What is RAID?
RAID stands for “Redundant Array of Independent Disks”. It is a data storage technology that combines multiple disk drive components into a logical unit. RAID takes advantage of the parallelism of arrays of disks to enhance data storage reliability and/or performance (https://www.britannica.com/dictionary/raid).
There are several different RAID levels that provide various combinations of increased data reliability and increased input/output performance. Some common RAID levels include:
- RAID 0 – Disk striping without parity or mirroring. Provides improved performance but no redundancy.
- RAID 1 – Disk mirroring without parity or striping. Provides redundancy by duplicating all data on secondary disks.
- RAID 5 – Block-level striping with distributed parity. Provides fault tolerance and improved performance.
A RAID system requires a RAID controller, which is a device that manages the RAID array. The controller handles the actual organization and reorganization of data placements across the disk drives. It also coordinates data recoveries in the event of a disk failure (https://www.merriam-webster.com/dictionary/raid).
Why do drives in a RAID fail?
Hard drives can fail for a variety of reasons, both mechanical and logical. Some of the most common causes of hard drive failure in a RAID array include:
Mechanical failures: Problems like the drive motor failing, heads getting stuck, or damage to platters can cause the drive to stop working properly from a hardware perspective. These types of failures are often unpredictable and increase as a drive ages.
Firmware bugs: Bugs in a drive’s firmware, which controls its basic operations, can also lead to unpredictable failures. Some drives are more prone to firmware issues than others.
According to Backblaze’s statistics on over 230,000 drives from Q1 2023, failure rates vary dramatically between drive models 1. For example, the Seagate ST4000DM004 4TB drive had an annualized failure rate of just 0.60%, while the Toshiba MG03ACA400 4TB reached 2.71%. High failure rate drives like this can increase the likelihood of rebuilding RAID arrays.
In addition to model differences, failure rates increase as drives age. Backblaze found drives were most reliable during their first 18 months of use. After 3 years, failure rates exceeded 1.5% per year for some models.
Understanding drive failure statistics helps choose reliable drives and anticipate failures before they happen. Monitoring drive SMART stats and replacing aging drives can help minimize RAID failures.
The RAID Rebuilding Process
When a drive fails in a RAID array, the RAID controller activates the rebuilding process to recreate the data that was on the failed drive. This involves reading all the data from the remaining drives and recalculating parity information. The process works like this:
First, the RAID controller identifies the failed drive and takes it offline. It then begins reading all the data blocks from the remaining drives in the array (Source). As it reads each data block, it recalculates the parity information for that stripe of data across the RAID drives. Parity allows the system to reconstruct missing data in case of a drive failure.
The controller writes the reconstructed data blocks from the failed drive onto a replacement drive. This rebuild process continues until all data blocks have been read from the surviving drives, parity recalculated, and missing data reconstructed and written to the new replacement drive. The rebuild is complete once all data has been reconstructed onto the replacement drive (Source).
The rebuild process aims to restore full redundancy and protection to the RAID array. However, during the rebuild, the system is vulnerable as there is no backup yet for the remaining drives. The controller prioritizes the rebuild to limit this vulnerability window.
Factors affecting rebuild times
The key factors that affect the duration of a RAID rebuild time include:
RAID level
The RAID level determines how data is distributed across the drives and the amount of redundancy built into the array. RAID levels like RAID 5 and 6 require more complex rebuilding since parity data needs to be recalculated and written across multiple drives. RAID 1 and 10 typically have faster rebuilds since they simply mirror data between pairs of drives.1
Number of drives
More drives in the array mean more data to rebuild across the set. A 12 drive RAID 6 array will take longer to rebuild than a 4 drive RAID 10 array, for example. The controller must read and write data across every drive in the array during a rebuild.2
Drive capacity
Higher capacity drives take longer to rebuild since there is more data that must be read and reconstructed on the new replacement drive. An 8TB drive will have a longer rebuild time than a 2TB drive.
Controller and connection speeds
Faster RAID controllers with more cache and processing power can help speed up rebuild times. Connections like SAS and PCIe provide higher throughput than SATA interfaces. Upgrading controllers and connections is an effective way to reduce rebuild times.3
Recommended rebuild times
When it comes to recommended RAID rebuild times, there are a few guidelines to follow from vendors, industry standards, and general rules of thumb:
Most storage vendors such as Dell, HPE, and NetApp publish recommended rebuild times for their specific hardware and software solutions. For example, Dell recommends 24 hours or less for most configurations, while HPE states RAID 5/6 rebuilds should take 1-2 days for drives up to 1TB. Consulting your vendor’s documentation is important to set proper expectations.
Industry standards suggest limiting rebuild times to 24-36 hours for typical multi-TB drives in order to minimize risk of a second drive failure during rebuild. Going beyond 36 hours significantly increases the chances of irrecoverable data loss. The Storage Network Industry Association (SNIA) recommends no more than 4-36 hours depending on array size.
As a general rule of thumb, expect 1TB per day as a conservative estimate. So an 8 TB drive would take about 8 days to rebuild. Higher-end RAID controllers with battery backups, caching, and optimized firmware can achieve faster rebuilds. But for most systems, budgeting 1 TB per day for rebuild is reasonable.
