With the exponential growth of data, scalability has become one of the most important considerations when designing storage infrastructure. Two major storage architectures, Direct Attached Storage (DAS) and Storage Area Network (SAN), each have their own advantages and limitations when it comes to scalability. In this article, we will compare DAS and SAN scalability to help you determine which solution best fits your needs.
What is DAS?
DAS refers to storage devices that are directly attached to a computer or server, usually via SATA, SAS or Fibre Channel. Some examples of DAS devices include hard drives, solid state drives, and drive enclosures. The main advantages of DAS include:
- Simple setup – DAS devices can be easily connected to and accessed by the host computer.
- Cost-effective – DAS solutions are typically less expensive than networked storage.
- High performance – Because DAS has a direct connection, it provides fast read/write speeds.
Scalability of DAS
The scalability of DAS is limited by the number of drive bays and ports on the host computer. Some ways DAS can scale include:
- Adding higher capacity drives to increase storage density.
- Upgrading to a larger DAS enclosure with more drive bays.
- Adding more host bus adapter (HBA) ports for connecting additional enclosures.
- Using shelf expanders to daisy-chain multiple enclosures together.
Most direct attached storage enclosures hold around 12-24 drives. Higher-end solutions can go up to 60 drives in a single enclosure. After maxing out drive bays and HBA ports, the only way to add more DAS is to add more servers or appliances.
What is SAN?
SAN (Storage Area Network) is a dedicated, high-speed network that provides access to consolidated, block-level storage. SAN typically uses Fibre Channel networking and protocols to connect servers and storage devices. Key benefits of SAN include:
- Centralized storage – Resources are pooled into a shared environment.
- Scalability – SAN storage can easily expand to meet demands.
- High bandwidth – Fibre Channel offers fast data transfer speeds.
- Flexibility – Servers can access any storage device on the SAN.
Scalability of SAN
One of the main advantages of SAN is its ability to scale-out to meet growing storage needs. SAN can be expanded in several ways:
- Add more SAN switches and HBAs to increase fabric ports.
- Upgrade to faster Fibre Channel speeds (16/32/64/128 Gbps).
- Use SAN fabric zoning to create more logical ports.
- Add disk enclosures and storage arrays to the SAN fabric.
- Increase capacity of existing arrays with more drives.
Enterprise SANs can be scaled out to connect tens of thousands of servers to petabytes of storage capacity across multiple data centers. The centralized nature of the SAN fabric makes expansion much easier compared to scaling out many separate DAS silos.
Comparing DAS vs. SAN Scalability
| Factor | DAS Scalability | SAN Scalability |
|---|---|---|
| Capacity | Limited by drive slots in attached enclosures, typically 10s to 100s of TBs | Virtually unlimited capacity by adding storage arrays, can scale to PBs |
| Performance | Fast, but constrained by HBA ports and host capabilities | Very high performance possible by adding SAN bandwidth and flash storage |
| Availability | Reliant on individual host and connectivity | No single point of failure, highly redundant |
| Manageability | Must be managed server-by-server | Centralized management for entire SAN fabric |
| Cost | Lower CapEx, but can require more siloed servers | Higher CapEx for SAN infrastructure |
As the table illustrates, SAN provides almost unlimited scalability while DAS capacities are limited to the capabilities of the individual servers. SAN also provides much better performance, availability, and centralized management as it scales. The tradeoff is the higher upfront investment required for the SAN fabric and shared storage systems.
Factors that Impact DAS Scalability
Several technical factors can limit how much DAS capacity can be added to a server:
- Drive interfaces – Faster drive interfaces like SAS allow more drives to be connected, but may require upgrading host bus adapter cards.
- Enclosure limits – The physical number of drive bays in attached enclosures restricts capacity.
- HBA ports – Servers only have a finite number of adapter card ports available for connecting DAS shelves.
- Host processing – The computing resources of the host will limit how many drives can be operated efficiently.
- Cabling – Longer distances and more drives may require fiber optic cabling.
If any of these resources become bottlenecks, adding more DAS can become difficult or impossible without major host upgrades.
Factors that Impact SAN Scalability
SAN scaling can also be impacted by the following technical factors:
- SAN fabric bandwidth – May require upgrades to faster Fibre Channel speeds.
- SAN fabric ports – Adding more switch ports allows connecting more devices.
- Storage controller processing – Storage processors must handle added capacity.
- Storage media performance – Disk or flash technologies affect overall throughput.
- Network cables – Longer cables may need fiber optic connections.
While these factors can constrain performance, with proper planning, a SAN can continue to expand to extremely large capacities. The centralized nature of the SAN fabric also simplifies management.
Conclusion
In summary, while DAS provides a simple, cost-effective storage solution, its scalability is severely limited by the capabilities of the individual servers. SAN storage can scale almost infinitely while also providing performance, availability and manageability benefits. For organizations that anticipate rapid data growth or need storage consolidation, the higher upfront investment in a SAN is usually worthwhile to avoid the scaling challenges inherent with DAS architectures.
When evaluating scalability, IT planners must consider not just immediate capacity requirements, but projected future needs over a 3-5 year period. This long-term outlook is critical to select a storage infrastructure that can readily expand without major disruptions. For most enterprise environments experiencing exponential data growth, a shared SAN storage platform will provide the most seamless and flexible scalability.
