Recovering data from a failed RAID 5 array can take anywhere from a few hours to several days, depending on the size of the array, the performance of the storage system, and the type of failure that occurred.
What is RAID 5?
RAID 5 is a common RAID (Redundant Array of Independent Disks) configuration that is used to provide fault tolerance and improve performance. In a RAID 5 array, data is striped across multiple disks, and parity information is distributed across the disks as well. This allows the array to continue functioning even if one of the disks fails. The parity information can be used to reconstruct the data that was on the failed disk.
How RAID 5 Protects Against Disk Failures
Here’s a quick overview of how RAID 5 protects against disk failures:
- Data is striped across multiple disks in chunks called “stripes.”
- Parity information is calculated and written across the disks.
- If one disk fails, the parity information can be used to reconstruct the data that was on the failed disk.
- This provides fault tolerance – the array can still operate with one failed disk.
The distribution of parity information is the key to RAID 5’s ability to withstand a single disk failure without data loss. When a disk fails, the data that was on that disk can be rebuilt using the parity info.
Factors That Affect RAID 5 Recovery Time
Several key factors determine how long it will take to recover from a failed disk in a RAID 5 array:
Array Size
Larger arrays with more disks and more data will take longer to recover than smaller arrays. Rebuilding a 6 TB RAID 5 array will take much longer than rebuilding a 2 TB array.
Disk Performance
Recovery time depends heavily on the performance characteristics of the drives in the array. Fast enterprise-class SSDs can rebuild much more quickly than slow HDDs. Multi-terabyte SATA drives take longer than smaller SAS or NVMe drives.
Storage System Performance
The processing power and cache memory capabilities of the RAID controller or HBA also impact rebuild times. Higher performance storage systems with multicore processors and more cache will rebuild arrays faster.
Failure Type
The type of disk failure also affects recovery time. Rebuilding after a total disk failure is generally faster than recovering from latent sector errors that require extensive disk scanning and error recovery techniques.
Factors that Increase RAID 5 Recovery Time
There are also some factors that can substantially increase the time needed to rebuild a RAID 5 array after a disk failure:
Degraded Performance
During the rebuild process, the array is running in a degraded state on the remaining disks. This can significantly reduce performance, which slows the rebuild process.
Rebuilding Larger Disks
As disk capacities grow larger, rebuild times also increase. Rebuilding a 4 TB drive can take hours, while rebuilding a 16 TB drive can take days.
Multiple Disk Failures
If additional disks fail before the rebuild completes, it may be impossible to recover the array without extensive disk recovery procedures.
Background Activity
Heavy loads on the storage system from other applications and users will compete for resources and can substantially slow rebuilds.
Estimating RAID 5 Rebuild Times
It’s difficult to provide precise estimates for how long RAID 5 rebuilds will take, but here are some general guidelines:
| Array Size | Average Rebuild Time |
|---|---|
| 2 TB | 2 – 6 hours |
| 4 TB | 4 – 10 hours |
| 8 TB | 8 – 24 hours |
| 16 TB | 16 – 48 hours |
| 32 TB | 32 – 96 hours |
These are just estimates – actual rebuild times can vary significantly based on the factors discussed earlier. With higher performance systems, rebuilds may be 2-3X faster. Slow systems or adverse conditions could also make rebuilds take 2-3X longer.
How to Speed Up RAID 5 Rebuild Times
If RAID 5 rebuild times are excessively long for your needs, here are some things you can do to improve rebuild performance:
Use Higher Performance Disks
Upgrading to faster SAS, NVMe or SSD drives will significantly reduce rebuild times over SATA HDDs.
Distribute Data Evenly
When setting up the array, distribute data evenly across all disks. This allows rebuilding any failed drive faster.
Add Spare Drives
Having hot spare drives allows the array to automatically start rebuilding on the spare, improving availability.
Ensure Proper Ventilation
Good airflow and cooling keeps drives running at peak performance during rebuilds.
Consider RAID 6
RAID 6 can withstand two disk failures by using a second parity stripe. But rebuilds take even longer than RAID 5.
