The answer is RAID 1. RAID 1, also known as disk mirroring, provides fault tolerance by writing identical data to two or more drives simultaneously. This allows the array to tolerate the failure of up to one drive (in a two-drive array) without any loss of data. If one drive fails, the data can still be accessed from the remaining operational drive(s).
What is RAID?
RAID stands for Redundant Array of Independent Disks. It is a data storage technology that combines multiple disk drives into a logical unit. Data is distributed across the drives in one of several ways called RAID levels, depending on the level of redundancy and performance required.
The different RAID levels provide different combinations of increased data reliability and/or increased input/output performance. The most commonly used RAID levels are:
- RAID 0 – Data is striped across drives for increased performance, but provides no redundancy. If one drive fails, all data will be lost.
- RAID 1 – Disk mirroring as described above. Provides redundancy but no performance gain.
- RAID 5 – Data is striped across drives with parity information distributed across the drives. Can tolerate the loss of one drive without data loss.
- RAID 6 – Similar to RAID 5 but can tolerate the loss of up to two drives.
- RAID 10 – A combination of RAID 1 and RAID 0. Provides both improved performance and redundancy.
How does RAID 1 disk mirroring work?
With RAID 1, identical copies of data are written to two or more separate drives simultaneously. This is referred to as disk mirroring or duplexing. For example, in a two-drive RAID 1 array, whenever data is written to the first drive, that same data is also written to the second drive at the same time. This produces a mirror image of the data on the second drive.
All read and write operations can be performed in parallel to both drives, providing increased performance compared to a single drive. But the main benefit of RAID 1 is fault tolerance. If one drive fails, the data is still accessible from the other mirrored drive(s).
RAID 1 arrays can continue operating with one failed drive. If a drive does fail, it can be replaced and the data rebuilt from the existing mirrored drive(s) to restore fault tolerance. RAID 1 can withstand a number of drive failures up to N-1, where N is the total number of drives in the array.
Advantages of RAID 1
- Excellent read performance – data can be read in parallel from both mirrored drives.
- Very good write performance – data is written to both drives simultaneously.
- Fault tolerance for drive failure – automatic recovery from a single drive failure.
- Easy to implement – only requires two identical drives.
Disadvantages of RAID 1
- Increased cost – requires at least two drives but provides the capacity of only one drive.
- No increase in performance for single drive reads and writes.
- Heavily reliant on controller for rebuild after a disk failure.
When to use RAID 1
RAID 1 is ideal for any application where fault tolerance and high availability are critical. The ability to continue operating with a failed drive makes it suitable for mission-critical systems that require continuous uptime.
Some examples of good uses for RAID 1 include:
- Database servers
- Email servers
- File servers
- Application servers
- Web servers
- Virtualization hosts
Any system where downtime would impact business operations or revenue generation would benefit from the redundancy of RAID 1. The performance improvements are also advantageous for databases and other transactional applications.
Implementing RAID 1
RAID 1 can be implemented in several ways depending on the required platform:
Hardware RAID
A hardware RAID controller manages the RAID array and performs the mirroring of data across drives. The operating system interacts with a single logical volume. Hardware RAID provides the best performance but requires purchasing a RAID controller.
Software RAID
RAID is implemented in software without additional hardware. This uses the processing power of the CPU for RAID calculations and management. Software RAID provides redundancy without additional hardware costs but has a higher processing overhead.
Operating System RAID
Some operating systems have built-in RAID capabilities, such as Windows Dynamic Disks, ZFS or Linux MD RAID. This approach is similar to software RAID but may be easier to configure and manage through the OS tools.
Fake RAID
“Fake RAID” or “RAID onboard” uses RAID software integrated into the motherboard BIOS or firmware. This provides RAID without a hardware controller but requires motherboard support and has some limitations.
RAID 1 vs other RAID levels
Compared to other common RAID levels, RAID 1 provides the best data protection but lower storage efficiency:
| RAID Level | Fault Tolerance | Storage Efficiency |
|---|---|---|
| RAID 0 | None | 100% |
| RAID 1 | 1 drive | 50% |
| RAID 5 | 1 drive | 67%-94% |
| RAID 6 | 2 drives | 50%-88% |
RAID 5 and RAID 6 provide fault tolerance for drive failure with better storage efficiency than mirroring, but rebuild times are slower and there is a greater risk of unrecoverable read errors during degraded mode operation.
Best practices when using RAID 1
To maximize the benefits of RAID 1, following these best practices is recommended:
- Use identical drives from the same manufacturer and model for each mirrored pair.
- Configure a RAID monitoring utility to receive alerts for drive failures.
- Have spare drives available for immediate replacement of failed drives.
- Schedule regular drive health tests using S.M.A.R.T. tools.
- Monitor rebuild times and drive errors to identify potential drive issues.
- Ensure the RAID controller has a battery-backup cache.
- Disable disk write-caching if using software RAID without BBU.
Conclusion
RAID 1 disk mirroring provides a simple and effective form of fault tolerance by maintaining two or more identical copies of data on separate drives. The redundancy of RAID 1 allows continuous operation in the event of a single drive failure. Although it doubles the storage cost, RAID 1 remains a popular choice for mission-critical server applications where uptime and reliability are essential.
The performance improvements of parallel disk access provide additional benefits for busy transactional databases and applications. With proper management and monitoring, RAID 1 delivers excellent protection against hardware failures alongside improved performance over a single drive.