RAID 1, also known as disk mirroring, is a redundancy method that duplicates all data from one hard disk to a second hard disk in real-time. This method provides fault tolerance and improves performance for read operations, but does not improve write performance.
What are the benefits of RAID 1?
There are several key benefits to using RAID 1 disk mirroring:
- Data redundancy – If one drive fails, the system can instantly switch to the other mirrored drive without any loss of data or service interruption. This provides excellent protection against hardware failures.
- Improved read performance – Reads can be distributed across both drives, effectively doubling read speeds.
- Easy to implement – Setting up RAID 1 is straightforward and does not require any special hardware.
What are the drawbacks of RAID 1?
There are also some potential downsides to consider with RAID 1:
- No write performance gains – Write speeds remain the same as a single disk because data has to be written twice.
- Higher hardware costs – You need twice the storage capacity to mirror drives.
- No protection against software failures or human error – Viruses, software bugs, accidental deletion etc. will be mirrored to both drives.
How does RAID 1 work?
The RAID 1 mirroring process works in the following way:
- Data is written to the primary hard disk.
- The RAID controller duplicates this data and writes it to the secondary disk in real-time.
- If the primary disk fails, the system transparently switches to the secondary mirrored disk.
- The failed disk can be replaced and the mirror rebuilt to the new drive.
This constant duplication results in 100% redundancy between the two drives at all times. If one drive fails, the system can instantly cut over to the other with zero downtime and no data loss.
What is the minimum number of hard disks for RAID 1?
RAID 1 requires a minimum of 2 hard disks in order to provide mirroring and redundancy. One disk contains the primary copy of the data, while the second disk contains the mirrored copy.
Some RAID 1 implementations allow you to have more than 2 disks. This is referred to as RAID 10. The additional disks can enhance performance, but 2 disks is the absolute minimum to get data redundancy benefits.
Can RAID 1 use different size hard disks?
Yes, RAID 1 can use two hard disks of different sizes. However, the total capacity available will be equal to the smaller drive.
For example, if you had a 120GB drive and a 240GB drive, the total usable RAID 1 space would be 120GB. The extra capacity on the larger drive would not be accessible and would go unused.
For best performance and capacity utilization, it is recommended to use two identical drives for RAID 1 configurations.
Can you have more than 2 hard disks in RAID 1?
Yes, it is possible to have more than 2 hard disks in a RAID 1 array. This is sometimes referred to as RAID 10.
In a RAID 10 configuration, you would mirror pairs of disks. For example, with 4 disks you would have 2 mirrored pairs. This provides the redundancy of RAID 1 plus the improved read performance of spreading reads across multiple disks.
The total usable capacity in a RAID 10 is 50% of the total disks, just like a standard 2-disk RAID 1. However, performance scales better by mirroring across multiple disks.
What types of RAID offer redundancy other than RAID 1?
There are several other standard RAID levels that offer redundancy and fault tolerance:
- RAID 5 – Block-level striping with distributed parity data across all disks. Can withstand one disk failure without data loss.
- RAID 6 – Block-level striping with double distributed parity data. Can handle up to two disk failures.
- RAID 10 – Stripe of mirrored disk pairs. Allows some disks to fail without data loss.
- RAID 50/60 – Combination of RAID 5/6 and 0 for larger arrays.
Of all the redundant RAID levels, RAID 1 is the simplest to implement and provides the fastest mirroring. But other options like RAID 5 and 6 offer more cost-efficient redundancy for larger arrays.
Can you convert a RAID 0 array to RAID 1?
Yes, it is possible to convert from a RAID 0 array to a RAID 1 array. However, this will involve the following steps:
- Backup all data from the RAID 0 array.
- Add capacity by attaching additional disks.
- Create a RAID 1 array with the new larger capacity.
- Restore the backup to the new RAID 1 array.
