology that combines multiple physical hard drives into one or more logical storage units. Through mechanisms such as data striping, mirroring, and parity, RAID can strike a balance between improving performance, increasing capacity, or providing fault-tolerant protection.
RAID technology was first proposed in 1987 by Patterson, Gibson, and Katz at the University of California, Berkeley, and initially defined six levels from RAID 0 to RAID 5. Today, RAID has been widely used in servers, NAS network storage devices, workstations and enterprise data centers, and is an indispensable basic technology in modern storage architecture.
The core goals of RAID include:Improve data access speed(through parallel reading and writing),Provide fault tolerance protection(Avoid data loss due to single hard drive failure), andIncrease storage capacity(Consolidate space on multiple hard drives). Different RAID levels have different trade-offs between these three goals.
RAID 0, 1, 5, 6, 10 differences
RAID 0 — Striping
Spread data across all hard drives for optimal read and write performance and 100% capacity utilization. But there is no fault tolerance - if only one hard drive fails, all data will be completely lost. Suitable for temporary data storage, video editing workspace and other scenarios where loss can be tolerated.
RAID 1 — Mirroring
Completely copy data to another hard drive to provide complete data backup. The writing speed is slower (two copies need to be written at the same time), but the reading speed can be improved (can read from two hard drives at the same time). Capacity utilization is only 50%, but fault tolerance is excellent. Suitable for system disk and important file storage.
RAID 5 - Distributed Parity
Store data and parity information dispersedly across all hard drives, sacrificing the capacity of one hard drive in exchange for fault-tolerant protection. The reading speed is good, but the writing speed is slightly lower depending on the parity. It requires at least 3 hard drives and is currently the most widely used RAID level for home NAS and small and medium-sized enterprises. The capacity utilization rate is (N-1)/N.
RAID 6 - Double Parity
Adding a second parity check on the basis of RAID 5 can tolerate the simultaneous failure of up to two hard drives. Sacrificing the capacity of two hard drives for higher data security, the write performance is lower than RAID 5, but it is especially important for arrays with large-capacity hard drives (4TB or more). Suitable for enterprise NAS and long-term data archiving.
RAID 10 — Striping + Mirroring (Striping of Mirrors)
Combine the advantages of RAID 0 and RAID 1: mirror the hard drives in pairs, then stripe the mirrored pairs. It has excellent read and write performance and complete fault-tolerance protection, but the capacity utilization rate is only 50%. At least 4 hard drives are required (and must be an even number), and the rebuild speed is extremely fast. Suitable for databases, virtual machines, and high-performance applications.
Can RAID replace backups?
No. RAID is definitely not a replacement for backups. This is a common but dangerous misconception. RAID provides "High Availability", allowing the system to continue operating even if a single (or multiple) hard drive fails; while Backup provides "Data Recovery", allowing users to retrieve historical versions after data is lost.
RAID cannot protect you from:
- Accidentally deleted files — RAID mirroring is immediately synchronized with deletion operations
- Ransomware/malware attacks — All data on the hard drive will be encrypted at the same time
- Logic errors/software bugs — Data corruption will be immediately replicated to all images
- Natural Disaster/Fire/Water Damage — RAID array co-located
- Multiple hard drive failures — RAID 5 can only tolerate one failure, and a second failure during rebuilding results in a total loss
The correct approach is:RAID + Backup = Complete Data Protection. Use RAID to ensure uninterrupted service and keep your data safe with a 3-2-1 backup strategy (3 copies of data, 2 media, 1 offsite).
Which RAID is best for NAS?
RAID selection for your NAS depends on the number of hard drives, performance requirements, and data criticality. The following are common recommended configurations:
- 2-hard drive NAS: RAID 1 (mirror) recommended. Although the capacity is only half, it can ensure that data will not be lost in the event of a hard drive failure.
- Home NAS with 3~4 hard drives: RAID 5 is recommended. The best balance between capacity, performance and protection for family photos, videos and multimedia files.
- Small and medium-sized enterprise NAS with 4~6 hard drives: RAID 6 or RAID 10 recommended. RAID 6 is suitable for large-capacity storage needs, and RAID 10 is suitable for scenarios that require high-performance database applications.
- Enterprise NAS with more than 8 hard drives: It is recommended to use RAID 6 with Hot Spare, or consider more advanced RAID 50/60 and other nested RAID configurations.
