What are storage controllers?

The amount of data that human beings produce is staggering. As of 2024, the estimated global datasphere clocks in at a whopping 149 zettabytes, with projections swelling to 181 zettabytes of data by the end of 2025.

To put that into perspective, one singular zettabyte is the equivalent of 1,000 exabytes, 1 billion terabytes or 1 trillion gigabytes. Put another way, one zettabyte is the equivalent of 250 billion DVDs.  

All that to say, data management is a major component of all computing systems, including enterprise data centers, cloud platforms, IoT devices and digital platforms. But not all data is created equal.

Much of the data we create is transient, meant to be used and discarded in a moment, while other types are more enduring. Some data is kept for mid-term storage and retrieval, while other types are kept for long-term archival or backup purposes. 

Depending on the purpose of the data created and stored, different types of storage media are better suited than others. From simple options like diskettes and USB flash drives to robust and complicated options like storage area network (SAN) and network attached storage (NAS) systems, computers rely on storage controllers (also known as disk controllers or storage processors) to write and retrieve data between storage devices and the computer’s main CPU. 

In most instances, storage controller cards are physical hardware units, sometimes integrated directly into a computer’s system board. However, when dealing with virtual machines (VMs) or other virtualized environments, a software-based virtual storage controller can be employed to simulate the function of a physical controller. In these types of computer systems, virtualized storage controllers are components of the overarching hypervisor software used to manage storage resources pooled from multiple physical devices.   

These are the main components of a storage controller:

• Host interface: The physical connection that enables a storage controller to communicate with the host system. Internal storage devices use standards like PCI Express (PCIe) for high-speed connections, while SANs use protocols like FibreChannel (FC) or NVMe over Fabrics for external connectivity.
• Processor: Storage controllers contain their own dedicated processor to manage input/output (I/O) operations. 
• Cache memory: Storage controller cache memory is usually composed of volatile DRAM. This high-speed, temporary storage is found on the controller itself and stores frequently accessed data that can be accessed more quickly than data stored on the connected storage device or devices. 
• Disk interface: The disk interface is the physical connection between the storage controller and the storage devices. 
• Battery or capacitor backup unit: For storage controllers with the ability to write to on-board cache memory, a battery backup unit (BBU) or capacitor-plus-flash module is a critical component. It allows the controller to preserve any data written to the volatile flash memory in the event of a sudden power outage. They’re effectively in-unit power supplies.

In facilitating data transfer, storage controllers perform several tasks to take advantage of the benefits of any particular storage media. They also mitigate various potential challenges and obstacles that might prevent a computer system from fully optimizing its storage capabilities. 

To accomplish smooth and efficient storage access, the main functionalities of a storage controller include the following:

• Data transfer and management: The most basic function of a storage controller is to act as a bridge between a storage device and the computer’s operational memory. Depending on the specific type of storage media and associated interfaces and protocols, storage controllers will optimize data transfer for speed and efficiency, promoting and maintaining overall system performance while combating latency.
• Data validation and fault-tolerant processing: Storage controllers can verify the accuracy of transferred data and perform fault-tolerant processing of erroneous data to ensure data accuracy and reduce interruptions caused by errors.
• Compression and decompression: When necessary, storage controllers can compress and decompress data to improve transmission speeds and reduce data pressure caused by large files.
• Data protection and backup services: Many storage controllers include features designed for data protection and redundancy, such as redundant array of independent disks (RAID) configurations and erasure coding. These features help secure and back up data and can be extremely valuable for improving fault tolerance and recovery times. 
• Interface compatibility: When pairing the wide variety of storage solutions with the myriad configurations of operating systems and computer hardware, compatibility becomes a major concern. Storage controllers help resolve compatibility issues by managing the different protocols between different storage device interfaces (for example, serial ATA (SATA), serial attached SCSI (SAS), peripheral component interconnect express (PCIe)) and different hardware and operating systems (for example, Microsoft Windows, Linux). 
• Cache management: While not standard across all options, built-in cache memory is a common feature among storage controllers. Built-in cache with dynamic automatic management stores frequently accessed data for quick access and helps systems optimize for quick responsive performance.
• Logical volume management support: Logical volume management (LVM) is a crucial technology for efficiently partitioning, organizing and using available system storage. LVM is used to create flexible and dynamic virtual storage pools, known as logical volumes, by pooling physical disks into volume groups (VGs) from the shared storage space. Storage controllers give system admins the ability to manage and resize storage volumes in real-time, create backup snapshots and consolidate multiple physical disks. This consolidation can become a sharable pool for optimal storage capacity with minimal downtime.
• Monitoring and reporting: Overseeing storage availability and performance, storage controllers are extremely useful for providing real-time monitoring and snapshots, offering valuable insight into storage health, performance, capacity and operational status of connected storage devices. Armed with this information, system administrators are better prepared to perform proactive maintenance and identify and remediate any potential issues to avoid any system downtime or critical data loss. 

Storage controllers can be divided into two main groups, based either on their interface or function.

While all storage controllers are responsible for managing the communication between the CPU and the storage devices, each type is specialized for unique purposes and environments. 

Designed specifically for interface compatibility, some interface-based storage controllers include the following:

• SATA (AHCI) controllers: SATA (AHCI) controllers are used for managing SATA-based storage devices, including SATA HDDs and SSDs. Cost-effective and well-suited to general-purpose computing, SATA is a common interface used in consumer-grade systems, including desktops, laptops, gaming devices and personal servers. Home and small-business environments with low storage demands and less need for high-speed data transfer often use SATA storage and SATA storage controllers.
• SAS controllers: SAS controllers are used with serial attached SCSI (SAS) HDDs and SSDs. Used for high-performance computing tasks, SAS supports higher transfer rates than SATA. SAS controllers are also frequently compatible with SATA devices, offering a storage setup with greater flexibility and options. These types of controllers and devices are commonly found in enterprise-level environments. They’re used for large-scale databases and complex virtualization tasks when reliable and high-speed data transfer is mission-critical.

SAS controllers are also well suited for daisy-chaining multiple storage devices, making them a good choice for operations that value scalable solutions. In these situations, a hardware component called an SAS expander can be employed to increase the number of SAS or SATA disk drives a system’s host bus adapter (HBA) can accept.

Function-based controllers are designed to accommodate storage devices based on interface protocols and intended functionality. These are a few examples:

• RAID controllers: RAID storage technology combines multiple storage drives into a single storage pool for improved performance and redundancy. Through techniques like striping (which splits data into smaller parts for speedy simultaneous writing across multiple drives) and mirroring (which duplicates data for redundancy), RAID provides safeguards against drive failure and increases data throughput. RAID controllers are designed to support multiple RAID levels—such as RAID 0, RAID 1, RAID 5 up to RAID 60—each offering various levels and combinations of performance and redundancy.
• Storage array controllers: Storage array controllers are used for managing huge arrays of storage devices designed for high-end systems requiring fast and highly available data transfer with reliable redundancy. Storage array systems that use this type of controller might actually have multiple controllers for improved functionality and reliability.