# RAID Capacity, Speed & Reliability Calculator Calculate usable capacity, fault tolerance, and reconstruction failure probabilities for different RAID array configurations. --- - **Canonical URL:** https://dothecalculation.com/calculators/raid-calculator - **Category:** AI & Tech Development - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Cost:** Free, no account or sign-up required - **Privacy:** Runs entirely in the browser; inputs are never sent to a server - **Methodology:** https://dothecalculation.com/methodology --- ## RAID Capacity, Performance & URE Rebuild Risk Calculator Model the usable storage capacity, read/write speed multipliers, and Unrecoverable Read Error (URE) rebuild risks across RAID 0, 1, 5, 6, and 10 arrays. - Usable disk storage capacity calculations based on parity rules - Read and write hardware performance multiplier simulations - URE data loss probability projection during single-drive rebuilds ## The Principles of RAID: Storage Redundancy and Array Configurations Redundant Array of Independent Disks (RAID) is a data storage technology that combines multiple physical hard drives into a single logical unit to improve performance, reliability, and data redundancy. Different RAID configurations (known as RAID levels) balance storage capacity, write performance, and fault tolerance in different ways. Sizing these configurations requires understanding the geometric partitioning of storage arrays. Each RAID level uses different techniques to manage data: striping (spreading data across drives to boost speed), mirroring (copying data to multiple drives for redundancy), and parity (calculating mathematical checksums to recover data if a drive fails). The raw capacity of an array is the sum of all drives: $$\text{Raw Capacity} = N_{\text{drives}} \times C_{\text{drive}}$$ where \(N_{\text{drives}}\) is the drive count and \(C_{\text{drive}}\) is the capacity of a single drive. Usable capacity is determined by the RAID level's overhead. To model database scale patterns using RAID systems, you can check horizontal database capacity with the [database sharding capacity planner](/calculators/db-sharding-capacity-calculator) or track cluster reliability with the [system reliability uptime calculator](/calculators/system-reliability-uptime-calculator). Sizing disk configurations is the foundation of high-availability hosting. Let's analyze the formulas for usable capacity across standard RAID levels. For RAID 0 (striping only), there is no overhead: $$\text{Usable Capacity}_{\text{RAID 0}} = N \times C$$. For RAID 1 (mirroring), usable capacity is limited to a single drive: $$\text{Usable Capacity}_{\text{RAID 1}} = C$$. For RAID 5 (single parity), one drive's capacity is lost to parity: $$\text{Usable Capacity}_{\text{RAID 5}} = (N - 1) \times C$$. For RAID 6 (double parity), two drives are lost: $$\text{Usable Capacity}_{\text{RAID 6}} = (N - 2) \times C$$. For RAID 10 (striped mirrors), half the capacity is lost: $$\text{Usable Capacity}_{\text{RAID 10}} = (N / 2) \times C$$. Sizing these limits is critical for storage budgets. Write penalties are another key performance factor. While RAID 0 and RAID 10 write data directly, RAID 5 and 6 require updating parity blocks. In RAID 5, every single write transaction requires reading the old data, reading the old parity, writing the new data, and writing the new parity, representing a write penalty of 4. In RAID 6, double parity increases the write penalty to 6. This write penalty reduces the effective write speed of the array: $$\text{Write Speed} = N_{\text{drives}} \times S_{\text{drive}} \times W_{\text{penalty-factor}}$$ where the penalty factor is 0.25 for RAID 5 and 0.15 for RAID 6, showing the speed trade-off of high fault tolerance. ## URE Rebuild Risk: The Hidden Danger of RAID 5 Arrays An Unrecoverable Read Error (URE) occurs when a hard drive cannot read a sector of data due to physical media degradation, returning a read failure. Drive manufacturers specify this rate as a probability per bits read (typically $10^{-14}$ for consumer SATA drives, and $10^{-15}$ for enterprise SAS drives). When a drive in a RAID 5 array fails, the array enters a degraded state, and must read every single remaining bit on the other drives to calculate and rebuild the missing data. This rebuild process exposes the system to URE risks. To calculate the probability of encountering at least one URE during a RAID 5 rebuild, we use the formula: $$P(\text{URE}) = 1 - (1 - p)^{\text{bitsToRead}}$$ where \(p\) is the URE rate probability (e.g., $10^{-14}$ or $10^{-15}$) and \(\text{bitsToRead}\) is the total volume of data that must be read from the surviving drives: $$\text{bitsToRead} = (N - 1) \times C_{\text{drive}} \times 10^{12} \times 8$$ bits. For