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 provides redundancy and improves performance by distributing data across multiple disks. This allows data to be accessed in parallel, increasing input/output speeds. If a drive fails, the data can be recovered from the other drives in the array.
There are several different RAID levels that provide various combinations of performance, redundancy, and efficiency:
- RAID 0 stripes data across multiple drives without parity or mirroring. It provides fast performance but no redundancy.
- RAID 1 mirrors data across drives. It provides redundancy but lower storage capacity.
- RAID 5 stripes data and parity information across disks. It provides good performance and storage efficiency while allowing recovery from a single disk failure.
- RAID 6 stripes data and dual parity for protection against the failure of two disks.
In summary, RAID allows multiple disk drives to act as a single storage unit, providing redundancy and/or performance improvements over a single drive (cite: Raid Definition). The different RAID levels provide various tradeoffs between redundancy, performance, and storage capacity.
How does RAID protect data?
RAID protects data through techniques like disk striping, parity, and fault tolerance. Disk striping spreads data across multiple drives to improve performance and redundancy. Parity allows for data reconstruction in case of disk failure by calculating and storing parity information on additional disks. Fault tolerance means RAID can withstand a certain number of disk failures without losing access to data. For example, RAID 6 can handle up to two disk failures by using double distributed parity (Newsoftwares). By incorporating redundancy, RAID safeguards against data loss in the event of drive failure.
When is RAID rebuilding needed?
One of the most common reasons for RAID rebuilding is a disk failure. RAID is designed to withstand and recover from disk failures through data redundancy across multiple disks. When a disk in a RAID fails or becomes corrupted, the RAID controller detects the failed drive and automatically begins rebuilding the data from the remaining disks.
Many RAID levels like RAID 1, 5, 6 and 10 can withstand a single disk failure without data loss. When a disk fails in these configurations, the system will continue operating in a degraded state using the data from the surviving disks. A RAID rebuild is initiated by replacing the failed hard drive with a new drive. The RAID controller then rebuilds the data that was on the failed drive onto the new replacement drive. This rebuild process restores full redundancy and protection to the RAID.
Rebuilding is an automatic background process and the priority is restoring redundancy. The rebuild rate can be configured in the controller settings but will not impact normal system operation. Rebuilding large RAID arrays with many terabytes of data can take hours or days depending on the performance of the controller and disks. The system remains vulnerable to a second disk failure during rebuilding so the process should be monitored and failed disks replaced promptly.
According to Vivotek, if automatic rebuilding does not start, it may be necessary to initiate RAID rebuilding manually through the controller configuration utility.
The RAID rebuilding process
The RAID rebuilding process involves reading all the data from the surviving disks, recalculating parity, and writing the data to the replacement disk. This allows the RAID array to recover full redundancy and protection against future disk failures.
During the rebuild, the RAID controller will:
- Read all data blocks and parity from the surviving disks
- Recalculate the missing data or parity based on the RAID level configuration
- Write the rebuilt data or parity to the replacement disk
This rebuild process is transparent and done in the background. The RAID continues to service I/O requests during the rebuild, but performance may be degraded. The length of time for a rebuild depends on the RAID level, size of disks, and workload. A RAID 5 or 6 rebuild takes longer than a RAID 1 rebuild due to parity recalculation.
It’s crucial not to interrupt the rebuild process. If the system or array is powered off, the rebuild stops and may not resume where it left off. A failed or interrupted rebuild could leave the array in a degraded state. Patience is required to let the rebuild fully complete.
Overall, the rebuild process reads data from the existing disks and writes to the new disk. No data is lost or deleted during a standard RAID rebuild.
Does rebuilding delete data?
No, rebuilding a RAID array does not delete or overwrite existing data on the healthy drives. The rebuilding process only overwrites the failed or replaced drive with the parity data calculated from the remaining healthy drives.
As the RAID controller rebuilds the array, it uses the parity data distributed across the surviving disks to recreate the data that existed on the failed drive and write it to the replacement drive. This parity data allows the array to recover and rebuild the data without needing to access the original data disks. The original data remains intact on the healthy drives.
Some key points on why rebuilding does not affect data:
- The original data disks are not accessed or modified during a rebuild.
- Only the failed disk data is recalculated and rewritten.
- RAID is designed to enable rebuilds without altering existing data.
- The rebuilding process overwrites only the disk being rebuilt.
Therefore, rebuilding a RAID does not cause data loss or deletion on the functioning disks. The existing data remains available and accessible during the rebuild process. Only the failed disk’s data gets reconstructed and replaced (Source).
