RAID stands for Redundant Array of Independent Disks. It is a data storage technology that combines multiple disk drives into one logical unit. RAID takes advantage of multiple disks to provide increased storage capacity, speed, and reliability over a single disk (Source).
There are several different RAID levels, each with its own benefits:
- RAID 0 – Data striping for increased performance
- RAID 1 – Disk mirroring for fault tolerance
- RAID 5 – Block-level striping with parity for fault tolerance and increased storage capacity
Some key benefits of RAID include (Source):
- Increased storage capacity – Combining multiple disks expands storage
- Improved performance – Spreading I/O across disks increases speed
- High availability – Redundancy keeps data available if a disk fails
- Scalability – RAID can be expanded by adding more disks
By combining multiple disks, RAID aims to provide greater performance, capacity, and reliability compared to single disks.
What is RAID 5?
RAID 5 is a form of RAID (Redundant Array of Independent Disks) that provides data protection through a distributed parity scheme (Definition from TechTarget). It requires a minimum of 3 drives to implement. Data is striped across multiple drives like in RAID 0, but parity information is also calculated and written across the drives. The parity information allows the array to reconstruct data if one of the drives fails.
In RAID 5, parity data is distributed evenly across all drives in the array. This provides performance benefits compared to RAID 4 which has a dedicated parity drive. If one drive fails in a RAID 5 array, the missing data can be calculated using the parity information from the other drives (How RAID 5 Works – Open-E).
The main pros of RAID 5 are:
- Good read performance due to striping
- Ability to withstand a single drive failure
- Efficient use of storage capacity
The main cons are:
- Slower write performance due to parity calculations
- Vulnerable to data loss with multiple drive failures
- Long rebuild times after drive failure
What is a Hot Spare?
A hot spare drive is an idle drive that is available to instantly replace a failed drive and rebuild a RAID array. It is a key component to provide fault tolerance and minimize downtime in a RAID-based storage system [1].
The purpose of a hot spare drive is to reduce the risk of data loss or downtime when a drive fails in a RAID 5 or RAID 6 array. Since RAID 5 and 6 can only withstand 1 or 2 drive failures before data loss occurs, having a hot spare allows the array to automatically rebuild itself using the spare drive [2]. This eliminates the need for manual drive replacement and rebuilding, providing continuous availability.
Once a hot spare is used to replace a failed drive, it becomes part of the active RAID array. A new hot spare should then be added to maintain fault tolerance. Hot spare drives are an important redundancy component for maximizing RAID array uptime and data protection.
How Many Drives for RAID 5 with Hot Spare?
The minimum number of drives required for RAID 5 with a hot spare is 5. This allows for the 4 drives needed for RAID 5, plus 1 hot spare drive. However, the recommended number of drives for RAID 5 with hot spare is 6 or more.
Some key factors to consider when determining drive count for RAID 5 with hot spare:
- More drives increases fault tolerance – With 5 drives, you can only handle 1 failure before data loss. With 6 drives, you can handle 2 failures if using a hot spare. More drives means more redundancy.
- More drives increases performance – RAID 5 performance depends on how many drives it can stripe/parallelize across. More drives means better read/write speeds.
- Hot spares reduce rebuild times – When a drive fails, the rebuild onto a hot spare is faster than replacing and rebuilding on a new drive.
- Cost vs redundancy tradeoff – More drives adds cost, but also improves redundancy and performance. Balance your budget with needed fault tolerance.
Most recommendations are to use at least 6 drives for RAID 5 with hot spare in order to handle 2 drive failures. Adding more drives further improves redundancy and performance.
Setting Up RAID 5 with Hot Spare
To set up RAID 5 with a hot spare drive, there are a few key considerations regarding the RAID controller and drives:
The RAID controller must support RAID 5 and hot spare functionality. Most hardware RAID controllers like those from Dell, HP, and LSI support configuring hot spares for RAID 5 arrays. Software RAID solutions like Windows Storage Spaces also allow RAID 5 with hot spare.
For the drives, best practice is to use identical drives in terms of capacity, speed, and type for all the main array drives and the hot spare. The hot spare should be the same size or larger than the main array drives. Using non-identical drives can lead to performance issues when rebuilding the array.
The basic steps to set up RAID 5 with hot spare are:
- Install the RAID controller and connect the drives.
- Enter the RAID controller configuration utility, usually during boot or via BIOS.
- Create a RAID 5 array with at least 3 drives.
- Assign a hot spare drive in global or dedicated mode.
- Initialize and format the RAID 5 array.
Once configured, the hot spare remains idle until a drive in the main RAID 5 array fails. Then the rebuild process automatically occurs using the hot spare drive.
Rebuilding RAID 5 from Hot Spare
One of the main benefits of configuring a hot spare drive with a RAID 5 array is that it allows for automatic rebuilding if a disk fails. Without a hot spare, rebuilding the array requires manually replacing the failed disk and then initiating the rebuild process.