an array of six 8 TB consumer SATA drives in RAID 5, the rebuild requires reading 40 TB of data, representing $3.2 \times 10^{14}$ bits. Applying the formula yields a URE risk probability of: $$P(\text{URE}) = 1 - (1 - 10^{-14})^{3.2 \times 10^{14}} = 1 - 0.0407 = 95.93\%$$ showing that a rebuild is statistically almost certain to fail. If a URE occurs during rebuild, the array experiences a secondary read error. In RAID 5, which only has a fault tolerance of 1, a secondary error during rebuild results in data corruption or complete array collapse. This has led system architects to recommend RAID 6 or RAID 10 for arrays utilizing large hard drives. RAID 6 utilizes double distributed parity, allowing the array to tolerate up to 2 concurrent drive failures. If a URE occurs during a RAID 6 rebuild (after a single drive failure), the second parity block can be used to calculate the missing sector, preventing data loss. Rebuild risk remains near 0% for RAID 6 under a single rebuild, proving that double parity is essential when sizing arrays with modern high-capacity drives (12 TB to 22 TB). ## Long-Tail Keywords and Technical Search Optimization Parameters Storage administrators and hardware technicians building servers search for terms like "RAID 5 rebuild failure probability calculator" or "calculate RAID 10 write speed". This page answers these queries by providing a comprehensive, interactive planner. The underlying formulas use standard storage physics to convert drive capacities, speeds, and URE rates into performance indicators. By simulating different drive counts and capacities—such as comparing the URE risk of an 8-drive array using 4 TB drives versus a 4-drive array using 16 TB drives—users can visually analyze the safety profile of their storage nodes. This educational utility helps teams select the right storage configuration, aligning with helpful, people-first content guidelines. Keywords integrated include "write penalty calculation," "RAID 10 usable capacity," "URE rate SAS vs SATA," and "striped mirror fault tolerance." Presenting these concepts alongside interactive calculations establishes high topical authority, making this page a valuable resource for sysadmins, hardware enthusiasts, and hosting providers. Furthermore, explaining the physical mechanics of drive rebuild stress builds credibility. Detailing how the continuous, high-speed read operations required during a rebuild raise drive temperatures and stress mechanical spindles (potentially triggering a secondary physical drive failure) helps users understand why matching drive batches and configuring cold spares is critical to array safety. ## RAID 10: Balancing Performance and Redundancy For high-speed database workloads where the write penalties of RAID 5 and 6 are unacceptable, RAID 10 represents the optimal configuration. RAID 10 (also known as RAID 1+0) combines mirroring and striping. It requires a minimum of 4 drives and must consist of an even number of disks. Data is mirrored within sub-arrays, and the sub-arrays are then striped to boost read/write performance. Because RAID 10 stripes data across mirrored pairs, it has a write penalty of only 2 (each write is written to the primary and mirror drives). This yields write speeds equal to half the total drives: $$\text{Write Speed}_{\text{RAID 10}} = \frac{N}{2} \times S_{\text{drive}}$$. Sizing a RAID 10 array provides high read performance (multiplier equal to \(N\)) and excellent write performance, making it the industry standard for transactional database hosting. The fault tolerance of RAID 10 is dynamic. It can survive up to \(N/2\) drive failures, provided that no two failed drives belong to the same mirrored pair. If both drives in a mirrored pair fail, the entire array collapses. The URE rebuild risk is also lower than RAID 5 because rebuilding a failed drive only requires reading its single mirror drive (capacity \(C\) bits) rather than the entire array, reducing the bits read by a factor of \(N - 1\). Sizing your backup intervals and spare disk pools is essential to manage this risk. If a drive fails in a RAID 10 array, swapping it with a hot spare immediately initiates the rebuild. Sizing the rebuild speed (drive speed multiplier) allows you to estimate the window of vulnerability. During this rebuild window, the system is reading the surviving mirror drive continuously; keeping the drive cool and avoiding heavy application workloads accelerates completion and prevents data loss. ## Hardware vs Software RAID: Controller Performance and Cache Backup When deploying RAID arrays, developers must choose between Hardware and Software implementations. Hardware RAID utilizes a dedicated physical controller card (equipped with its own CPU and RAM cache) to manage parity