For a small home ZFS or NAS system, the practical starting point is two identical data drives in a mirror. Choose three or more drives for RAIDZ parity: RAIDZ1 uses one parity drive, RAIDZ2 uses two, and RAIDZ3 uses three. Your complete build also needs a 64-bit host, memory, a dedicated SSD boot device, enough SATA or SAS connections, a suitable case and power supply, cooling, network connectivity, and a separate backup.
There is no single correct drive count. The right number depends on usable capacity, how many drive failures you want to survive, the workload, drive size, replacement time, and whether you can expand the pool later. The figures below use OpenZFS and TrueNAS documentation, including the TrueNAS 25.10 hardware baseline.
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Drive-count quick reference
Each RAID layout trades capacity, protection and performance differently. The table shows nominal capacity before filesystem overhead for equal-sized drives.
| Layout | Minimum drives | Approximate usable capacity | Drive failures tolerated | Best fit |
|---|---|---|---|---|
| Two-way mirror | 2 | About one drive | 1, provided the other drive is healthy | Simple home storage, virtual machines and small random reads |
| RAIDZ1 | 2 by the mathematical minimum; 3 is the practical starting point | (number of drives − 1) × one drive | 1 | Capacity-focused pools where single-drive protection is acceptable |
| RAIDZ2 | 3 by the mathematical minimum; 4 or more is more useful | (number of drives − 2) × one drive | 2 | Important data and larger pools needing stronger protection |
| RAIDZ3 | 4 | (number of drives − 3) × one drive | 3 | Large or especially risk-sensitive pools |
OpenZFS defines the minimum width as one more device than the number of parity devices and recommends 3–9 devices in a RAIDZ group. TrueNAS advises not exceeding 12 disks per vdev and also recommends 3–9. These are layout guidelines, not guarantees of a particular performance level.
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How many drives do you need?
Two drives: a mirror
A two-drive mirror writes the same data to both devices. If one drive fails, the pool remains available from the surviving drive. Its nominal usable capacity is the size of one drive, so two 8 TB drives provide roughly 8 TB before filesystem overhead rather than 16 TB.
Mirrors consume more raw capacity than parity layouts, but TrueNAS generally describes them as better for small random reads. That makes a mirror a sensible first pool for documents, photos, application data or virtual machines when simplicity and responsiveness matter more than maximum capacity efficiency. You can later add another mirror vdev to a pool, although expansion options depend on the storage platform and pool design.
Three drives: RAIDZ1
Three equal drives in RAIDZ1 dedicate roughly one drive’s worth of space to single parity, leaving about two drive capacities for data. A three-drive set therefore tolerates one failed device. OpenZFS permits a two-device RAIDZ1 group mathematically, but three drives produce a more useful balance of data and parity.
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TrueNAS warns against RAIDZ1 with drives larger than 1 TiB. Treat that as TrueNAS guidance rather than a universal rule: the risk of a second failure during replacement depends on drive workload, age, error rates, replacement speed and the importance of the data. If a second failure during a rebuild would be unacceptable, use RAIDZ2 or mirrors plus a strong backup plan.
Four drives: RAIDZ2 or a pair of mirrors
Four drives in RAIDZ2 provide about two drive capacities of nominal space and tolerate any two device failures in the vdev. This is a common protection-oriented starting point for a small NAS.
The same four drives can instead be arranged as two mirror vdevs. That also provides roughly two drive capacities, but failure tolerance depends on which disks fail: one disk can fail in each mirror, while losing both members of the same mirror loses that vdev. Mirrors typically offer stronger small-block random performance, whereas RAIDZ2 uses parity more efficiently for sequential storage.
Six drives: a wider RAIDZ2 group
Six equal drives in RAIDZ2 provide approximately four drive capacities and tolerate two failures. For six 12 TB drives, the simple planning estimate is about 48 TB before formatting and ZFS overhead. Actual available space can be lower because RAIDZ allocation depends on sector size, record size and stripe layout; small records can increase parity overhead.
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RAIDZ3 and larger groups
RAIDZ3 reserves three parity devices and needs at least four drives. A seven-drive RAIDZ3 group, for example, has an approximate data capacity of four drives and can survive three device failures. Triple parity is most relevant when a pool contains many high-capacity drives, replacement may be slow, or the cost of losing availability is high.
OpenZFS recommends keeping a RAIDZ group between three and nine devices. TrueNAS says a vdev should not exceed 12 disks and asks administrators to consider resilver time as vdevs grow. Wider groups can hold more data, but a failed high-capacity disk may take longer to reconstruct and leaves less margin for another failure during that process.
How RAIDZ capacity is calculated
For equal-sized drives, the planning formula is approximately (N − P) × drive capacity, where N is the number of devices and P is the number of parity devices. RAIDZ1 has P=1, RAIDZ2 has P=2 and RAIDZ3 has P=3.
| Example | Raw capacity | Nominal RAIDZ capacity | Protection |
|---|---|---|---|
| 3 × 8 TB RAIDZ1 | 24 TB | About 16 TB | 1 failed drive |
| 4 × 8 TB RAIDZ2 | 32 TB | About 16 TB | 2 failed drives |
| 6 × 12 TB RAIDZ2 | 72 TB | About 48 TB | 2 failed drives |
| 7 × 12 TB RAIDZ3 | 84 TB | About 48 TB | 3 failed drives |
These are nominal figures, not the formatted space shown to applications. ZFS needs space for metadata, and allocation varies with record and sector layout. Leave free capacity for snapshots and normal operation instead of planning to fill the calculated total.
