What is RAID 1?
RAID 1, also known as disk mirroring, is a RAID configuration that provides redundancy by duplicating all data from one hard drive to a second hard drive (Source). This essentially “mirrors” the data between the two drives. When data is written to one drive, it is simultaneously written to the other drive as well. This provides fault tolerance in the event that one of the hard drives fails.
The main benefits of RAID 1 are increased reliability and redundancy. Since the data is mirrored between two drives, the array can continue operating if one drive fails. This prevents data loss and downtime. RAID 1 also provides improved performance for read operations, since the reads can be distributed across the two drives. However, write performance does not improve since all writes have to be completed on both drives (Source).
Overall, RAID 1 provides simple data protection through mirroring while avoiding some of the complexity and overhead of other RAID levels. The tradeoff is that it requires double the disk capacity. RAID 1 is commonly used for mission critical data that requires high availability.
How does drive failure impact a RAID 1 array?
With RAID 1, data is mirrored across two identical drives. If one of the drives in the array fails, the RAID system is designed to continue operating using the surviving drive [1]. This means reads can continue to be served without any data loss since an intact copy still exists on the functional drive.
However, with one failed drive, the array is operating in a degraded state. Writes cannot be completed since there is no longer a mirror drive to write the data to redundantly. The array will continue to operate in this degraded state, unable to process write requests, until the failed drive is replaced and the array rebuilt [2].
Detection of drive failure
RAID controllers and disk utilities can detect drive failure in a RAID 1 array through several methods:
The RAID controller continuously monitors the hard drives and can identify issues like high error rates, slow response times, and failed self-tests that indicate an impending drive failure. The controller will log these issues and send alerts or error messages to the system administrator (Source: https://serverfault.com/questions/100301/how-does-raid-detect-a-faulty-hd).
Many RAID controllers have LED status lights on each drive bay that will turn red or amber to indicate a failed or degraded drive. Checking these visual indicators is a quick way to identify a failed drive (Source: https://www.linkedin.com/advice/1/how-do-you-identify-which-hard-drive-failed-raid-array-skills-raid).
Disk utility software can also monitor SMART drive statistics and report predicted failures. The system admin can run manual checks with these tools to confirm a failed or failing drive (Source: https://www.stellarinfo.com/blog/common-symptoms-of-raid-array-failures/).
Replacing the failed drive
When a drive fails in a RAID 1 array, the failed drive needs to be replaced to restore full redundancy. Many RAID controllers and server motherboards support hot-swappable drives that allow the failed drive to be replaced without powering down the system.
Once the failed drive has been physically replaced, the new drive will be automatically detected by the RAID controller and added to the RAID 1 array. The controller will start rebuilding the data from the still-functional drive to the replacement drive. This rebuild process syncs the data to bring the replacement drive up to date with the rest of the array.
Hot-swappable drives make it easy to replace the failed drive without any system downtime. The RAID controller handles adding the new drive and rebuilding the data automatically. Within a few hours or less, depending on the size of the drives, the RAID 1 array will be back to full redundancy.
Sources:
[One drive failed in my RAID 1 array, am I safe to replace it without losing data?](https://serverfault.com/questions/908917/one-drive-failed-in-my-raid-1-array-am-i-safe-to-replace-it-without-losing-data)
[Raid 1 degraded: How do I go about replacing defective drive?](https://www.dell.com/community/en/conversations/storage-drives-media/raid-1-degraded-how-do-i-go-about-replacing-defective-drive/647f1c2ef4ccf8a8def471a3)
The rebuild process
When a failed drive in a RAID 1 array is replaced with a new, blank drive, the data needs to be rebuilt on the new drive to restore redundancy. This is done automatically by the RAID controller once the new drive is inserted into the array.
The rebuild process involves copying all the data from the surviving drive over to the replacement drive. As noted by IBM, this rebuild ensures the RAID 1 array is fully redundant again with two identicial drives mirroring the data.
The length of time it takes to rebuild the RAID 1 array depends on the size of the drives. Larger drive capacities mean a longer rebuild time, as there is more data to copy over. The rebuild process can take hours or even days on very large RAID 1 arrays.
During the rebuild, the array is operating in a degraded state with only one functional drive. This leaves the array vulnerable to a second drive failure and potential data loss. It’s important to complete the rebuild process as quickly as possible.
