Configure NAS drives for RAID by matching a supported layout to your NAS model, number of disks, usable-capacity target, and acceptable disk-failure risk. First verify the model’s supported RAID types, drive compatibility rules, and expansion limits. Then select the disks, create the storage pool or RAID group, create the required volume or dataset, and set up shares, monitoring, and a separate backup.
There is no universal “best RAID.” A two-bay NAS has different choices from an eight-bay system, and a TrueNAS ZFS pool is built differently from a Synology DSM storage pool or a QNAP QTS RAID group. The terminology and migration options also vary by firmware version and model.
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What RAID does in a NAS
RAID combines multiple physical drives into a logical storage arrangement. Depending on the layout, it can duplicate data, reserve parity information for recovery, improve sequential throughput, or simply combine capacity. The trade-off is always among usable space, failure tolerance, performance, cost, and future expansion.
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RAID protects against some drive failures; it is not a complete backup. A deleted file, ransomware, fire, theft, filesystem corruption, or an administrator mistake can affect every disk in one array. Keep at least one separate, verified copy of important data.
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Stripe (RAID 0)
A stripe writes portions of each file across several disks. It offers the combined capacity of the member disks, but there is no redundancy. TrueNAS states: “A data VDEV with a stripe layout irretrievably loses all stored data if a single disk in the VDEV fails.” Use a stripe only for disposable data or workloads where another complete copy exists and downtime is acceptable.
Mirror (RAID 1 or a mirrored VDEV)
A mirror stores identical data on two or more disks. In a TrueNAS mirror, at least two disks are required and usable capacity is approximately one disk’s capacity in that VDEV. A mirror can continue after a disk failure, but it sacrifices more capacity than parity layouts.
Parity layouts
Parity layouts calculate recovery information and distribute it across the disks. One parity disk-equivalent is reserved in a single-parity layout, two in a dual-parity layout, and three in a triple-parity layout. Recovery still depends on a healthy pool, compatible replacement media, and a successful rebuild.
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|---|---|---|---|
| Stripe | Not stated as a fixed minimum in the cited guidance | 0 | No parity or mirror overhead; one failure loses the data VDEV |
| Mirror | 2 | Depends on mirror width and which disks fail | Usable capacity is approximately one disk in a two-way mirror |
| RAIDZ1 | 3 | 1 | Approximately one disk-equivalent reserved for parity |
| RAIDZ2 | 4 | 2 | Approximately two disk-equivalents reserved for parity |
| RAIDZ3 | 5 | 3 | Approximately three disk-equivalents reserved for parity |
The RAIDZ failure counts apply to the specified TrueNAS VDEV design. They should not be transferred automatically to similarly numbered layouts on another platform.
How do I choose a RAID layout?
Start with the failure tolerance you actually need, then check whether the NAS can deliver it with the disks and bays you have. A layout that cannot be expanded or repaired under your model’s rules may be a poor choice even when its initial capacity looks attractive.
Choose by required protection
- For no-failure tolerance, a stripe is technically possible but unsuitable for irreplaceable data.
- For a small two-disk NAS, a mirror is the straightforward redundant option.
- For three or more disks in TrueNAS, RAIDZ1 tolerates one failed disk.
- For four or more disks in TrueNAS, RAIDZ2 tolerates two failed disks and reserves two disk-equivalents for parity.
- For five or more disks in TrueNAS, RAIDZ3 tolerates three failed disks and reserves three disk-equivalents for parity.
Parity is not a guarantee that every failure sequence is safe. A second disk can fail while a degraded pool is rebuilding, and an unrelated hardware or filesystem problem can still make data unavailable.
Choose by workload
Performance depends on workload, controller or CPU, network speed, drive type, block size, caching, and the implementation. Do not rely on a universal ranking such as “RAID 5 is always faster” or “mirrors are always best.” File serving, virtual machines, surveillance recording, media streaming, and backup jobs stress storage differently.
Choose by future expansion
Some systems expand by replacing every disk with a larger one; others allow adding disks to an existing group; some require a new pool or vdev. The RAID type, pool health, free bays, and exact disk requirements all constrain expansion. Confirm the supported path before buying fewer disks with the expectation that any later disk can simply be added.
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How many drives do I need for RAID?
