Shared Storage (NAS, SAN, iSCSI) – CompTIA Server+ Guide
Why Shared Storage Matters
In modern data centers and enterprise environments, servers rarely operate in isolation. They need access to centralized, reliable, and high-performance storage that can be shared across multiple hosts. Shared storage is the backbone of virtualization, clustering, high availability, disaster recovery, and efficient data management. Without shared storage, features such as live migration of virtual machines, failover clustering, and centralized backup would be extremely difficult or impossible to implement. Understanding shared storage technologies is essential for any server administrator and is a key topic on the CompTIA Server+ exam.
What Is Shared Storage?
Shared storage refers to storage resources that are accessible by multiple servers or hosts simultaneously over a network. Rather than each server relying solely on its own internal hard drives, shared storage pools disk resources into a centralized system that can be provisioned, managed, and accessed by many systems. The three primary shared storage technologies you need to know are:
1. NAS (Network Attached Storage)
NAS is a dedicated file-level storage device connected to a network that provides data access to heterogeneous clients (Windows, Linux, macOS). NAS devices use standard network protocols to share files.
Key Characteristics:
- Operates at the file level – clients access files and directories, not raw disk blocks.
- Connects via standard Ethernet (1 GbE, 10 GbE, or faster).
- Uses file-sharing protocols such as NFS (Network File System, commonly used with Linux/UNIX), SMB/CIFS (Server Message Block / Common Internet File System, commonly used with Windows), and AFP (Apple Filing Protocol).
- Easy to set up and manage; often plug-and-play.
- Ideal for general file sharing, home directories, media storage, and backup targets.
- NAS devices typically have their own operating system and manage their own file systems.
- Performance is dependent on the network bandwidth and latency of the Ethernet connection.
2. SAN (Storage Area Network)
A SAN is a high-speed, dedicated network that provides block-level access to storage. Unlike NAS, a SAN presents raw storage volumes (LUNs – Logical Unit Numbers) directly to servers, which then format and manage their own file systems on top of the storage.
Key Characteristics:
- Operates at the block level – the server sees the storage as if it were a locally attached disk.
- Typically uses Fibre Channel (FC) as the interconnect, running at speeds of 8, 16, 32, or 64 Gbps.
- Components include HBAs (Host Bus Adapters) in the servers, FC switches (forming the SAN fabric), and storage arrays.
- Uses zoning (on switches) and LUN masking (on storage arrays) to control which servers can see which storage volumes.
- High performance and low latency, making SANs ideal for databases, transactional applications, virtualization datastores, and mission-critical workloads.
- More complex and expensive to implement compared to NAS.
- Supports advanced features such as snapshots, replication, thin provisioning, and multipathing.
- WWN (World Wide Name) is used to uniquely identify devices in a Fibre Channel SAN, similar to how MAC addresses work in Ethernet.
3. iSCSI (Internet Small Computer Systems Interface)
iSCSI is a protocol that enables block-level storage access over standard TCP/IP Ethernet networks. It essentially encapsulates SCSI commands inside TCP/IP packets, allowing servers to access remote storage as if it were a local disk – but over regular network infrastructure instead of expensive Fibre Channel.
Key Characteristics:
- Operates at the block level, similar to Fibre Channel SAN.
- Uses existing Ethernet infrastructure (1 GbE, 10 GbE, 25 GbE, etc.), reducing cost compared to Fibre Channel.
- iSCSI Initiator: The client/server that sends SCSI commands (can be software-based or hardware-based via an iSCSI HBA/TOE card).
- iSCSI Target: The storage device or server that receives and processes the SCSI commands and provides the storage volumes.
- Uses IQN (iSCSI Qualified Name) to uniquely identify initiators and targets.
- Supports CHAP (Challenge-Handshake Authentication Protocol) for authentication between initiators and targets.
- A cost-effective alternative to Fibre Channel SANs, suitable for small to midsize deployments or environments where FC infrastructure is not justified.
- Performance can be enhanced by using jumbo frames (MTU 9000), dedicated VLANs for iSCSI traffic, and MPIO (Multipath I/O) for redundancy and load balancing.
- iSCSI traffic should ideally be isolated on a dedicated network or VLAN to prevent congestion and ensure consistent performance.
