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A shared disk array is a physical storage array whose logical storage can be accessed by more than one host server, usually over a storage area network (SAN). In a cluster, that shared access lets an application restart on another eligible node and still reach its data after a node fails. The array makes the storage available to several servers, but it does not by itself provide a shared filesystem or permit safe simultaneous writes. Cluster software, the filesystem or application, and access controls must coordinate who writes and when.
What the term means
The phrase combines two ideas that are easy to blur together: the hardware and the way hosts use it.
- The array is the hardware. Dell describes a storage array as an enclosure of physical disks accessed through RAID controller modules, with one or more attached host servers able to reach the data. That definition comes from Dell’s PowerVault MD3860i Series Deployment Guide, so it describes that product family rather than every array on the market.
- Sharing describes host access. An array becomes a shared disk array when logical storage on it is presented to more than one host, and those hosts are meant to use it together, typically as nodes of a cluster.
Because the word “shared” describes access, not hardware design, the same physical array can be shared in one deployment and dedicated to a single server in another.
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How hosts reach the storage
In a common enterprise pattern, disks and controllers sit in the array, the array presents logical units to servers, and a storage network carries the traffic. Veritas’s documentation for its Cluster Server product (Solaris, version 9.2.2 Administrator’s Guide, last published September 18, 2026) describes a basic shared-storage cluster in which a single cluster shares access to a storage device, typically over a SAN. The table below separates the terms that are often used interchangeably.
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| Term | What it refers to | Typical access style |
|---|---|---|
| Disk array / storage array | An enclosure or system with multiple physical disks and controller functions, presenting logical storage | Depends on the attached network and protocol |
| Shared disk / shared storage | A storage device or logical resource reachable from more than one host | Block-level access, with access restricted or coordinated by cluster software |
| SAN | A storage network through which servers reach block storage | Block storage (hosts see disks) |
| NAS | Network-attached file storage | File access (for example SMB or NFS), where the storage system manages the filesystem |
Microsoft’s failover clustering guidance groups SAN and NAS together as networked storage options for clusters, but they present storage differently. Treat “SAN,” “NAS,” and “disk array” as separate concepts, not synonyms.
Shared access is not the same as safe shared writing
The most common misreading is that connecting one disk to several hosts makes ordinary filesystems safe for concurrent use. It does not. A shared disk architecture says only that nodes share storage. It says nothing about whether all nodes may write at the same time, so the cluster, filesystem, and application rules decide that.
- Cluster manager. Microsoft’s guidance for shared managed disks in Azure states that a cluster manager handles node communication and write locking.
- Fencing. SAP’s guidance on failover clusters describes fencing, which keeps a failed host from continuing to write to shared files after failover. This is platform-specific guidance from SAP, not a universal rule for every cluster product.
- Cluster-aware filesystem or application. A shared block device may need a filesystem or application designed for multi-host access before more than one host can use it safely.
Shared block storage versus shared file storage
A shared disk exposes block storage: each host sees disks and formats or mounts them according to its own software. That is different from a shared folder. Microsoft’s Azure documentation, “Share an Azure managed disk across VMs,” says: “Shared disks offer shared block storage that can be accessed from multiple VMs, these are exposed as logical unit numbers (LUNs).” The same page notes that shared managed disks do not natively provide a fully managed SMB or NFS filesystem. If your application expects a shared directory, you need a file service or a cluster-aware filesystem layered on the block device.
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Failover on shared storage follows a sequence. Exact behavior depends on the cluster product, so verify each step against your platform’s documentation.
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- The cluster detects that a node has stopped responding or has failed a health check.
- The cluster fences the failed node so it cannot keep writing to the shared storage.
- The cluster brings the application’s storage resources online on another eligible node that has access to the same logical units.
- The application starts on the new node and recovers its data using its own consistency mechanisms.
The array keeps the data reachable for the surviving node. It does not decide which node is allowed to write; the cluster software does that.
What the array does and does not protect
Redundancy is a design choice, not something the term guarantees. Dell notes that multiple physical paths between hosts and the array can keep access available if one path fails. Microsoft’s failover clustering storage guidance describes SAN and NAS availability and resiliency as provided by the storage platform, and it recommends removing single points of failure. A complete design therefore also considers:
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- host bus adapters or network adapters, switches, and cabling,
- redundant storage controllers and multipath configuration,
- power supplies and site-level dependencies,
- cluster software compatibility with the array and host operating system,
- fencing behavior and tested recovery procedures.
An array with redundant controllers still fails if the only path to it runs through one switch, and a cluster that never tests failover may not recover as expected.
Checks before you rely on a shared disk design
- Confirm which cluster product and version support the array and host operating system combination, using the vendor’s compatibility documentation.
- Identify the component that prevents conflicting writes: a cluster manager, a fencing mechanism, or application-level ownership.
- Check whether the filesystem is cluster-aware if more than one host will mount the same logical unit.
- Map every path from each host to the array and remove single points of failure.
- Test a planned failover on the real configuration before production use.
Cloud features change, so verify current limits for shared-disk services such as Azure shared managed disks, because the Microsoft page used here is marked as previous-version documentation.
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Where a shared disk array is used, the exact software behavior, supported configurations, and throughput or scale limits depend on the platform involved. Confirm them against the vendor documentation for your own versions rather than assuming they carry over from another product.
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