The key is avoiding excessively long rebuilds beyond 3-4 days, as that increases the risk of a second disk failure. Consulting vendor recommendations, industry standards, and rules of thumb provides rebuild time guidelines to aim for.
Monitoring rebuild progress
It is important to monitor the progress of a RAID rebuild to ensure the process completes successfully. There are a few ways to check the status and estimate time remaining for a rebuild:
To check the rebuild status, you can use disk management utilities like hpacucli or ssacli to view the percentage complete. Commands like hpacucli ctrl all show config detail will show details on any active rebuilds. The percentage complete gives an idea of the overall progress.
Estimating the time remaining can be done by noting the current rate of completion and calculating how long it will take to reach 100% at that rate. However, rebuild speeds often fluctuate so estimates may not be completely accurate. Track the rate of progress over time to refine the estimate.
Most RAID controllers can also be configured to send alerts if a rebuild is abnormally long. For example, on an HPE Smart Array controller you can set the Long Rebuild Warning threshold in minutes. The controller will warn if a rebuild exceeds this time limit.
See this HPE reference for more details on monitoring RAID rebuilds. Keeping an eye on rebuild progress is important to identify any issues as early as possible.
Best practices
There are several best practices that can help ensure successful and timely RAID rebuilds.
Hot spares
One of the most important practices is to configure hot spare drives. Hot spares are extra drives that are not part of the main RAID array. If a drive in the array fails, the hot spare is automatically swapped in to replace it without needing to wait for a new drive to be installed. This allows the rebuild process to start immediately rather than waiting on getting a replacement drive. According to guides on SalvageData and TTR Data Recovery, hot spares can significantly reduce rebuild times.
Battery backups
Another key practice is to use an uninterruptible power supply (UPS) or battery backup for the RAID system. This protects against data loss or corruption if power is lost during a rebuild. According to discussions on ArsTechnica forums, battery backups should be standard procedure for any production RAID implementation.
Scheduled rebuilds
Scheduled rebuilds involve proactively rebuilding the RAID array on a periodic basis, before any drives have actually failed. This spreads out the load instead of waiting for failure to trigger a full rebuild. Rebuilds can be scheduled during off-peak hours to minimize impact. TTR Data Recovery recommends scheduled rebuilds for any mission critical RAID setups.
SMART monitoring
Using SMART (Self-Monitoring, Analysis and Reporting Technology) to monitor drive health can provide early warnings about potential disk failures before they happen. This allows preemptive drive replacements. According to SalvageData, proactive drive monitoring with SMART helps avoid failed drives during rebuilds, reducing the risk of data loss.
Troubleshooting long rebuilds
There are a few key things to check if a RAID rebuild is taking longer than expected:
Identify bottlenecks. Check the utilization on critical components like the RAID controller, CPU, memory and network. A saturated component can slow down rebuild times. Upgrading firmware, drivers or hardware may help.
Prioritize the rebuild process. The rebuild process can be intensive. Consider adjusting priorities so the rebuild gets sufficient resources. Pausing or throttling other I/O activities can allow the rebuild to complete faster.
Recover failed rebuilds. If the rebuild fails entirely, identify and replace any failed drives. You may need to reinitialize the array and restart the rebuild. Be prepared with spare drives as rebuilds increase the chance of additional failures.
Monitor progress closely and consult the storage vendor’s documentation for troubleshooting advice. Long rebuilds strain the array so resolving any issues quickly is critical.
Alternatives to rebuilding
While rebuilding a failed drive in a RAID array is a common recovery method, there are alternatives that may better suit some use cases:
Non-RAID redundancy like erasure coding can provide protection against drive failures without the rebuild requirements of RAID. Erasure coding splits data into fragments and encodes it across drives, allowing for efficient recovery of corrupted or lost data.
Auto-repairing file systems like ZFS use advanced integrity checking and self-healing capabilities to detect and repair corrupted data. This avoids the need for lengthy rebuilds after a drive failure compared to traditional RAID.
Maintaining comprehensive backups either locally or in the cloud provides a way to restore lost data without relying on RAID rebuilds. While slower to restore large amounts of data, backups provide an alternative to rebuilt times that can stretch to days or weeks.
Weighing the rebuild time against the importance of continuous uptime can help determine if alternatives like erasure coding, ZFS, or backups are a better choice than rebuilding a failed RAID array.
Conclusions
In summary, RAID rebuild times can vary greatly depending on the RAID level, drive size, system load, drive interface, and other factors. As a general guideline, rebuild times should be in the range of 2-4 hours per TB for most RAID 5 arrays. For larger drives and arrays, allow up to 24 hours per TB. Anything longer than 24 hours per TB may indicate problems.
To minimize impact on performance, aim to keep rebuilds under 72 hours. Rebuild times over 1 week often lead to further drive failures. Monitor rebuild progress closely and take action if it stalls or slows down significantly.
Consider hot spares, stronger RAID levels like RAID 6, upgrading drive interfaces, and spread data across multiple volumes to improve redundancy and rebuild times. Schedule regular patrol reads to identify problems early.