Because RAID 0 has no redundancy, the data must be backed up first before changing the RAID level. The capacity also needs to be doubled to support RAID 1 mirroring. So there is some downtime involved in migrating arrays.
What happens if only one disk fails in a RAID 1 array?
If one disk fails in a RAID 1 array, the system will instantly switch to using the surviving mirror disk with no interruption in service or loss of data. All reads and writes will continue as normal, serviced by the remaining disk.
The failed disk should be replaced as soon as possible, and the RAID controller will automatically rebuild the mirror to the new disk after replacement. During the rebuild process, the array is still vulnerable to a second disk failure. So the faulty disk should be swapped out promptly.
Can RAID 1 arrays suffer data loss? When does that happen?
Yes, RAID 1 arrays can still suffer data loss in certain failure scenarios:
- If both mirrored disks fail simultaneously, all data will be lost. The mirror provides redundancy but no backup.
- If there is a failure during a rebuild, such as the second disk failing before the new mirror is restored, data can be corrupted or lost.
- Buggy firmware, a virus, or human error could replicate corrupted data to both mirrored disks.
To fully protect against data loss, RAID 1 should be combined with a solid backup strategy and recovery process. The mirror protects against hardware faults, but not against software or user errors.
How long does it take to rebuild a RAID 1 array after drive failure?
The time it takes to rebuild a RAID 1 array after a drive failure depends on several factors:
- Size of the disks
- Performance of the disks and RAID controller
- Amount of load on the system
In general, rebuilding a 1TB drive could take 2-5 hours. Rebuilding larger 3-4TB drives could take 6-10 hours. Heavy activity during the rebuild will also prolong the process.
To minimize downtime, replace failed drives promptly. Use higher performing components and avoid taxing the system during rebuilds.
Can you access data during a RAID 1 rebuild?
Yes, data remains fully accessible and operational during a RAID 1 rebuild after a disk failure. The surviving disk remains online and services all read and write requests as normal.
The rebuild process duplicates data from the operational drive to populate the new replacement drive. This occurs in the background without any disruption to normal use. Access remains unaffected.
Does RAID 1 require identical drives? Can they be different makes and models?
Strictly speaking, RAID 1 does not require identical drives. It is possible to mirror different makes, models, and sizes of drives.
However, for best performance and stability, identical drives are strongly recommended. Mixing drives from different manufacturers or with different speeds and capacities can cause a number of issues:
- The array capacity is limited to the smallest drive size.
- Faster drives may need to throttle down to match slower speeds.
- Different error rates and drive ages may increase rebuild times and likelihood of failure during rebuilds.
- Different drive firmware versions could lead to compatibility issues.
Some RAID controllers support mixing drive types, but come with the above caveats. Consulting your controller documentation is advised.
Can you have a 2 drive RAID 1 array with different size drives?
Yes, it is technically possible to create a 2-drive RAID 1 array with different capacity drives. However, there are some significant downsides to this approach:
- The total capacity will be equal to the smaller drive. The excess capacity of the larger drive will not be used.
- Performance will be limited by the slower of the two drives.
- Rebuilds will take longer and have a higher risk of failure.
- More prone to compatibility and firmware issues.
While not recommended, a RAID 1 with mixed drive sizes can work in less critical roles where redundancy is more important than capacity or peak performance.
What happens if I lose power to a RAID 1 array?
Losing power to a RAID 1 array will shut down the disks safely once battery backup power is depleted. No data loss will occur.
When power is restored, the RAID controller will start up normally and the disk mirror will resynchronize any cached data that was not written out during the power loss. The resync should complete quickly once power is back online.
To avoid issues, use an Uninterruptible Power Supply (UPS) to keep RAID systems up for a short time after power failures.
Can RAID 1 arrays be accessed without a RAID controller?
RAID 1 arrays are dependent on a RAID controller for the mirroring logic that keeps both disks synchronized. Without the controller, the disks are simply two independent drives.
