Network-attached storage (NAS) is a type of dedicated file storage device that enables multiple users and client devices to retrieve data from centralized disk capacity. Users on a local area network (LAN) access the storage via a standard Ethernet connection. NAS systems contain one or more hard disk drives that are arranged into logical, redundant storage containers or RAID arrays.
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. Data is distributed across the drives in one of several ways called RAID levels, depending on what level of redundancy and performance is required.
The different RAID levels provide various combinations of increased data reliability and/or increased input/output (I/O) performance relative to single drives. The different levels work by writing data across multiple disks in a pattern called data striping. RAID arrays use disk mirroring or parity to provide fault tolerance so that data can be recovered if a drive fails.
Key benefits of RAID
- Increased data reliability and fault tolerance – If a drive fails, data can be recovered from the remaining drives
- Improved I/O performance – Spreading I/O across drives increases throughput
- Capacity scalability – Disks can be added to array as needed
Common RAID levels
There are several different widely used RAID levels, each optimized for a particular use case:
RAID 0
- Also called disk striping
- Data is split across drives in blocks
- Fast performance but no redundancy
- Ideal for non-critical data
RAID 1
- Also known as disk mirroring
- Data is duplicated on a second drive
- Provides redundancy if one drive fails
- 2x storage cost
RAID 5
- Data and parity stripes distributed across drives
- Parity allows recovery from a single drive failure
- Good performance and redundancy
RAID 6
- Like RAID 5 but with dual distributed parity
- Protects against loss of two drives
- Slower write speeds than RAID 5
- Recommended for large arrays
RAID 10
- Stripes data across mirrored sets
- Combines performance of RAID 0 with fault tolerance of RAID 1
- 4x storage cost of single disk
How does RAID protect data?
RAID provides increased data reliability through redundancy. This means data is duplicated or additional parity data is stored, allowing recovery in case of a drive failure. The specific methods used depend on the RAID level:
- Mirroring (RAID 1, 10) – Data is copied in full to a second drive.
- Parity (RAID 5, 6) – Exclusive OR arithmetic is used to calculate and store parity information that can rebuild data after a drive failure.
- Dual distributed parity (RAID 6) – Provides two independent parity calculations to allow rebuilding after loss of two drives.
Because data is distributed across multiple disks, RAID also protects against sector errors and drive defects or failures. If a physical sector goes bad, data can be rebuilt using redundancy mechanisms. This helps improve overall data integrity and availability.
How does RAID improve performance?
Many RAID levels deliver increased input/output (I/O) performance by distributing or “striping” data across multiple drives in chunks or “stripes.” Spreading the load across drives enables multiple read and write operations to happen simultaneously, thereby increasing overall throughput.
Specific performance optimization mechanisms include:
- Striping (RAID 0, 5, 6, 10) – Large sequential I/O requests can be split and serviced concurrently across drives for faster throughput.
- Distributed parity (RAID 5, 6) – Parity calculation load is distributed across drives instead of being on a single dedicated drive.
- Read performance (RAID 1, 10) – Identical data blocks on mirrored drives can be read in parallel.
- Write performance (RAID 0, 10) – Writes are done concurrently since parity calculations are not required.
In addition, the use of multiple drives increases overall disk capacity. Larger RAID arrays can facilitate faster data transfers and improved bandwidth for demanding applications.
RAID in network-attached storage (NAS)
Network-attached storage (NAS) devices commonly use RAID technology to protect data and improve performance. NAS servers contain multiple internal hard disk drives that can be grouped together into customized RAID arrays based on capacity and redundancy requirements.
One of the benefits of using RAID with NAS is the ability to replace individual failed drives without turning off the NAS or disrupting users. Because data is distributed across the array, storage remains accessible during drive swaps. Many NAS devices also support hot spares – idle drives that can automatically rebuild data if an active drive fails.
Combining RAID with NAS offers affordable, flexible shared storage and data protection for small and midsize businesses. It provides centralized file services, storage capacity, backup, disaster recovery, and fault tolerance without the expense of a storage area network (SAN).
