RAIDZ1
What Is RAIDZ1?
RAIDZ1 is a ZFS RAIDZ layout that provides single-parity protection. A RAIDZ1 VDEV can tolerate the failure of one member device without losing access to the data stored in that VDEV.
If a second device in the same RAIDZ1 VDEV becomes unavailable before redundancy is restored, the VDEV can fail. Because top-level data VDEVs are fundamental components of a zpool, losing a data VDEV can result in loss of the pool.
How Does RAIDZ1 Work?
RAIDZ distributes data and parity information across the devices in the VDEV. RAIDZ1 uses the equivalent of one device's worth of parity protection across the group.
Data Is Written
ZFS writes data across the devices participating in the RAIDZ1 VDEV.
Parity Is Generated
Parity information is calculated so missing data can be reconstructed after one device failure.
Data & Parity Are Distributed
RAIDZ does not dedicate one physical disk permanently as the parity disk. Data and parity are distributed across the VDEV.
One Failure Can Be Reconstructed
If one member device fails, ZFS can reconstruct the missing information from the surviving data and parity.
Example RAIDZ1 VDEV
Consider four equal-size drives arranged as one RAIDZ1 VDEV. Capacity equivalent to approximately one drive is devoted to parity before additional filesystem and storage overhead.
How Many Drives Does RAIDZ1 Need?
RAIDZ1 requires multiple storage devices because both data and parity must be distributed across the VDEV. A minimum of three devices is commonly used to create a practical RAIDZ1 layout.
Larger RAIDZ1 VDEVs can provide greater usable capacity, but increasing the number and capacity of drives also increases the amount of data exposed while the VDEV is operating without full redundancy after a failure.
RAIDZ1 Usable Capacity
For a simplified group of equal-size drives, RAIDZ1 usable raw capacity can be approximated as the combined capacity of all drives minus approximately one drive's worth of parity.
N represents the number of equal-size devices in the RAIDZ1 VDEV. Actual usable space is lower because filesystem, metadata, allocation behavior, units and operational free space must also be considered.
RAIDZ1 Capacity Examples
| Drives | Drive Size | Raw Capacity | Simplified RAIDZ1 Capacity | Drive Failures Tolerated |
|---|---|---|---|---|
| 3 | 4 TB | 12 TB | ≈ 8 TB before overhead | 1 |
| 4 | 8 TB | 32 TB | ≈ 24 TB before overhead | 1 |
| 5 | 12 TB | 60 TB | ≈ 48 TB before overhead | 1 |
| 6 | 16 TB | 96 TB | ≈ 80 TB before overhead | 1 |
These examples are intentionally simplified and should not be treated as exact formatted capacity calculations.
RAIDZ1 Can Tolerate One Device Failure
RAIDZ1 provides one-device failure tolerance within the VDEV. After one device fails, the VDEV is operating without its normal redundancy until the missing device is replaced and the required data is restored.
A second device failure in the same RAIDZ1 VDEV during that vulnerable period can exceed the VDEV's parity protection. This is one reason larger-capacity drives and critical storage environments may justify considering RAIDZ2 or RAIDZ3 instead.
What Happens After a RAIDZ1 Drive Fails?
When a failed device is replaced, ZFS restores the information required on the replacement device through a process commonly called resilvering.
The time required depends on factors such as the amount of allocated data, drive performance, workload activity and storage hardware. During this period, RAIDZ1 does not have another parity level available to tolerate an additional device failure in that VDEV.
Is RAIDZ1 the Same as RAID 5?
No. RAIDZ1 and RAID 5 are often compared because both provide single-parity protection, but they are not the same storage implementation.
RAIDZ is part of ZFS and works within ZFS's integrated storage and filesystem architecture. Traditional RAID 5 is generally implemented as a block-level RAID array beneath a separate filesystem. Their data layout, write behavior and data-integrity mechanisms therefore differ.
RAIDZ1 vs RAIDZ2
| Characteristic | RAIDZ1 | RAIDZ2 |
|---|---|---|
| Parity Level | Single parity | Double parity |
| Device Failures Tolerated | 1 per VDEV | 2 per VDEV |
| Capacity Efficiency | Higher | Lower because more capacity is used for parity |
| Failure Protection | Lower | Higher |
| Typical Decision | Favor capacity efficiency when one-device tolerance is acceptable | Favor additional failure tolerance for more critical storage |
When Can RAIDZ1 Make Sense?
When Should You Consider RAIDZ2 Instead?
RAIDZ2 provides additional protection by allowing two member devices in the VDEV to become unavailable without exceeding its parity tolerance.
It may be preferable for larger-capacity drives, larger VDEVs, important production data, systems where replacement may be delayed, or environments where the risk of another failure during resilvering is unacceptable.
RAIDZ1 Performance Considerations
RAIDZ1 performance depends on the member drives, VDEV width, workload, ZFS configuration, memory, record sizes and the number of data VDEVs in the pool.
Should RAIDZ1 Drives Be the Same Size?
Using drives of equal or closely matched capacity simplifies RAIDZ planning and avoids wasting capacity. In a mixed-size VDEV, larger devices may not contribute all of their physical capacity if smaller devices limit the effective layout.
Drives should also be selected based on more than capacity. Performance, endurance, interface type, sector characteristics and intended workload should be considered.
RAIDZ1 Drive Technologies
RAIDZ1 Is Not a Backup
RAIDZ1 provides storage redundancy against a single member-device failure, but it does not protect against every cause of data loss. Accidental deletion, application corruption, administrative errors, malware, theft, catastrophic hardware damage and site-level events can still affect the data.
Important data should be protected by an independent backup strategy designed around the required recovery objectives.
RAIDZ1 FAQ
What is RAIDZ1?
RAIDZ1 is a ZFS VDEV layout using single-parity protection.
How many drives can RAIDZ1 lose?
A RAIDZ1 VDEV can tolerate one unavailable member device.
Is RAIDZ1 the same as RAID 5?
No. Both use single-parity concepts, but RAIDZ is part of ZFS and differs from conventional RAID 5 in architecture and data handling.
How much capacity does RAIDZ1 lose to parity?
A simplified equal-drive model allocates approximately one drive's worth of raw capacity to parity before additional overhead.
Is RAIDZ1 safe?
RAIDZ1 provides single-device fault tolerance. Whether that protection level is appropriate depends on drive size, VDEV width, workload, recovery time and the value of the data.
Is RAIDZ2 safer than RAIDZ1?
RAIDZ2 provides an additional parity level and can tolerate two device failures within the VDEV.
Does RAIDZ1 need a dedicated parity disk?
No. RAIDZ distributes parity and data across the member devices rather than assigning one permanent parity disk.
What happens when a RAIDZ1 disk fails?
The VDEV operates in a degraded state until the device is replaced and redundancy is restored through resilvering.
Can RAIDZ1 use SSDs?
Yes. RAIDZ1 can be built from supported SSDs or other block storage devices, subject to the platform and ZFS implementation.
Is RAIDZ1 a backup?
No. RAIDZ1 is storage redundancy and should not replace an independent backup strategy.