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A shared disk array is a storage array whose disks can be reached by more than one server, usually over a storage area network (SAN). In a cluster, that shared access is what lets an application restart on another eligible node and still reach its data after a node fails. The array alone does not make simultaneous writes safe. Cluster software, the filesystem or application, and access controls must coordinate who may write, and when.
What the term means
A disk array is an enclosure or system that contains multiple physical disks and the controller hardware that manages them. It usually presents logical storage units to attached hosts rather than raw physical drives. Dell describes an array this way in its PowerVault MD3860i Series Deployment Guide: an enclosure of physical disks accessed through RAID controller modules, with one or more host servers able to access the data. That description comes from one product family, so controller count, drive layout, and management features vary between products.
“Shared” describes how hosts access the array, not what the hardware is. A disk array becomes a shared disk array when more than one host is configured to access the same logical storage. The term is often confused with several neighboring concepts, which are easier to separate in a table.
| Term | What it refers to | Common confusion |
|---|---|---|
| Disk array / storage array | An enclosure or system holding multiple physical disks and controller functions, presenting logical storage to hosts (Dell, PowerVault MD3860i Series Deployment Guide) | Assuming every product has identical components or features |
| Shared disk / shared storage | Storage that more than one host can access. In a cluster, access may be restricted or coordinated by cluster software (Veritas InfoScale, Cluster Server 9.2.2 Administrator’s Guide for Solaris; Microsoft Learn) | Treating shared access as the same as safe simultaneous writing |
| SAN (storage area network) | A network through which servers access block storage. Veritas describes shared storage in a basic cluster as typically reached over a SAN | Using “SAN” to mean “shared folder” |
| NAS (network-attached storage) | File storage presented over the network, accessed through file protocols such as SMB or NFS | Using “NAS” and “SAN” as synonyms |
| Shared disk architecture | A compute design in which cluster nodes share the same storage | Assuming it says whether all nodes can write at once. It does not; the cluster, filesystem, and application rules decide that |
How shared access works in a cluster
In a typical enterprise setup, disks and controllers sit in the array, the array presents logical units to servers through a SAN, and cluster software decides which node runs a given workload. The sequence below shows the general pattern. Exact behavior depends on the cluster platform and vendor.
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- Storage is built in the array. Physical disks are grouped into logical units (LUNs) that the array presents to hosts.
- Hosts connect over the SAN. Each eligible cluster node has a path to the same logical units.
- Cluster software assigns the workload to one node. That node runs the application and uses the shared data.
- A failure triggers a move. If the active node fails, the cluster software starts the workload on another eligible node that can see the same storage.
- The failed node is fenced off. Fencing prevents a host that has failed or lost cluster membership from continuing to write to shared files. SAP describes this as part of its failover guidance for multiple-host systems.
Paths and controllers
Dell notes that multiple physical paths between hosts and the array can keep data reachable if one path fails. Microsoft describes SAN and NAS availability and resiliency as functions of the storage platform, and recommends removing single points of failure. Redundant paths help only if each layer, including host adapters, switches, cables, and controllers, is also redundant.
Shared block storage is not a shared folder
A shared disk exposes block storage: raw capacity that a host formats and manages. It does not automatically provide a file namespace that many machines can browse at once. Microsoft’s documentation for Azure shared managed disks states the distinction directly: “Shared disks offer shared block storage that can be accessed from multiple VMs, these are exposed as logical unit numbers (LUNs).” Microsoft also states that these disks do not natively provide a fully managed SMB or NFS filesystem.
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In practice, a shared block device usually needs one of three things before several hosts can use it safely: a cluster-aware filesystem, application-level coordination that gives one owner write access at a time, or a clustered service that manages the volume on behalf of the hosts. Which one applies depends on the platform, so confirm it before design work begins.
What the term does not guarantee
Because the phrase describes access rather than behavior, several assumptions readers often make do not follow from it.
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- Simultaneous writes are safe. Attaching the same disk to several hosts does not make an ordinary filesystem safe for concurrent writing. Microsoft assigns communication and write locking to the cluster manager.
- Failover is automatic. Shared access makes failover possible. Something must still detect the failure, fence the failed host, and start the workload elsewhere.
- The array is the only availability layer. Host connectivity, controllers, paths, power, software compatibility, and recovery procedures all affect whether data stays available.
- Every product behaves the same. Component details and supported configurations come from individual vendors and platforms.
Comparing the main design choices
When a project moves from definition to design, the useful comparison is between shared block access over a SAN and file access over NAS, measured along the axes below. No universal winner exists, because the right choice depends on the workload and platform.
| Axis | Shared block storage over SAN | File access over NAS (SMB or NFS) |
|---|---|---|
| Presentation | Logical units presented to hosts, which format and manage them (Microsoft Learn; Veritas) | Shared files and directories presented over the network (Microsoft Learn, Failover Clustering Storage Architectures in Windows Server) |
| Write coordination | Handled by the cluster manager, a cluster-aware filesystem, or the application. Microsoft requires a cluster manager for node communication and write locking on Azure shared managed disks | Handled by the file service and its protocol; the sources reviewed do not describe the internal locking model for each product |
| Failure handling | Node failover and fencing; controller and path redundancy (Dell; SAP) | Depends on the storage platform’s availability design. Microsoft describes SAN and NAS resiliency as provided by the platform |
| Compatibility and scale | Depends on cluster software, host adapters, network, and vendor-supported configurations | Throughput and scale not stated in the sources reviewed; they are platform-specific |
This comparison does not rest on performance or market figures. The sources cited here are product and platform documentation, so specific throughput or scale numbers should come from the vendor’s current specifications for the exact product and version.
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- Intelligent Temperature Control: The D2-320 HDD enclosure has an intelligent temperature-controlled and low-noise fan that automatically adjusts its speed based on the temperature of the hard disk. This feature ensures that the hard disk operates at its best temperature and provides better heat dissipation
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Quoted vendor wording
Veritas describes the basic shared-storage cluster in its InfoScale Cluster Server 9.2.2 Administrator’s Guide for Solaris, published September 18, 2026: “In this configuration, a single cluster shares access to a storage device, typically over a SAN.” That sentence describes one cluster topology, not every shared-disk design.
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Before you rely on a shared disk array
- Confirm that your cluster software explicitly supports the shared-disk configuration you plan to use.
- Identify the component that prevents conflicting writes: cluster-managed locking, a cluster-aware filesystem, or application ownership.
- Verify fencing behavior and test failover before depending on it for production data.
- Check redundancy at every layer between host and array, not only at the array.
- Confirm host adapters, switches, and array models against the vendor’s compatibility or certification list. A generic listing does not make a storage array cluster-ready.
- For cloud deployments, check the current feature limits. Microsoft’s Azure shared-disk page is marked as previous-version documentation, so the cited limits may have changed.
Sources
- Veritas InfoScale, “About basic shared storage cluster,” Cluster Server 9.2.2 Administrator’s Guide for Solaris, published September 18, 2026.
- Dell, “Introduction to Storage Arrays,” PowerVault MD3860i Series Deployment Guide.
- Microsoft Learn, “Share an Azure managed disk across VMs” (marked as previous-version documentation).
- SAP Help Portal, “Multiple-Host System Concepts.”
- Microsoft Learn, “Failover Clustering Storage Architectures in Windows Server.”
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