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Choosing between mirrors and RAIDZ
Compare the layouts on four questions before buying disks.
- How many failures must the pool survive? A mirror survives one failed member; RAIDZ1, RAIDZ2 and RAIDZ3 survive one, two and three failed devices respectively.
- How much usable capacity is required? Mirrors give roughly half of raw capacity per two-disk vdev. RAIDZ uses one, two or three parity devices per group, improving capacity efficiency as the group widens.
- What is the workload? TrueNAS notes that mirrors generally perform better for small random reads. RAIDZ is often attractive for large sequential files, media libraries and backups, but the actual result depends on record size, clients and applications.
- How will you expand? ZFS pools are built from vdevs. You can combine multiple vdevs in one pool, but changing a vdev’s structure later is constrained by the platform. Plan the initial width and future replacement strategy before purchase.
For a turnkey NAS, the vendor’s current model documentation matters. Synology’s support material, for example, describes model-specific RAID minimums and expansion restrictions. Do not assume that a RAID layout or expansion path available on one NAS model is supported on another; check the exact compatibility list and storage-manager documentation.
Understanding vdevs and pools
In ZFS, a vdev is a group of disks arranged as a mirror or RAIDZ group. A pool combines one or more vdevs and presents their combined storage to datasets and shares. The pool’s redundancy is the redundancy of its vdevs: if one vdev is lost, the pool is unavailable even when other vdevs are healthy.
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Drives within a vdev should normally be equal in size. TrueNAS supports differing capacities, but each device contributes only as much as the smallest device allows. Mixing a 12 TB disk with 8 TB disks in one vdev therefore wastes the extra capacity on the larger disk. Use matching models or at least matching usable capacities whenever possible.
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Hardware checklist for a home NAS
Storage drives
Buy drives intended for continuous operation and the expected RAID workload. TrueNAS says ordinary desktop SATA drives are not designed or warrantied for continuous operation or RAID groups. NAS-rated families are specified for different enclosure sizes; its hardware guide gives WD Red Plus as designed for up to eight drives, WD Red Pro for up to 16, and WD UltraStar for systems beyond 16. Those limits are vendor- and model-specific, so verify the current manufacturer specification for the exact SKU.
Keep at least one tested spare or a realistic replacement plan for important pools. A replacement must be at least as large as the failed device’s required capacity; nominal labels do not always translate to identical usable sectors, so matching models simplify replacement.
CMR versus SMR recording
Check whether each hard drive uses conventional magnetic recording (CMR) or shingled magnetic recording (SMR). TrueNAS cautions that SMR can produce slower writes and less reliable, longer ZFS resilvers. Its guidance says DM-SMR and HA-SMR function poorly and that HM-SMR is unsupported. Product-family names alone may not identify the recording method; check the exact model number on the manufacturer’s specification page before buying.
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TrueNAS 25.10 lists a minimum of a 2-core 64-bit Intel processor or AMD x86_64 processor and 8 GB of memory. That is a platform baseline, not a universal build recommendation. Add memory for applications, virtual machines, encryption, many simultaneous users or large caches. Processor needs also rise when the system transcodes media, runs containers or performs heavy compression.
Boot device
The same TrueNAS 25.10 guide specifies a 16 GB SSD boot device and discourages using a spinning disk or USB stick for boot. Keep the operating system on a separate device from the data pool so a data-disk replacement does not also remove the boot environment. Maintain a copy of the NAS configuration outside the machine.
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Drive connections and controllers
Count the motherboard’s compatible SATA or SAS ports before selecting a layout. If you need more ports, an HBA can provide additional connections. Verify that the card supports your operating system, has suitable firmware and exposes disks individually.
When a hardware RAID controller is used with ZFS, TrueNAS recommends JBOD or pass-through operation so ZFS controls the individual drives. A controller that hides disks behind a proprietary virtual volume can prevent ZFS from seeing drive health and managing redundancy correctly. Confirm passthrough behavior, cabling, cooling and operating-system support before purchasing an HBA or controller.
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Select a case with enough physical bays for the initial drives and planned replacements. Check whether it accepts 3.5-inch or 2.5-inch devices, provides vibration control and leaves room to route data and power cables without blocking airflow.
Size the power supply from the host, controller, fans and every installed drive. Hard disks draw their highest startup current when spinning up, so use the manufacturer’s peak figures rather than only idle consumption. Provide direct airflow across the drive cage and monitor temperatures after installation. There is no universal case, cooling or wattage specification: the correct choice depends on the exact parts and local electrical conditions.