The answer depends on the implementation, not just the RAID number. TrueNAS documents minimums of three disks for RAIDZ1, four for RAIDZ2, and five for RAIDZ3, while a TrueNAS mirror requires at least two. Synology DSM and QNAP QTS expose different combinations and may impose model-specific bay or firmware limits.
| Situation | Practical starting point | What to verify |
|---|---|---|
| Two-bay NAS needing redundancy | Two-disk mirror or the model’s equivalent | Whether the model supports the chosen mirror type and how replacement works |
| Three-disk TrueNAS pool | RAIDZ1 | Three-disk minimum and one-disk failure tolerance |
| Four-disk TrueNAS pool | RAIDZ2 or another supported layout | Whether the lower capacity of dual parity is acceptable |
| Five-disk TrueNAS pool | RAIDZ3, RAIDZ2, or a mirror design | Workload, rebuild exposure, and future expansion plan |
| Large TrueNAS array | RAIDZ is generally preferred over dRAID when a dRAID VDEV would contain fewer than 10 data-storage devices | Whether the array is large enough for the dRAID use case and whether its rebuild behavior fits your operations |
Check the NAS model and drives before creating anything
- Record the hardware. Write down the exact NAS model, number of bays, installed memory if relevant to the platform, current operating-system or firmware version, and every disk’s model, capacity, interface, and health status.
- Read the model specification. Confirm supported RAID types, maximum drive count, expansion methods, and whether storage pools, volumes, datasets, or RAID groups are separate objects.
- Use the compatibility list. Vendors can restrict supported drives by model, capacity, interface, or firmware. Synology advises checking its product compatibility list, and its storage-pool guidance requires pool drives to be the same drive type. “Same drive type” is different from “same capacity”: SHR can optimize volume size when different capacities are combined, subject to the specific model’s rules.
- Check drive health. Do not build a new pool around a disk reporting SMART errors, an unstable link, or an unknown history. A disk that passes a quick test can still fail during a long parity build, so test and monitor all media before trusting the pool.
When shopping, search for compatible NAS hard drives or drives for a NAS only after checking the exact model’s approved list. A drive sold for NAS use is not automatically supported by every enclosure.
Capacity planning before you click Create
Use the vendor’s capacity calculator or the estimate shown in the management interface. Raw capacity is not the same as usable space: mirrors duplicate data, and parity layouts reserve one, two, or three disk-equivalents. Filesystem metadata, reserved space, snapshots, and system overhead reduce the amount available to shares.
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In many parity arrangements, the smallest disk limits the usable contribution of larger disks in the same group. QNAP’s RAID comparison documentation highlights the effect of mismatched capacities. Do not calculate capacity by simply adding the label sizes of all disks. Check how the NAS handles a larger replacement disk and whether unused capacity can be reclaimed later.
Capacity example
Four equal disks in a TrueNAS RAIDZ2 design provide roughly two disks’ worth of raw data capacity before filesystem overhead because two disk-equivalents are used for parity. The exact displayed value depends on decimal versus binary units and the platform’s reservations. A two-way mirror made from two equal disks provides roughly one disk’s worth of usable capacity.
Platform differences: TrueNAS, Synology DSM, and QNAP QTS
TrueNAS and ZFS
TrueNAS organizes storage as a pool containing one or more VDEVs. You select disks and create a VDEV layout such as mirror, RAIDZ1, RAIDZ2, RAIDZ3, stripe, or, for specific very large arrays, dRAID. A pool’s reliability and performance depend on the VDEV design; adding a second VDEV does not turn an existing stripe into a redundant layout.
TrueNAS documentation currently recommends RAIDZ generally and specifically favors RAIDZ when a dRAID VDEV would contain fewer than 10 data-storage devices. Treat that as platform guidance, not a universal rule for other NAS operating systems.
Synology DSM
Synology separates the storage pool from the volume created on that pool. The pool holds the RAID arrangement; the volume is where shared folders and applications use space. DSM offers conventional RAID types and Synology Hybrid RAID (SHR). Synology describes SHR as automated RAID management that can optimize volume size when drives of different sizes are combined, but support and expansion behavior depend on the model.
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Do not assume that an SHR pool can be imported into a ZFS system or that a conventional RAID group can be changed to SHR without rebuilding. Follow the model-specific migration and expansion procedure shown by DSM.
QNAP QTS
QNAP documents RAID 1, 5, 6, 10, 50, 60, and additional types depending on the product and use case. QTS may present a storage pool, RAID group, and volume as separate choices. Its terminology and supported operations are not interchangeable with a TrueNAS VDEV workflow, so follow the QTS wizard for the exact enclosure and firmware.
Step-by-step: create a new NAS RAID configuration
1. Make a separate backup
Copy important files to another device or service and verify that the copy can be opened. Creating a pool can erase selected disks, and a rebuild or migration is not a substitute for a second copy.
2. Confirm the target disks
In the storage-management screen, identify unused, eligible disks by bay number and serial or model information. Remove any disk that is incompatible, unhealthy, or still contains data you need.
3. Select the storage object
Open the NAS’s storage or pool-management area. TrueNAS asks you to build a pool from one or more VDEVs; DSM generally asks you to create a storage pool and then a volume; QTS may ask you to create a RAID group or storage pool before a volume.
4. Choose the layout
Select the supported RAID or SHR option that matches your failure target and capacity plan. Read the wizard’s warning about minimum disks, parity, usable capacity, and data destruction. If the desired layout is absent, stop rather than substituting a similarly named option.