Best practices for RAID rebuilding
To minimize the risk of permanent data loss, it is crucial to follow best practices when rebuilding a RAID array. Here are some key tips:
Replace failed disks quickly – At the first sign of a disk failure, replace the faulty drive right away. Leaving a failed drive in place puts significant stress on the remaining disks during rebuilds and leaves no protection if another drive fails.
According to source, replacing failed disks promptly is critical to avoiding a total array failure during rebuilding. The source recommends hot swapping failed drives if supported by the RAID controller.
Schedule regular parity checks – For RAID levels that use parity (e.g. RAID 5, 6), performing periodic parity checks helps identify issues before disk failures occur. The parity check scans drives and validates the parity data matches the stored data. This can reveal pending drive issues.
As recommended by this source, scheduling weekly or monthly parity checks provides early warning of problems. This allows preventative replacement of questionable disks.
Risks during rebuilding
Rebuilding a RAID array comes with some inherent risks that should be considered. The two main risks are additional disk failures and extended rebuild times.
If a disk in a RAID array fails, it puts additional stress on the remaining disks during the rebuild process. This increases the likelihood that another disk could fail before the rebuild completes. According to one source, “During the rebuild process, the remaining drives are under more strain as they are having to work harder to reconstruct the data from the failed drive across the remaining drives. This leads to an increased risk of multiple drive failures occurring during the rebuild” (source).
In addition, rebuild times can be quite long depending on the size of the disks and the RAID level. The larger the disks and the higher the RAID level, the longer the rebuild will take. Extended rebuild times exacerbate the risk of multiple disk failures. One source notes that their advanced drive monitoring software “prevents the time needed and potential risks of RAID rebuilding, thus greatly improving system reliability” (source).
In summary, the two primary risks during RAID rebuilds are additional disk failures due to increased strain on the remaining disks, and extended rebuild times that compound this risk. Proper monitoring and backups are critical to mitigate these risks.
Monitoring RAID Health
Monitoring the health of a RAID regularly is crucial to identify issues before they cause data loss or downtime. There are several key ways to monitor RAID health:
Disk scans – Most RAID controllers allow scheduling periodic disk scans to check for bad sectors or other disk errors. Scans help identify failing drives before they lead to degraded arrays or outright failure.
SMART data – Self-Monitoring, Analysis and Reporting Technology (SMART) provides health stats and predictive failure alerts for drives. Monitoring SMART attributes like reallocated sectors can reveal issues.
Parity checks – RAID levels like 5 and 6 provide parity data for redundancy. Regular parity checks validate data integrity across the array. If parity inconsistencies are found, it may indicate impending drive issues.
By scheduling periodic disk scans, watching SMART attributes, and performing parity checks, admins can keep a close eye on RAID health. Proactive monitoring helps avoid destructive failures.
When to consider backups
While RAID provides redundancy and protects against disk failures, it is not a substitute for a proper backup solution (Source). Backups serve as an additional layer of protection by creating copies of your data that are stored separately from your primary RAID system.
Some key reasons why backups are still recommended with RAID include:
- Backups protect against catastrophic system failure – If the entire RAID array is lost due to hardware failure, theft, fire or other disaster, backups provide recovery options.
- Backups protect against data corruption – If files get corrupted on a RAID system, backups allow you to restore an uncorrupted version.
- Backups provide point-in-time recovery – You can restore previous versions of files from backup that may have been deleted or changed on the RAID.
- Backups facilitate recovery from user error – Accidental file deletion or overwrites on RAID can be reversed via backup.
For optimal protection, experts recommend implementing both RAID and a backup solution using the 3-2-1 backup strategy – having at least 3 copies of your data, on 2 different media types, with 1 copy offsite (Source). This provides redundancy against hardware failure, as well as protection if disaster strikes your onsite RAID system.
Summary
In summary, rebuilding a RAID does not delete your data, but it does put your data at risk during the rebuilding process. The rebuilding process recreates the redundant data on a new drive to restore fault tolerance and protection. While no data is deleted, there is increased vulnerability to data loss from additional drive failures or errors during rebuilding. It’s crucial to monitor RAID health to catch issues early and have backups available in case problems do occur. Rebuilding is a necessary maintenance task for RAID, but backups provide an extra layer of protection and the ability to restore data if needed. Following best practices for monitoring, maintenance, and backups is key to ensuring data integrity when rebuilding RAID.