With a dedicated hot spare drive, the rebuild process happens automatically. As soon as a disk in the RAID 5 array fails, the hot spare immediately takes its place and rebuilding begins using the parity information spread across the remaining disks. This helps minimize downtime and avoids potential data loss from an additional disk failure during rebuild.
According to a post on Server Fault, the instant the array rebuild completes, the hot spare returns to its ready state, available to replace any other failed disks. The failed disk can then be safely replaced without any impact on the running array. This allows for replacing multiple failed disks one at a time.
While automatic rebuilds require no administrative intervention, it’s still important to monitor the process. Rebuilding a failed RAID 5 array puts additional stress on the remaining disks. It’s recommended to replace the failed disk as soon as possible after a failure to reduce this stress.
Performance Impact
When a drive in a RAID 5 array fails, the array goes into a degraded state where read performance remains unaffected but write performance will be slower. This is because the array has to calculate the missing drive’s parity data in order to write new data.
When the hot spare kicks in to start rebuilding the failed drive, this further impacts performance. The rebuilding process requires significant read and write operations as data from the surviving drives is read to recalculate parity and rebuild the failed drive. This puts additional strain on the array. According to a study by Dell, rebuild times for a 1 TB drive in various RAID configurations averaged around 80-120 minutes depending on the number of drives. But total time to complete rebuild can take hours or days for larger drives [1].
After the rebuild is complete, the RAID 5 array goes back to optimal state with full redundancy and performance is restored. The hot spare is still present as a standby in case another drive fails. Overall, the temporary decrease in performance during rebuild is worth it for the protection against a double disk failure that having a hot spare provides.
Alternatives to Hot Spare
While using a hot spare is a common approach for providing fault tolerance in RAID 5 arrays, there are some alternatives worth considering:
Distributed Spare
Instead of dedicating an entire drive as a hot spare, some RAID implementations allow you to use distributed spare capacity across all the drives in the array. This allows the array to re-stripe and rebuild itself in the event of a failure without requiring a whole dedicated hot spare drive. The downside is that you lose some overall usable capacity since space is reserved on each drive.[1]
RAID 6
RAID 6 provides fault tolerance by using double distributed parity, allowing the array to sustain up to two drive failures without data loss. This removes the need for a hot spare, but comes at the cost of reduced overall capacity and write performance compared to RAID 5. RAID 6 is preferable for large arrays where rebuilds take longer and the likelihood of a second drive failure during rebuild is higher.
More Frequent Monitoring
Rather than relying on hot spare drives, some administrators prefer to simply monitor RAID 5 arrays more frequently using tools like email alerts and automatic monitoring scripts. This allows detecting failures early and replacing failed drives promptly before additional failures occur. However, this requires diligent monitoring and may not be feasible in all environments.
Overall, hot spares still remain a popular choice to balance cost, usable capacity, and rebuilding reliability for RAID 5. But alternatives like distributed spare, RAID 6, and increased monitoring provide options to consider based on your specific storage needs and environment.
[1] https://www.open-e.com/blog/raid-5-raid-6-or-other-alternativee/
Best Practices
When using a hot spare drive with RAID 5, there are some best practices to follow for optimal performance and data protection:
Monitor drive health closely. Use disk monitoring tools like S.M.A.R.T. to get early warnings about potential drive issues. This allows you to replace questionable drives before failure occurs.
Consider using dedicated hot spares if you have many RAID arrays. That way, if a drive fails in one array, the hot spare is ready to rebuild that array specifically instead of being potentially tied up rebuilding another array.
Make sure the hot spare is at least the same size or larger than the RAID 5 drives. The spare won’t work if it’s too small to fully rebuild the array.
Keep hot spares in the same chassis and connected to the same RAID controller as the array it will potentially rebuild. This avoids delays if the spare needs to be moved or reconnected.
Use hot swap bays and trays if possible for easy drive swapping. Avoid having to shut down the server to access the hot spare physically.
Consider staggering drive purchases when building the array so they won’t all fail at once later from old age.
Test the rebuilding process periodically to verify drive assignments and rebuild times.
Have a spare on hand to swap in once the hot spare takes over a failed drive, to avoid being unprotected.
Conclusions
In summary, RAID 5 requires a minimum of 3 drives with optional hot spares depending on the number of drives in the array. Hot spares provide fault tolerance by automatically rebuilding the array if a drive fails. The tradeoff is reduced storage capacity.
When implementing RAID 5, best practices are to use enterprise-grade drives, monitor drive health, and replace drives proactively. RAID 6 offers an alternative to hot spares through a second distributed parity drive.
Key takeaways:
- RAID 5 requires a minimum of 3 drives
- Hot spares improve fault tolerance but reduce storage capacity
- Follow best practices like using quality drives and monitoring drive health
- RAID 6 offers distributed dual parity as an alternative to hot spares