5. Review the estimate
Check the listed disks, estimated usable capacity, redundancy level, and any warning about unequal sizes. Confirm that the selected disks are in the intended bays and that no disk containing the only copy of a file is included.
6. Create and wait for initialization
Confirm creation. Initializing, formatting, parity calculation, or resilvering can run for many hours and can reduce performance while active. Keep the NAS powered and connected to reliable power, and do not remove disks unless the vendor’s procedure explicitly calls for it.
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7. Create the filesystem layer and shares
After the pool is healthy, create the required TrueNAS dataset, Synology volume, or QNAP volume. Then create shared folders, users, permissions, snapshots where supported, and backup jobs. A pool by itself does not create a usable network share.
8. Enable monitoring
Configure disk-health alerts, pool-degraded notifications, capacity warnings, and scheduled scrubs or consistency checks offered by the platform. Record the bay order and replacement procedure so another administrator can act during an outage.
Replacing a failed disk and expanding later
Replacing a failed disk
First confirm which physical disk is failed by matching the NAS alert to the bay indicator and serial number. Use a disk that meets the platform’s compatibility and capacity requirements. Follow the vendor’s replace or repair action, then monitor the resilver, rebuild, or reconstruction until the pool returns to a healthy state.
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Expanding capacity
Expansion can mean adding disks, replacing disks one by one with larger models, extending a RAID group, adding a VDEV, or creating a new pool. The permitted method depends on RAID type, current health, available bays, disk-size rules, and firmware. A larger disk may initially contribute only the capacity of the old member until every member is replaced, and some layouts cannot use the extra space without a separate expansion step.
Read the model’s expansion instructions before purchasing replacement disks. Never assume that adding a disk to a pool is supported merely because an empty bay is available.
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Creating a stripe for valuable files
Symptom: the wizard shows maximum capacity and little or no redundancy. Fix: cancel and choose a mirror or parity design, or proceed only when the data is disposable and a verified independent copy exists. Changing an existing stripe usually requires moving the data and recreating storage.
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Symptom: a disk is rejected, the pool option is unavailable, or the estimated capacity is lower than expected. Fix: check the exact compatibility list, drive type, interface, firmware, and minimum capacity. Do not force an unsupported disk into service.
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Assuming RAID is backup
Symptom: a deleted or encrypted file cannot be recovered even though every disk is healthy. Fix: restore from a separate backup and test that backup periodically. RAID handles selected hardware failures; it does not preserve historical versions or protect against every incident.
Starting a rebuild without enough free space or time
Symptom: the pool remains degraded, performance is poor, or a replacement disk cannot be accepted. Fix: verify that the replacement meets the vendor’s size rule, keep the system available for the complete rebuild, and avoid heavy nonessential workloads until health is restored.
Expecting cross-platform portability
Symptom: disks from one NAS are not recognized as an importable pool by another. Fix: treat TrueNAS VDEVs, Synology pools, and QNAP RAID groups as platform-specific formats. Plan a file-level migration with temporary storage instead of moving disks blindly.
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Questions to answer before finalizing the pool
- How many disks can the enclosure hold today, and how many will remain free after creation?
- Can the chosen layout survive the number of simultaneous disk failures your data requires?
- What is the usable capacity after parity, mirroring, filesystem reservations, and snapshots?
- Does the vendor support adding disks, replacing disks with larger ones, or both?
- Are all selected disks on the model’s compatibility list and the same required drive type?
- Where is the independent backup, and has a restoration actually been tested?
Frequently Asked Questions
Can I convert an existing single disk or RAID layout to another RAID type without deleting data?
There is no vendor-neutral guarantee. Conversion and migration behavior depends on the NAS model, firmware, filesystem, and target layout. Plan a verified backup and, when required, a file-level migration to newly created storage.
Is a hot spare the same as backup?
No. A hot spare is an extra disk that may accelerate replacement or rebuilding after a failure. It does not provide an independent copy of files or protection from deletion, corruption, ransomware, or site loss.
Can I use different-sized disks in one NAS pool?
Sometimes, but the rules vary. Synology SHR is designed to optimize volume size with different capacities, while pool drives still must meet the model’s drive-type requirement. Other layouts may limit each disk to the smallest member’s usable size. Check the exact compatibility and capacity guidance for your model.
Should I choose RAIDZ2 instead of RAIDZ1 for a TrueNAS pool?
Choose according to the number of failures you must tolerate, the capacity you can surrender to parity, and your rebuild exposure. TrueNAS RAIDZ1 tolerates one failed disk; RAIDZ2 tolerates two and requires at least four disks. The safer choice for one workload may not be the best capacity choice for another.
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What should I do if the NAS reports a degraded pool?
Identify the affected disk from the alert and bay indicator, verify its serial number, confirm that the pool is actually degraded, and follow the platform’s documented replace or repair action. Do not remove another disk speculatively; monitor the rebuild until the pool reports healthy.
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