computer science

Storage virtualization

Storage virtualization is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Storage virtualization rather than just read about it. In short: In computer science, storage virtualization is "the process of presenting a logical view of the physical storage resources to" a host computer system, "treating all storage media (hard disk, optical disk, tape, etc.) in the enterprise as a single pool of storage." A "storage system" is also known as a storage array, disk array, or filer. Storage systems typically use special hardware and software along with disk dri…

Key takeaways

Storage virtualization 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 Storage virtualization to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Storage virtualization from memory before moving on to harder problems.

Reference excerpt

In computer science, storage virtualization is "the process of presenting a logical view of the physical storage resources to" a host computer system, "treating all storage media (hard disk, optical disk, tape, etc.) in the enterprise as a single pool of storage." A "storage system" is also known as a storage array, disk array, or filer. Storage systems typically use special hardware and software along with disk drives in order to provide very fast and reliable storage for computing and data processing. Storage systems are complex, and may be thought of as a special purpose computer designed to provide storage capacity along with advanced data protection features. Disk drives are only one element within a storage system, along with hardware and special purpose embedded software within the system. Storage systems can provide either block accessed storage, or file accessed storage. Block access is typically delivered over Fibre Channel, iSCSI, SAS, FICON or other protocols. File access is often provided using NFS or SMB protocols. Within the context of a storage system, there are two primary types of virtualization that can occur:

Block virtualization used in this context refers to the abstraction (separation) of logical storage (partition) from physical storage so that it may be accessed without regard to physical storage or heterogeneous structure. This separation allows the administrators of the storage system greater flexibility in how they manage storage for end users. File virtualization addresses the NAS challenges by eliminating the dependencies between the data accessed at the file level and the location where the files are physically stored. This provides opportunities to optimize storage use and server consolidation and to perform non-disruptive file migrations.

Block virtualization

Address space remapping Virtualization of storage helps achieve location independence by abstracting the physical location of the data. The virtualization system presents to the user a logical space for data storage and handles the process of mapping it to the actual physical location. It is possible to have multiple layers of virtualization or mapping. It is then possible that the output of one layer of virtualization can then be used as the input for a higher layer of virtualization. Virtualization maps space between back-end resources, to front-end resources. In this instance, "back-end" refers to a logical unit number (LUN) that is not presented to a computer, or host system for direct use. A "front-end" LUN or volume is presented to a host or computer system for use. The actual form of the mapping will depend on the chosen implementation. Some implementations may limit the granularity of the mapping which may limit the capabilities of the device. Typical granularities range from a single physical disk down to some small subset (multiples of megabytes or gigabytes) of the physical disk. In a block-based storage environment, a single block of information is addressed using a LUN identifier and an offset within that LUN – known as a logical block addressing (LBA).

Metadata The virtualization software or device is responsible for maintaining a consistent view of all the mapping information for the virtualized storage. This mapping information is often called metadata and is stored as a mapping table. The address space may be limited by the capacity needed to maintain the mapping table. The level of granularity, and the total addressable space both directly impact the size of the meta-data, and hence the mapping table. For this reason, it is common to have trade-offs, between the amount of addressable capacity and the granularity or access granularity. One common method to address these limits is to use multiple levels of virtualization. In several storage systems deployed today, it is common to utilize three layers of virtualization. Some implementations do not use a mapping table, and instead calculate locations using an algorithm. These implementations utilize dynamic methods to calculate the location on access, rather than storing the information in a mapping table.

I/O redirection The virtualization software or device uses the metadata to re-direct I/O requests. It will receive an incoming I/O request containing information about the location of the data in terms of the logical disk (vdisk) and translates this into a new I/O request to the physical disk location. For example, the virtualization device may :

Receive a read request for vdisk LUN ID=1, LBA=32 Perform a meta-data look up for LUN ID=1, LBA=32, and finds this maps to physical LUN ID=7, LBA0 Sends a read request to physical LUN ID=7, LBA0 Receives the data back from the physical LUN Sends the data back to the originator as if it had come from vdisk LUN ID=1, LBA32

Capabilities Most implementations allow for heterogeneous management of multi-vendor storage devices within the scope of a given implementation's support matrix. This means that the following capabilities are not limited to a single vendor's device (as with similar capabilities provided by specific storage controllers) and are in fact possible across different vendors' devices.

Replication

Data replication techniques are not limited to virtualization appliances and as such are not described here in detail. However most implementations will provide some or all of these replication services. When storage is virtualized, replication services must be implemented above the software or device that is performing the virtualization. This is true because it is only above the virtualization layer that a true and consistent image of the logical disk (vdisk) can be copied. This limits the services that some implementations can implement – or makes them seriously difficult to implement. If the virtualization is implemented in the network or higher, this renders any replication services provided by the underlying storage controllers useless.

Remote data replication for disaster recovery Synchronous Mirroring – where I/O completion is only returned when the remote site acknowledges the completion. Applicable for shorter distances (<200 km) Asynchronous Mirroring – where I/O completion is returned before the remote site has acknowledged the completion. Applicable for much greater distances (>200 km) Point-In-Time Snapshots to copy or clone data for diverse uses When combined with thin provisioning, enables space-efficient snapshots

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Storage virtualization

Start with the simplest possible case. Write down what Storage virtualization 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 Storage virtualization 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 Storage virtualization 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 Storage virtualization

In research: Storage virtualization 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 Storage virtualization 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: Storage virtualization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer data storage, Information technology management, Storage virtualization, so understanding it makes those chapters shorter.
In everyday life: Look for Storage virtualization 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.

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How to study Storage virtualization in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Storage virtualization 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 Storage virtualization out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Storage virtualization in simple terms?

Why does Storage virtualization 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 Storage virtualization?

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 Storage virtualization.