Common NAS RAID configurations
Some typical ways that RAID can be implemented on network-attached storage include:
- RAID 1 – Mirroring for enhanced read performance and redundancy
- RAID 5 – Good balance of capacity and redundancy for general use
- RAID 6 – Ideal for large NAS arrays needing protection against dual drive failures
- RAID 10 – Combination of RAID 0 striping and RAID 1 mirroring for optimal performance
Choosing a RAID level
Deciding which RAID level to implement requires understanding the use case and weighing factors such as:
- Storage efficiency – RAID 0 provides 100% efficiency since no space is used for parity. RAID 5 uses 1 drive for parity for every 5 data drives. RAID 1 mirrors disks so requires 2x storage.
- Fault tolerance – RAID 0 has no redundancy while RAID 1 can tolerate one drive failure. RAID 6 provides the highest tolerance, surviving up to two failed drives.
- Read performance – RAID 0 and 10 provide the fastest read speeds by striping/mirroring data across drives that can operate in parallel.
- Write performance – Writes are fastest for RAID 0 since parity does not need to be calculated before writing data.
- Capacity scalability – RAID levels 5, 6, and 10 are most scalable – additional drives can be added as needed.
For NAS implementations, quick performance is often a priority so RAID 10 is a popular choice. If parity performance is acceptable, RAID 5 offers a good balance of capacity and redundancy. For large arrays needing high fault tolerance, RAID 6 is preferable despite slower writes.
RAID management
Managing and monitoring RAID storage requires specialized software and sometimes dedicated hardware cards. Key capabilities include:
- Configuration – Selecting drives, choosing RAID levels, defining arrays
- Monitoring – Tracking drive health, logging errors, alerts
- Maintenance – Marking bad sectors, scheduling consistency checks
- Recovery – Rebuilding failed drives, repairing arrays
- Reporting – Activity logs, storage utilization
Many NAS devices include built-in RAID management software for monitoring events and rebuilding arrays after drive swaps. However, more advanced management may require standalone tools or RAID controller cards.
Hardware vs. software RAID
RAID can be implemented in NAS devices using:
- Hardware/dedicated RAID card – Provides own processor and memory resources for RAID tasks. Allows OS to offload RAID processing.
- Software RAID – Managed by OS and drivers. Provides flexibility but adds load on main system CPU.
Hardware RAID generally provides better performance than software RAID since it has dedicated resources. However, software RAID implementation on modern multicore processors is often sufficient for NAS workloads. The choice depends on performance requirements and budget.
Expanding capacity
One advantage of using RAID on a NAS is the ability to expand storage capacity as needed. There are two main methods:
- RAID migration – Moving to a new RAID level with more drives. For example, migrating from 4-disk RAID 5 to 6-disk RAID 6.
- RAID expansion – Adding disks to existing array without changing RAID level. For example, expanding a 3-disk RAID 5 array to 4 disks.
The process for expanding capacity varies by NAS vendor but often can be done without taking data offline. Considerations include matching new drives in terms of size, speed, and type to existing array.
Best practices
Some best practices for implementing RAID with NAS include:
- Choose RAID level to match performance and capacity needs
- Use hot spare drives and hot swap bays for quick rebuilds
- Monitor drive health closely
- Test recovery process regularly
- Ensure proper ventilation to avoid overheating
- Buy matching drives when expanding arrays
- Scrub/check arrays periodically to identify bad sectors
- Backup NAS data regularly in case of failure
Conclusion
Combining RAID technology with network-attached storage provides organizations with a scalable, high-performance storage solution that includes data protection and fault tolerance. RAID safeguards against data loss due to drive issues and improves uptime by enabling rapid recovery after drive failures.
Choosing the right RAID level along with proper management and monitoring allows NAS systems to meet capacity, performance and availability requirements at a lower cost than traditional SAN storage. RAID enables NAS to serve as primary storage for file sharing, backup, archiving and other applications.