Restoring Full Redundancy
Once the failed drive has been replaced, the RAID 1 array will begin rebuilding the mirrored data onto the new drive. This rebuild process copies all the data from the surviving drive to the replacement drive so that full redundancy is restored. The rebuild time will depend on the size of the drives and the amount of data that needs to be copied. Typical rebuild times can range from a few hours for small arrays up to a day or more for large multi-terabyte arrays. (Source)
It’s important to note that until the rebuild operation completes, the RAID 1 array remains in a degraded state with no redundancy. During this time, if the surviving original drive were to also fail before the rebuild finishes, complete data loss would occur. The degraded rebuild period carries an increased risk of irrecoverable data loss. Once the rebuild finishes and full redundancy is restored, the array can again tolerate a failure of either drive without data loss.
Impacts during degraded state
When a drive in a RAID 1 array fails, it can have significant impacts on performance and redundancy until the failed drive is replaced and rebuilt. Some key impacts include:
Potential performance impact from only 1 drive: With only one functional drive, the array loses the performance benefit of spreading reads and writes across multiple disks. All I/O operations are limited to the capabilities of the single remaining drive [1]. This can lead to slower read and write speeds.
No redundancy until rebuild completes: If the remaining drive fails before the rebuild completes, complete data loss can occur. The system is vulnerable during this degraded state [2]. The rebuild process should be completed as soon as possible to restore protection against drive failure.
Preventing drive failures
One of the best ways to avoid drive failures in a RAID 1 array is to proactively monitor drive health and replace drives before they fail. Most enterprise-grade RAID controllers have built-in S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) capabilities that continuously monitor drive health statistics like read/write errors, bad sectors, spin-up time, etc. Monitoring these metrics can provide early warnings about potential drive issues.
When S.M.A.R.T. metrics indicate a drive is likely to fail soon, best practice is to preemptively replace the drive. Waiting for an actual failure causes undue stress on the array. According to TechTarget, replacing a drive at the first sign of failure reduces the possibility of catastrophic multiple drive failures.
Using enterprise-grade drives certified for RAID environments is also recommended. Consumer-grade drives are not designed to handle the heavy workloads and vibration issues common in RAID arrays. Enterprise drives typically have longer warranties, better vibration tolerance, and features like TLER to better handle RAID reconstruction after a failure.
Alternatives to RAID 1
While RAID 1 remains a popular option for data redundancy, there are some alternatives that provide similar or better protection in certain use cases:
RAID 5 offers parity protection, which allows the array to withstand a single drive failure without data loss. Unlike mirroring in RAID 1, parity doesn’t duplicate data but rather uses mathematical calculations to enable recreation of data if a drive fails. The tradeoff is you get more total capacity for your drives, but rebuild times are slower after failure. RAID 5 requires a minimum of 3 drives.
RAID 10 combines mirroring and striping for both performance and redundancy. It mirrors two drives together, then stripes data across the mirrors. This means up to half the drives can fail without data loss. The downside is you need at least 4 drives, and only get 50% of total capacity. But performance is excellent since data is striped across mirrors.
For home or small office use, drive pooling software like DrivePool, mergerFS, and UnRAID offer more flexible options. You can combine drives of different sizes, add capacity easily, and implement parity protection. Modern filesystems like ZFS and Btrfs also have built-in RAID-like features.
Backups to external drives or cloud storage provide an alternative to in-server redundancy. Solutions like Arq Backup, Backblaze, and CrashPlan make this easy to implement.
Key takeaways
When it comes to recovering from a failed drive in a RAID 1 array, there are a few key points to remember:
RAID 1 can withstand a single drive failure without data loss. Since the data is mirrored on the remaining drive, no data will be lost if one of the drives fails. The RAID will continue to operate in a degraded state until the failed drive is replaced.
Rebuilding the RAID restores full redundancy. Once the failed drive has been replaced, the RAID will need to rebuild the mirror by copying all data to the new drive. This restores the array to full redundancy again.
The rebuild process can impact performance, but a RAID 1 can safely operate during rebuilding. The goal is to replace the failed drive promptly, in order to minimize this degraded state.
Overall, RAID 1 provides excellent protection against a single drive failure. By understanding the rebuild process, users can recover quickly while avoiding data loss.