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A UPS can keep the NAS running through short outages and allow an orderly shutdown. Choose capacity from the system’s measured draw and the runtime you want; no single VA or runtime figure fits every build. A UPS does not replace backups.
Choose network speed according to clients and workload. A faster adapter helps only when the NAS, switch, cabling and client devices all support that speed. Large sequential transfers can benefit from faster links, while many home workloads are limited by disk layout, client storage or network contention.
A practical build sequence
- Define the data requirement. List current data, expected growth, snapshot space and the amount of free capacity you want to keep. Convert that total into nominal drive capacity using the mirror or RAIDZ formula, then add headroom.
- Choose the failure policy. Decide whether one, two or three simultaneous drive failures must be survivable. If the answer is two, do not build RAIDZ1 simply because it offers more nominal space.
- Choose a vdev width. Keep RAIDZ groups within the OpenZFS 3–9-device recommendation where practical, and account for the TrueNAS guidance not to exceed 12 disks per vdev.
- Check exact drive models. Confirm capacity, CMR or SMR technology, workload rating and the NAS platform’s compatibility list. Buy equal-sized devices for each vdev.
- Verify the host. Confirm the processor architecture, memory, SSD boot device, available drive ports, HBA compatibility, case bays, power budget and cooling path.
- Install and identify disks. Label physical bays and record serial numbers. Use the NAS operating system’s disk identifiers rather than relying only on bay order.
- Create the pool deliberately. Select mirror, RAIDZ1, RAIDZ2 or RAIDZ3 only after checking the displayed usable capacity and redundancy. Do not confuse a pool with a backup.
- Run health checks. Enable drive monitoring, schedule scrubs according to the platform’s guidance, and test alerts before placing important files on the system.
- Set up backup and recovery. Configure snapshots and automated replication, then perform a test restore to a separate system or location.
What happens when a drive fails?
Replacing a failed drive in a mirror or RAIDZ vdev starts a process TrueNAS calls resilvering. ZFS reconstructs the blocks that are in use and can resume an interrupted resilver. The elapsed time is not fixed: it varies with drive size, pool occupancy, workload, controller, vdev width and concurrent activity.
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A pool remains in a more vulnerable state during resilvering. TrueNAS notes that another disk can fail before a RAIDZ resilver completes. Keep the replacement drive ready, avoid unnecessary heavy workloads during recovery, and make sure the backup copy is current before a failure occurs. Never remove a healthy disk merely because it is older without first confirming the pool status and replacement procedure.
RAID is not a backup
RAID and disk redundancy protect against selected drive failures, not accidental deletion, malware, theft, fire, controller faults, filesystem mistakes or corruption that is replicated across all disks. TrueNAS explicitly states that RAID and disk redundancy are not substitutes for a reliable backup strategy.
Use snapshots for point-in-time recovery and automated replication to another system. Keep at least one copy separate from the NAS, and for irreplaceable files keep a copy in another physical location. Test restores: a backup that has never been restored is an assumption, not a verified recovery plan.
Worked planning examples
Small household file server
For documents, photos and a few computers, two identical 8 TB CMR NAS drives in a mirror are the simplest starting point. Plan for about 8 TB nominal capacity, one-drive failure tolerance, a two-core 64-bit host, at least 8 GB of memory, a 16 GB SSD boot device and a separate backup destination. Add a second mirror vdev only after confirming the pool’s expansion method and backup capacity.
Media and backup archive
A six-drive RAIDZ2 group can provide about four drive capacities while tolerating two failures. Six 12 TB drives yield a nominal planning figure of about 48 TB before filesystem overhead. This layout favors capacity and sequential transfers, but keep free space for snapshots and expect a high-capacity replacement to require a potentially lengthy resilver.
High-value data with slow replacement access
If obtaining a replacement drive may take days and the data cannot tolerate a second failure, use RAIDZ2 or RAIDZ3 rather than RAIDZ1. Match the parity level to the number of devices, keep a tested spare if practical, and maintain an independent backup so the pool is not the only copy.
Common mistakes to avoid
- Buying drives first and discovering that the case or motherboard lacks enough bays or ports.
- Mixing unmatched capacities in one vdev and losing the larger drive’s extra space.
- Using SMR drives without checking their ZFS resilver behavior.
- Putting ZFS behind a controller that does not expose disks in JBOD or pass-through mode.
- Treating the TrueNAS minimum hardware table as sufficient for virtual machines, encryption or many users.
- Filling the pool to its calculated nominal capacity and leaving no room for snapshots or normal allocation.
- Assuming a RAID level can always be expanded or converted in place.
- Skipping configuration backups, monitoring and restore tests.
- Confusing a UPS with a backup system.
The practical answer
Buy two identical CMR NAS drives and use a mirror when you want the simplest two-drive system and good small-file responsiveness. Choose at least three drives for RAIDZ1 only when single-drive protection and its capacity advantage match your risk tolerance. Choose four or more drives for RAIDZ2 when two-drive failure tolerance matters, and consider RAIDZ3 for larger or especially critical pools. Build around the exact NAS software, vdev width, drive technology, replacement plan and backup strategy rather than a headline capacity number.
Quick Recap
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