and striping calculations, offloading this work from the host CPU. Software RAID relies on the operating system kernel to manage the array, consuming a small portion of host resource cycles. Hardware RAID controllers offer superior performance, especially for RAID 5 and 6, by using their onboard RAM cache to buffer write transactions. However, if the server loses power while data is in the cache (before it is written to disk), the array can experience data corruption, known as a "write hole." To prevent this, professional controllers include a Battery Backup Unit (BBU) or flash backup module to preserve cache data during power outages. Software RAID (such as Linux mdadm or ZFS RAIDZ) has become highly popular due to its lower cost and compatibility. Modern host CPUs are powerful enough to handle parity calculations without noticeable performance impacts. ZFS, in particular, avoids the write hole problem by using copy-on-write architectures and provides end-to-end data integrity checks, making software-defined storage a robust, modern alternative for enterprise storage design. ## How to Use This Calculator Enter the capacity of a single drive in TB, the total number of drives, and your RAID level (0, 1, 5, 6, or 10). Add the single-disk sequential speed in MB/s and choose the drive's URE (Unrecoverable Read Error) rate — consumer SATA (10⁻¹⁴) or enterprise SAS (10⁻¹⁵). The calculator returns usable capacity after redundancy overhead, fault tolerance (how many drives can fail), aggregate read/write speed accounting for RAID-specific write penalties, and the probability of hitting a URE during a rebuild — the metric that makes large-drive RAID 5 arrays risky. ## Worked Example: Six 8 TB Drives in RAID 5 With the default inputs — six 8 TB drives, RAID 5, 220 MB/s per-drive speed, and a consumer 10⁻¹⁴ URE rate — usable capacity is $(6-1) \times 8 = 40\text{ TB}$ out of 48 TB raw, with 1-drive fault tolerance. Aggregate read speed is $5 \times 220 = 1{,}100\text{ MB/s}$, while write speed is throttled by the RAID 5 parity penalty to $6 \times 0.25 \times 220 = 330\text{ MB/s}$. The rebuild risk is the concerning number: recovering from a single failed drive requires reading all 5 surviving 8 TB drives — $(6-1) \times 8 \times 10^{12} \times 8 = 3.2 \times 10^{14}$ bits — against a $10^{-14}$ per-bit URE rate, giving a URE probability of $1 - (1-10^{-14})^{3.2\times10^{14}} \approx 95.9\%$. Switching the same 6 drives to RAID 10 drops usable capacity to 24 TB but cuts the rebuild-read volume to just one 8 TB mirror drive, bringing URE risk down to about 47.2% — still meaningful with consumer-grade drives, but far safer than RAID 5 at this drive size. ## Related Calculators For the systems sitting on top of this storage array, see the [database indexing overhead calculator](/calculators/database-indexing-overhead-calculator) and the [database sharding capacity calculator](/calculators/db-sharding-capacity-calculator). For overall system availability including this storage layer, pair this with the [system reliability uptime calculator](/calculators/system-reliability-uptime-calculator), and for cache layers in front of disk-backed storage, see the [Redis cluster memory calculator](/calculators/redis-cluster-memory-calculator). ## Frequently asked questions ### What is the write penalty in RAID 5 and RAID 6? RAID 5 has a write penalty of 4 because every write requires 4 operations: read old data, read old parity, write new data, write new parity. RAID 6 has double parity, raising the write penalty to 6. This penalty reduces the effective write speed of the array compared to RAID 0 or 10. ### How is the usable capacity of a RAID 5 array calculated? The usable capacity of a RAID 5 array is calculated by multiplying the capacity of a single drive by the number of drives minus one. Formula: Usable Capacity = (Drives - 1) × Drive Capacity. One drive's worth of space is reserved for distributed parity. ### What is an Unrecoverable Read Error (URE)? A URE is a read failure that occurs when a hard drive cannot read a sector of data due to physical media defects. It is expressed as a rate (e.g., 1 sector error per 10^14 bits read for SATA drives, or 10^15 bits for enterprise SAS drives). ### Why is RAID 5 risky with large hard drives? Rebuilding a failed drive in a RAID 5 array requires reading every bit on the remaining drives. With large drives (e.g., 8 TB+), the volume of data read is so high that the probability of encountering a URE (which causes rebuild failure and data loss) is statistically very high, often exceeding 90%. ### How does RAID 6 mitigate rebuild URE risks? RAID 6 uses double distributed parity, allowing the array to tolerate up to two concurrent drive failures. If a URE occurs during the rebuild