physics

Write once read many

Write once read many is a physics 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 Write once read many rather than just read about it. In short: Write once read many (WORM) describes a data storage device in which information, once written, cannot be modified. This write protection affords the assurance that the data cannot be tampered with once it is written to the device, excluding the possibility of data loss from human error, computer bugs, or malware.

Key takeaways

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

Reference excerpt

Write once read many (WORM) describes a data storage device in which information, once written, cannot be modified. This write protection affords the assurance that the data cannot be tampered with once it is written to the device, excluding the possibility of data loss from human error, computer bugs, or malware. On ordinary (non-WORM) data storage devices, the number of times data can be modified is limited only by the lifespan of the device, as modification involves physical changes that may cause wear to the device. The "read many" aspect is unremarkable, as modern storage devices permit unlimited reading of data once written. Historical exceptions include time-limited discs such as Flexplay, designed for short-term rental of movies; and early non-volatile memory technologies such as magnetic-core memory and bubble memory, from which reading data also erased it. WORM prevents important data being deleted or modified, helping to preserve its authenticity.

History

WORM drives preceded the invention of the CD-R, DVD-R and BD-R. An example was the IBM 3363. These drives typically used either a 5.1 in (13 cm) or a 12 in (30 cm) disc in a cartridge, with an ablative optical layer that could be written to only once, and were often used in places like libraries that needed to store large amounts of data. Interfaces to connect these to PCs also existed. Punched cards and paper tape are obsolete WORM media. Although any unpunched area of the medium could be punched after the first write of the medium, doing so was virtually never useful. Read-only memory (ROM) is also a WORM medium. Such memory may contain the instructions to a computer to read the operating system from another storage device such as a hard disk. The non-technical end-user, however, cannot write the ROM even once but considers it part of the unchangeable computing platform. WORM was utilized for broker-dealer records within the Financial Industry Regulatory Authority and the U.S. Securities and Exchange Commission.

Current WORM drives

The CD-R, DVD-R and BD-R optical discs for computers are common WORM devices. On these discs, no region of the disc can be recorded a second time. Through packet writing, which uses the Universal Disk Format (UDF) file system, these discs often use a file system that permits additional files, and even revised versions of a file by the same name, to be recorded in a different region of the disc. To the user, the disc appears to allow additions and revisions until all the disk space is used. The SD card and microSD card spec allows for multiple forms of write-protection. The most common form, only available when using a full-size SD card, provides a physical write protection switch which allows the user to advise the host card reader to disallow write access. This does not protect the data on the card if the card reader hardware is not built to respect the write protection switch. Multiple vendors beginning in the early 2000s developed magnetic WORM devices. These archival grade storage devices utilize a variation of RAID and magnetic storage technologies to secure data from unauthorized alteration or modification at both the hardware and software levels. As the cost of magnetic (and solid-state) storage has decreased, so has the cost for these archival storage technologies. These technologies are almost always integrated directly into a content/document management system that manages retention schedules and access controls, along with document level history. There are multiple vendors providing magnetic storage technologies including NetApp, EMC Centera, KOM Networks, and others. In 2013, GreenTec-USA, Inc. developed WORM hard disk drives in capacities of 3 TB and greater. Prevention of rewrite is done at the physical disk level and cannot be modified or overridden by the attached computer.

Research In the first decade of the 21st century, there had been a renewed interest in WORM based on organic components, such as PEDOT:PSS or other polymers such as PVK or PCz. Organic WORM devices, considered organic memory, can be used as memory elements for low-power RFID tags.

Worked examples

Example 1 — a first encounter with Write once read many

Start with the simplest possible case. Write down what Write once read many claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 Write once read many 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 Write once read many 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 Write once read many

In research: Write once read many appears in physics 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 Write once read many 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: Write once read many is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical computer storage, so understanding it makes those chapters shorter.
In everyday life: Look for Write once read many 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Write once read many in 20 minutes

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

Frequently asked questions

What is Write once read many in simple terms?

Why does Write once read many matter?

Because it connects several physics 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 Write once read many?

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 Write once read many.