Monitor Disk Health
Use S.M.A.R.T. monitoring and preemptively replace disks before they fail unexpectedly.
Limit Disk Loads
Reduce background activity during rebuilds to prioritize recovery performance.
Optimizing RAID 5 for Faster Rebuilds
If you are implementing a new RAID 5 array, there are some best practices you can follow to optimize the storage system for faster rebuilds:
Choose RAID Controller with Battery Backup
A RAID controller with battery-backup cache preserves data in the cache if power is lost. This reduces rebuild time.
Select Drives with Fast Sequential Speed
Drive throughput for sequential reads directly impacts rebuild time. Prioritize this specification.
Implement Flash-Backed Cache
Adding SSDs as a cache layer improves performance during rebuilds and normal operation.
Use a Redundant Controller
A redundant RAID controller improves performance if the primary fails during a rebuild.
Configure Hot Spares
Hot spares immediately start rebuilding failed drives for faster recovery.
Choose HDDs with Low Latency
Lower drive latency enables faster rebuilds so seek time and interface are important.
Limit Vibration with Anti-Vibration Mounts
Proper anti-vibration HDD mounting reduces vibration, ensuring peak rebuild performance.
Monitoring and Accelerating RAID Rebuilds
To help ensure RAID rebuilds complete successfully within an acceptable time frame, administrators should closely monitor the rebuild process and utilize available methods to accelerate rebuilds when needed.
Monitor Progress with Management Software
RAID management utilities will show rebuild progress, time remaining, and current throughput. This allows identifying any issues.
Prioritize Rebuild Activity
Temporarily halt or slow non-critical workloads so rebuilding has full system resources.
Ensure Proper Airflow to Drives
Maintain proper airflow and cooling to drives to prevent overheating during sustained rebuilds.
Add SSD Cache if Supported
Many RAID controllers support using SSDs as cache to accelerate rebuilds and improve throughput.
Adjust Stripe Size
Larger stripes mean reading more data from each drive. Adjust if throughput is an issue.
Replace Old/Slow Drives
Swap older drives with faster ones to significantly speed up rebuilds.
Alternative RAID Configurations for Faster Rebuilds
In situations where RAID 5 rebuild times are unacceptable, administrators may consider alternative RAID configurations that offer faster rebuilds.
RAID 10 (1+0)
RAID 10 mirrors data across pairs of drives. Rebuild only requires copying mirror contents to a new drive.
RAID 50
RAID 50 stripes RAID 5 sets together for the fault tolerance of RAID 5 with the performance of striping.
RAID 60
RAID 60 combines the multiple parity drives of RAID 6 with RAID 0 striping for faster rebuilds.
RAID-Z1/RAID-Z2
ZFS RAID-Z1 and RAID-Z2 offer the equivalent of RAID 5/6 with superior bit rot detection.
Distributed Spare Disks
DataRobot software distributes “virtual hot spares” across the drives for faster rebuilds.
Newer Advancements for Faster Rebuilds
Some newer technologies and techniques can also dramatically accelerate RAID rebuilds and recovery times:
In-Place Reconstruction
Increases performance by reconstructing failed data in-place rather than moving it.
Multithreaded Parity Calculations
Parallelizing parity calculations across multiple cores accelerates rebuilds.
Selective RAID Rebuilds
Only reading/writing required blocks can reduce needed drive I/O significantly.
Drive Sparing
Hot spare drives start rebuilding failed data immediately for better RTOs.
Machine Learning Prediction
ML utilizes historical data to predict and remediate drive failures preemptively.
Storage Class Memory
New persistent memory technologies like 3D XPoint offer performance between SSDs and RAM.
Conclusion
Recovering from a failed drive in a RAID 5 array can take anywhere from 2 hours to several days depending on the size, system performance, failure type, and other factors. Larger arrays, slower disks, multiple failures, and background activity all increase recovery times. Monitoring rebuild progress, optimizing systems for faster rebuilds, and new technologies like machine learning and storage class memory can help accelerate RAID 5 recovery significantly.
