RAID combines storage devices to distribute data, keep copies, or calculate parity. The right level depends on how much usable capacity you need, how many device failures the layout can withstand, and how your workload behaves. RAID can help keep an array available after certain drive failures, but it is not a backup of your files.
What RAID levels change
A RAID level describes how data and, where applicable, copies or parity are arranged across devices. That arrangement affects three practical questions: how much capacity is available, what happens when a device fails, and how the array handles reads and writes.
The names are not guarantees of identical behavior everywhere. A RAID level may be implemented by an operating system, filesystem, controller, or storage product; supported layouts and recovery behavior can differ. The details below distinguish general layout concepts from specific Linux md and OpenZFS documentation.
How the main RAID levels work
| Level or layout | How it arranges data | Capacity and device-failure trade-off | Key qualification |
|---|---|---|---|
| RAID0 | Stripes data across devices. | Uses the devices for data without redundancy. It does not tolerate a member failure; a failed member can make the array’s data unavailable. | Striping is not fault tolerance. Linux md describes consecutive chunks being striped across neighboring devices in its implementation: Linux kernel documentation on RAID arrays. |
| RAID1 or mirror | Stores replicated copies on two or more devices. | Copies consume raw capacity. In OpenZFS, an N-device mirror of size X holds X and can tolerate up to N−1 device failures before integrity is compromised. | The capacity and failure statement is specific to OpenZFS mirror documentation: OpenZFS zpool concepts. |
| RAID5 | Stripes data with single parity. | Parity uses some capacity to allow recovery from a device failure, but exact usable space depends on the implementation. | Linux md documents RAID5 parity and a possible write hole if a shutdown interrupts a multi-device stripe write. Its cache documentation describes implementation-specific mitigations: Linux kernel RAID 4/5/6 cache documentation. |
| RAID6 | Stripes data with two parity blocks. | Dual parity uses more capacity than single parity and supports recovery from more device failures than a single-parity arrangement when the implementation and failure pattern permit. | Linux md supports two parity blocks and specifies minimum-device constraints for its implementation. Check support and configuration for the system you plan to use: Linux kernel documentation on RAID arrays. |
| RAID10 | Combines striping with mirrored copies. | Spends capacity on copies while distributing I/O across the layout. Failure tolerance depends on which members fail and where their copies are located. | Linux md supports near, far, and offset layouts; do not assume all RAID10 configurations behave identically: Linux kernel documentation on RAID arrays. |
| RAIDZ1, RAIDZ2, RAIDZ3 | OpenZFS parity layouts with one, two, or three parity devices, respectively. | For a group of N disks of size X and P parity disks, OpenZFS gives approximate capacity of (N−P)X; the group can tolerate P device failures. | Actual usable space depends on sector size, record size, and dynamic stripe width. OpenZFS recommends groups of 3–9 devices for performance, not as a universal rule for every RAID implementation: OpenZFS RAIDZ documentation. |
Estimate capacity without mistaking it for formatted space
Capacity arithmetic is useful for comparing layouts, but it is an estimate rather than a promise of filesystem-usable space. Drive size, parity or copies, implementation overhead, and allocation behavior affect the result.
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- Mirrors: In the OpenZFS example, a mirror made from N devices of size X holds X, because the devices store copies rather than separate portions of the usable data.
- RAIDZ: OpenZFS approximates a group with N disks of size X and P parity disks as (N−P)X. The exact amount available to store files can be lower because sector size, record size, and dynamic stripe width matter.
- RAID5, RAID6, and RAID10: Do not assume one capacity formula applies across all products or implementations. Consult the documentation for the actual controller, operating system, or filesystem configuration.
OpenZFS’s approximate formula and its 3–9-device performance recommendation are technical guidance for RAIDZ, not the findings of a benchmark or a general rule for other RAID implementations.
Compare failure tolerance for the actual layout
Redundancy does not mean every combination of drive failures is safe. A mirror’s copies, a parity group’s placement, and RAID10’s mirror pairs determine whether the remaining devices still contain recoverable data. For RAID10 in particular, the number of failed devices alone does not establish whether the array survives: which members fail matters.
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For RAIDZ, OpenZFS describes RAIDZ1, RAIDZ2, and RAIDZ3 as single-, double-, and triple-parity layouts, respectively. The corresponding group can tolerate one, two, or three device failures. Treat that as a layout-level device-failure limit, not a guarantee against unrelated faults or data loss.
RAID0 has no redundancy. With parity and mirrors, the protection applies to device failures within the layout’s tolerance; it does not protect against accidental deletion, malware, theft, or loss of the entire system. Keep an independent backup of important files.
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Performance depends on workload and implementation
There is no universally fastest RAID level. Sequential versus random access, reads versus writes, drive type, stripe or chunk layout, cache behavior, and the specific implementation all affect results. Without comparable measurements for the system and workload in question, a blanket speed ranking would be misleading.
Linux kernel documentation says chunk size is relevant to striping levels 0, 4, 5, 6, and 10. OpenZFS notes that a RAIDZ write can touch every disk in a stripe and that worst-case write IOPS can be limited by the slowest disk. That is a documented RAIDZ consideration, not a benchmark showing that RAIDZ is always slower than another level.
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When comparing configurations, look at usable capacity, failure tolerance for the actual member layout, the read/write pattern of your workload, recovery behavior, and the features supported by your controller or software.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Write holes, recovery, and implementation-specific safeguards
In Linux md, an interrupted multi-device RAID4, RAID5, or RAID6 stripe write can leave data and parity inconsistent. This is known as the write hole. Linux md documents write-through and write-back journal modes as cache approaches; in write-back mode, losing the cache device can cause data loss, so the cache device is part of the configuration’s safety considerations: Linux kernel RAID 4/5/6 cache documentation.
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OpenZFS documents RAIDZ as eliminating the RAID5 write hole: OpenZFS RAIDZ documentation. Keep this distinction tied to the named implementations rather than assuming that every product bearing a RAID5 or RAIDZ label behaves the same way.
After a device failure, an array may resynchronize or recover to restore redundancy. Linux md distinguishes creating an array—which writes metadata to devices—from assembling it, which associates devices with a virtual md device; its documentation also describes resync and recovery states: Linux kernel documentation on RAID arrays. Check the recovery and health-monitoring procedures for your own system instead of relying on a RAID label to predict compatibility across vendors.
Choose by trade-off, not by number
- Choose RAID0 only when you accept no device-failure redundancy and the data can be recreated or recovered elsewhere.
- Consider a mirror when keeping copies on separate devices is the priority and the capacity cost is acceptable.
- Compare RAID5 and RAID6 by parity protection, capacity, workload, implementation support, and the documented safeguards against interrupted writes.
- Consider RAID10 when the combination of striping and mirrored copies suits the workload, but verify the exact layout and which failure combinations it tolerates.
- For OpenZFS RAIDZ, compare the single-, double-, or triple-parity choice and group size with the documented allocation behavior and your capacity needs.
For hardware selection, verify drive interface and compatibility, capacity, intended workload, and support in the target enclosure or controller. A RAID level name alone does not establish that a particular device combination is supported.
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