VSAN

Key takeaways

VSAN belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect VSAN to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of VSAN from memory before moving on to harder problems.

Reference excerpt

A virtual storage area network (virtual SAN, VSAN or vSAN) is a logical representation of a physical storage area network (SAN). A VSAN abstracts the storage-related operations from the physical storage layer, and provides shared storage access to the applications and virtual machines by combining the servers' local storage over a network into a single or multiple storage pools. The use of VSANs allows the isolation of traffic within specific portions of the network. If a problem occurs in one VSAN, that problem can be handled with a minimum of disruption to the rest of the network. VSANs can also be configured separately and independently.

Technology

Operation A VSAN operates as a dedicated piece of software responsible for storage access, and depending on the vendor, can run either as a virtual storage appliance (VSA), a storage controller that runs inside an isolated virtual machine (VM) or as an ordinary user-mode application, such as StarWind Virtual SAN, or DataCore SANsymphony. Alternatively it can be implemented as a kernel-mode loadable module, such as VMware vSAN, or Microsoft Storage Spaces Direct (S2D). A VSAN can be tied to a specific hypervisor, known as hypervisor-dedicated, or it can allow different hypervisors, known as hypervisor-agnostic. Different vendors have different requirements for the minimum number of nodes that participate in a resilient VSAN cluster. The minimum requirement is to have at least 2 for high availability.

All-flash versus hybrid VSAN Data center operators can deploy VSANs in an all-flash environment or a hybrid configuration, where flash is only used at the caching layer, and traditional spinning disk storage is used everywhere else. All-flash VSANs are higher performing, but as of 2019 were more expensive than hybrid networks.

Protocols For sharing storage over a network, VSAN utilizes protocols including Fibre Channel (FC), Internet Small Computer Systems Interface (iSCSI), Server Message Block (SMB), and Network File System (NFS), as well as proprietary protocols.

Applications A VSAN fills a similar role as physical SAN infrastructure, but is also used for workload virtualization which can include databases, virtual desktop infrastructure (VDI) environments, file servers, CRM systems, and other enterprise applications. VSANs can be used for remote branch office networks, using low-cost commodity hardware. There are also applications with DMZ, an isolated network that sits outside a company's firewall, and areas with space constraints, such as vessels, where a reduced hardware footprint 2-3 node deployment has a better chance of fitting. By being able to work on top of commodity hardware, a VSAN is often used for building highly available networks across two or more sites that are combined in a single cluster, a type of deployment known as stretched clustering. A VSAN can also create highly available hybrid cloud environments by combining existing on-premises resources with public cloud environments.

Advantages The primary advantage of a VSAN is its ability to provide shared storage. It also increases the resiliency of storage systems by allowing the creation of redundant data copies across the servers in a VSAN cluster. It does this while requiring less hardware and power consumption than a physical SAN. Using a VSAN for local storage provides other resiliency options including mirroring or erasure coding with different parity levels. Another advantage is that unlike a typical fabric that is resized switch-by-switch, a VSAN can be resized port-by-port.

Similar concepts A VSAN is often associated with hyperconvergence, a concept which refers to unifying computing, networking, and storage resources, within a single server. Similarly, a VSAN shifts away from having a separate physical storage layer and required networking stack, and instead combines storage from multiple nodes into a virtualized and consolidated storage layer. VSANs take control of the storage-related operations, access and features allowing the implementation of data deduplication, data compression, high availability and other features, on commodity x86 server hardware. VSAN is also associated with software-defined storage (SDS). However, VSAN can be also used in converged architectures where VSAN is deployed on dedicated storage nodes as a cluster, and presents the shared storage over a network to compute nodes.

See also Storage area network Fibre Channel Fibre Channel fabric VLAN, for analogous mechanism in Ethernet

References

Worked examples

Example 1 — a first encounter with VSAN

Start with the simplest possible case. Write down what VSAN claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to VSAN before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about VSAN ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of VSAN

In research: VSAN appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses VSAN in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: VSAN is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer networks, Data management, Fibre Channel, so understanding it makes those chapters shorter.
In everyday life: Look for VSAN outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study VSAN in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what VSAN means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain VSAN out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is VSAN in simple terms?

A virtual storage area network (virtual SAN, VSAN or vSAN) is a logical representation of a physical storage area network (SAN). A VSAN abstracts the storage-related operations from the physical storage layer, and provides shared storage access to the applications and virtual machines by combining…

Why does VSAN matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study VSAN?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on VSAN.