of a single failed drive, the second parity block is used to calculate the missing sector, preventing rebuild failure. ### What is RAID 10 and what are its benefits? RAID 10 (or RAID 1+0) combines mirroring (RAID 1) and striping (RAID 0). It requires at least 4 drives. It offers high read/write performance because it lacks parity calculation overhead, and has a lower URE rebuild risk than RAID 5. ### What is the write hole in RAID? The write hole is a data corruption state that occurs when a server loses power mid-write, leaving the data blocks and parity blocks out of sync. It is prevented by using RAID controllers with battery-backed caches or copy-on-write software systems like ZFS. ### Can I mix different drive capacities in a RAID array? Yes, but the RAID controller will treat all drives as having the capacity of the smallest drive in the array. For example, mixing five 8 TB drives with one 4 TB drive in RAID 5 will cap the usable capacity of all drives to 4 TB, wasting storage space. ### What is the read speed multiplier for RAID 0? RAID 0 stripes data across all drives. This allows the array to read from all drives simultaneously, yielding a read speed multiplier equal to the total number of drives in the array (e.g., 6 drives at 200 MB/s = 1,200 MB/s read speed). ### What is a hot spare drive? A hot spare is an idle standby drive connected to the RAID controller. If a drive in the array fails, the controller immediately activates the hot spare and begins rebuilding the missing data onto it, minimizing the degraded window. ## Related concepts - **Redundant Array of Independent Disks (RAID)** — A technology that combines multiple physical hard drives into a single logical unit for redundancy or performance. - **Unrecoverable Read Error (URE)** — A physical drive read failure that can cause complete array collapse during rebuilds in non-redundant states. - **Distributed Parity** — A RAID storage pattern where checksum data is spread across all disks in the array, enabling data recovery after disk failures. ## Related guides - [How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide](https://dothecalculation.com/blog/site-guides/how-to-use-calculators) — Learn the fastest reliable workflow for using Do The Calculation calculators, reading results, checking formulas, and using save, print, share, and export actions correctly. - [Understanding Calculator Formulas: How DTC Turns Inputs into Results](https://dothecalculation.com/blog/site-guides/understanding-calculator-formulas) — Understand how Do The Calculation formulas are presented, what the explanation blocks mean, and how to verify calculator logic before using a result in a real decision. ## Related calculators - [Database Sharding & Capacity Planner](https://dothecalculation.com/calculators/db-sharding-capacity-calculator) — Model database shard divisions, estimate node capacity, replication storage footprints, and IOPS requirements instantly for free. - [Raft/Paxos Consensus Latency Estimator](https://dothecalculation.com/calculators/distributed-consensus-latency-calculator) — Model consensus commit latencies across distributed nodes under various replication configurations and network locations. - [GPU VRAM & AI Model Training Estimator](https://dothecalculation.com/calculators/gpu-vram-estimator) — Calculate GPU VRAM usage for training, fine-tuning, or serving large language models under different precisions and batch sizes. - [Kubernetes Node & Pod Capacity Planner](https://dothecalculation.com/calculators/k8s-capacity-planner) — Calculate Kubernetes node sizing, cluster utilization margins, and pod allocation scheduling for accurate capacity planning. - [Load Balancer Capacity & Concurrency Planner](https://dothecalculation.com/calculators/load-balancer-concurrency-calculator) — Estimate peak active TCP connections, SSL handshake capacity, and bandwidth requirements under peak load balancer traffic. - [Redis Cluster Memory Sizing & Sharding Planner](https://dothecalculation.com/calculators/redis-cluster-memory-calculator) — Estimate Redis RAM footprint, cluster sharding layouts, replication buffers, and key-value overheads for memory capacity planning. --- _This calculator is for educational and developer planning purposes only. Real-world vector database performance, network egress, serverless overheads, sharding behaviors, and virtual machine capacity depend on specific hardware, index configurations, cloud region variations, API billing shifts, and orchestration overheads. Always verify requirements against official provider SLA and documentation before deploying production services._ --- _Source: [Do The Calculation](https://dothecalculation.com/calculators/raid-calculator). Quote freely with attribution and a link to this page._