How Shared Storage Works – A Comparative Overview
NAS:
Server → Ethernet Network → NAS Device (File-Level Access via NFS/SMB)
Fibre Channel SAN:
Server (HBA) → FC Switch (Fabric) → Storage Array (Block-Level Access via FC Protocol)
iSCSI:
Server (iSCSI Initiator) → Ethernet Network → iSCSI Target/Storage Array (Block-Level Access via TCP/IP)
Key Differences at a Glance
| Feature | NAS | SAN (Fibre Channel) | iSCSI |
|---|---|---|---|
| Access Level | File | Block | Block |
| Protocol | NFS, SMB/CIFS | Fibre Channel | iSCSI over TCP/IP |
| Network | Ethernet | Fibre Channel | Ethernet |
| Adapter | NIC | HBA | iSCSI HBA or NIC |
| Identifier | IP/hostname | WWN | IQN |
| Cost | Low-Medium | High | Low-Medium |
| Complexity | Low | High | Medium |
| Best For | File sharing, backups | High-performance DBs, VMs | Budget-friendly block storage |
Important Concepts to Understand
LUN (Logical Unit Number): A logical representation of a portion of storage on a SAN or iSCSI target. LUNs are presented to servers as if they were local disks.
Zoning: A SAN fabric configuration that restricts which HBAs (servers) can communicate with which storage ports. This improves security and reduces unnecessary traffic.
LUN Masking: A storage-side configuration that controls which servers (based on WWN or IQN) can access specific LUNs. Provides an additional layer of access control beyond zoning.
Multipathing (MPIO): Configuring multiple physical paths between a server and storage for redundancy and performance. If one path fails, traffic automatically fails over to another path.
Fibre Channel over Ethernet (FCoE): A technology that encapsulates Fibre Channel frames over Ethernet networks, converging storage and data traffic on a single network infrastructure. Requires Converged Network Adapters (CNAs) and lossless Ethernet (DCB – Data Center Bridging).
Jumbo Frames: Ethernet frames with an MTU of 9000 bytes (vs. the default 1500). Used in iSCSI and NAS environments to reduce overhead and improve throughput. All devices in the path must support the same MTU size.
CHAP Authentication: Used in iSCSI to authenticate initiators and targets, preventing unauthorized access to storage resources.
Thin Provisioning: Allocating storage to a LUN on-demand rather than reserving the full amount upfront. This maximizes storage utilization efficiency.
Snapshots and Replication: Storage arrays often support point-in-time snapshots for data protection and replication to remote sites for disaster recovery.
Exam Tips: Answering Questions on Shared Storage (NAS, SAN, iSCSI)
1. Know the access level distinction: NAS = file-level access. SAN and iSCSI = block-level access. This is one of the most commonly tested concepts. If a question mentions file sharing or protocols like NFS/SMB, it is referring to NAS. If it mentions LUNs, block-level access, or raw disk volumes, it is referring to SAN or iSCSI.
2. Identify the network type: Fibre Channel SAN uses a dedicated FC fabric with HBAs and FC switches. iSCSI and NAS both use Ethernet. If a question mentions HBAs, WWNs, or FC switches, think Fibre Channel SAN. If it mentions iSCSI initiators, IQNs, or CHAP, think iSCSI.
3. Remember the unique identifiers: Fibre Channel = WWN. iSCSI = IQN. NAS = IP address or hostname.
4. Understand zoning vs. LUN masking: Zoning is configured on the FC switch (fabric level). LUN masking is configured on the storage array. Both are used together for comprehensive access control.
5. iSCSI best practices: Expect questions about isolating iSCSI traffic on a dedicated VLAN or network, using jumbo frames for performance, enabling MPIO for redundancy, and using CHAP for security.
6. Cost considerations: If a question asks about a cost-effective block-level storage solution, the answer is likely iSCSI, since it uses existing Ethernet infrastructure. Fibre Channel is the highest-performing but most expensive option.
7. Virtualization scenarios: Shared storage (especially SAN and iSCSI) is essential for VM live migration (e.g., VMware vMotion, Hyper-V Live Migration) and failover clustering. If a question describes a scenario requiring VMs to move between hosts without downtime, shared block-level storage is required.
8. NAS use cases: NAS is best suited for file sharing, user home directories, media libraries, and backup repositories. If the question is about serving files to multiple users across different operating systems, NAS is the answer.
9. FCoE awareness: Know that FCoE converges FC and Ethernet traffic, requires CNAs, and needs lossless Ethernet (DCB). It is a way to reduce cabling and infrastructure by running storage and data traffic on the same physical network.
10. Process of elimination: Many questions will describe a scenario and ask you to choose the best technology. Quickly determine: Is it file-level or block-level? Is it using Ethernet or Fibre Channel? What is the budget? What is the performance requirement? These answers will guide you to the correct choice.
11. Multipathing matters: Questions about redundancy and path failover in storage environments will likely involve MPIO. Know that MPIO works with both Fibre Channel and iSCSI, and that it requires multiple physical connections between server and storage.
12. Watch for keyword traps: "Network Attached" does not mean block-level just because it is on a network. NAS is network-attached but provides file-level access. Similarly, iSCSI uses the network but provides block-level access. Always focus on the access level, not just the connection method.
By mastering these concepts and practicing scenario-based questions, you will be well-prepared to answer any CompTIA Server+ exam question related to shared storage technologies including NAS, SAN, and iSCSI.