However, the disks can potentially be accessed independently without the RAID controller:
- The primary disk should still have all data intact and accessible as a standalone drive.
- The second mirror disk should also have a complete copy of all data that you may be able to access directly.
This would allow you to potentially recover data if the controller fails. But you lose the redundancy and have to rely on the disks independently.
Can you implement RAID 1 via software?
Yes, RAID 1 can be implemented through software without a hardware RAID controller. This is known as “software RAID”.
Operating systems like Windows, Linux, and macOS include software RAID functionality. The OS manages the mirroring and synchronization process between the disks.
The main advantages of software RAID 1 are:
- Lower cost since a hardware RAID controller is not required.
- Flexibility to manage and configure the RAID through software.
The disadvantages are:
- Higher CPU overhead to manage the RAID in software.
- Lack of battery-backup cache on most motherboards.
- No firmware optimizations or custom features that hardware controllers provide.
For home use or small arrays, software RAID 1 works well. But hardware RAID controllers are recommended for mission critical setups.
How do you monitor the health of RAID 1 arrays?
To monitor the health of RAID 1 arrays, you should check the following:
- Review controller logs for any detected errors or warnings.
- Check the synchronization status – ensure mirrors are up-to-date.
- Monitor SMART drive statistics via controller or OS tools.
- Watch for rebuild alerts after swapping failed drives.
- Perform read tests on both disks to check for bad sectors or performance issues.
Running regular disk health checks as preventive maintenance is recommended. This can pickup potential problems before failures occur.
What are the typical RAID 1 storage capacities supported?
RAID 1 can support a wide range of storage capacities. However, typical implementations tend to range from:
- 2 TB to 16 TB – For PC and consumer-grade setups using 3.5″ hard drives.
- 200 GB to 1.6 TB – For enterprise servers using 2.5″ drives.
- 4 TB to 64 TB – For larger arrays using multiple drives and expansions.
Most RAID 1 deployments are in the small to mid-size range. Larger capacities typically use other RAID levels. But capacities ultimately depend on the number and size of physical drives used.
What RAID configurations provide better performance than RAID 1?
There are several RAID levels that can provide better performance than RAID 1 in certain workloads:
- RAID 0 – Block-level striping. Optimized for peak read/write speeds but has no redundancy.
- RAID 5 – Block-level striping with distributed parity. Reads faster with some redundancy.
- RAID 10 – Mirrored stripes for combined performance and redundancy.
- RAID 50 – RAID 5 arrays striped together. High performance and capacity.
RAID 1 optimizes for simple redundancy. But needs to duplicate writes. RAID levels designed for performance gain speed by spreading data across multiple disks.
What are typical real-world RAID 1 performance benchmarks?
Real-world RAID 1 performance benchmarks typically show:
- 180-220 MB/s read speeds in sequential tests.
- 120-180 MB/s write speeds for sequential writes.
- 4-8x faster random read IOPS versus standalone disks.
- Similar or slower random write IOPS compared to standalone disks.
Performance varies based on the RAID controller, number of drives, and types of drives used. But doubling of read speeds is common for RAID 1 versus single disks.
How critical is a battery backup or cache module for RAID 1?
Battery backups and cache modules provide some benefits for RAID 1 arrays in terms of:
- Preserving data in cache if power is lost.
- Improving speed of write operations.
However, they are not as critical for RAID 1 as other RAID levels. The mirrored disks already provide redundancy if power is lost before cached data can be written.
Cache can boost performance. But RAID 1 is not as write-dependent as striped RAID levels. The batteries provide nice protection, but are not mandatory.
Conclusion
In summary, RAID 1 delivers straightforward disk mirroring for redundancy. It provides 100% fault tolerance from disk failures and improves read speeds. But lacks gains for write performance.
RAID 1 is easy to implement and works well for protecting smaller server and PC arrays. It ensures maximum uptime and prevents data loss from hardware faults. For larger deployments focused on performance, other RAID levels may be more suitable.