Special note: If you use a large-capacity hard drive above 4TB, it is strongly recommended to choose RAID 6 instead of RAID 5. The rebuilding time of RAID 5 for large-capacity hard drives may last for days or even weeks. During this period, if a second hard drive fails (UBER error), the data of the entire array will be lost.
How long does a RAID rebuild take?
There is no fixed answer to RAID rebuild time and is affected by many factors. In general, the following are rough estimates:
- Hard drive capacity: The bigger it is, the slower it is. Rebuilding a 1TB drive takes approximately 2 to 4 hours, a 4TB drive may take 12 to 24 hours, and a drive above 12TB may take several days.
- RAID type: RAID 1 is the fastest to rebuild (only needs to copy data), RAID 5 is the second (needs to recalculate parity), and RAID 6 is the slowest (needs to calculate double parity). RAID 10 rebuilds are extremely fast because only copies are made from the mirror pair.
- Controller performance: Hardware RAID cards typically rebuild faster than software RAID and have dedicated processors and cache memory.
- load during rebuild: If the RAID array continues to provide services (read and write operations) during the rebuild period, the rebuild speed will be significantly reduced.
- Hard drive interface and speed: SAS drives are generally faster than SATA drives, and enterprise-class SSDs can rebuild much faster than traditional HDDs.
RAID Frequently Asked Questions FAQ
Is RAID 5 safe?
RAID 5 is relatively safe for small-capacity hard drives (under 2TB), but as hard drives get larger, the risks of RAID 5 increase. The reason is that the probability of URE (Unrecoverable Read Error) is proportional to the amount of data read. An 8TB hard drive needs to read 8TB of data during rebuilding, which significantly increases the chance of URE. It is recommended to use RAID 6 for hard drives larger than 4TB.
Is RAID 6 worth it?
It is worth it, especially in scenarios with a large number of hard drives or a single hard drive with a large capacity (more than 4TB). RAID 6 sacrifices the capacity of two hard drives in exchange for the protection that can tolerate simultaneous failure of two hard drives. Considering the long rebuild time of large-capacity hard drives (possibly several days), during which a second failure can occur at any time, the additional protection of RAID 6 is crucial.
Why is RAID 10 only half capacity?
RAID 10 (also known as RAID 1+0) first combines two hard drives into RAID 1 mirror pairs, and then stripes these mirror pairs in RAID 0 mode. Because each pair of mirrors stores exactly the same data, only the capacity of one hard disk is available in each pair of mirrors. If there are 4 hard drives, the available capacity is 4 × capacity ÷ 2 = the capacity of 2 hard drives. Although capacity utilization is only 50%, it combines the speed of RAID 0 with the protection of RAID 1.
Will RAID increase read and write speeds?
Depends on RAID type. RAID 0 and RAID 10 can significantly increase read and write speeds (due to parallel access by multiple hard drives), RAID 5 has good read speeds but slower writes (needs to calculate parity), RAID 1 has improved read speeds but slower write speeds (needs to write two copies at the same time), and RAID 6 has the slowest write speed (needs to calculate double parity). Regardless of RAID, the actual performance improvement also depends on the RAID controller, hard drive interface (SATA/SAS/NVMe) and system bottlenecks.
What is the recommended RAID for home NAS?
Generally, it is recommended to use RAID 5 (3~4 hard drives) or RAID 1 (2 hard drives) for home NAS. If you only have 2 hard drives, RAID 1 is the only reasonable choice. If you have 4 hard drives and the data is very important (such as family photos, important documents), you can consider RAID 6 or RAID 10. If you have sufficient budget and value performance, RAID 10 is a good choice.
Can RAID hard drives of different capacities be mixed?
Yes, but not very efficient. When you mix hard drives of different capacities in a RAID array, the array is configured based on the smallest capacity drive. For example, if three hard drives of 4TB, 4TB, and 6TB are used to form a RAID 5, the actual total available capacity will be calculated based on 4TB: 4TB × (3-1) = 8TB, and the extra 2TB space will be wasted. It is recommended to use hard drives of the same capacity and model to build RAID to obtain the best capacity utilization and stability.
Can RAID continue to be used during rebuilding?
Yes, most RAID controllers and NAS systems support continued service during a rebuild (online rebuild). But be aware: read and write performance will drop significantly during rebuilds, and rebuild times will be extended due to sustained I/O load. It is recommended to perform reconstruction during periods when the system load is low (such as at night), and to avoid large-scale data movement or high-intensity access during the reconstruction period.