science

LogFS

LogFS is a 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 LogFS rather than just read about it. In short: LogFS is a Linux log-structured and scalable flash file system, intended for use on large devices of flash memory. It is written by Jörn Engel and in part sponsored by the CE Linux Forum.

Key takeaways

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

Reference excerpt

LogFS is a Linux log-structured and scalable flash file system, intended for use on large devices of flash memory. It is written by Jörn Engel and in part sponsored by the CE Linux Forum. LogFS was introduced in the mainline Linux kernel in version 2.6.34, released on May 16, 2010. It was removed from the codebase during the merge window of version 4.10, in December 2016, because it was "unmaintained for years and seemingly unused".

History As of November 2008, LogFS was mature enough to pass its entire test suite, and was subsequently included in the mainline Linux kernel, marked as 'experimental', in version 2.6.34 released on May 16, 2010. However, it did not attract a large user base and was removed from the kernel in December 2016.

Operation LogFS was motivated by the difficulties of JFFS2 with larger flash-memory drives. LogFS stores the inode tree on the drive; JFFS2 does not, which requires it to scan the entire drive at mount and cache the entire tree in RAM. For larger drives, the scan can take dozens of seconds and the tree can take a significant amount of main memory. LogFS avoids these penalties, but it does do more work while the system is running and uses some of the drive's space for holding the inode tree. LogFS stores a file's inode tree on the drive, which means on a write to the file, each ancestor node in the tree must be rewritten. This is done by a "wandering tree" update. The lowest node in the tree (i.e., the data) is written first, each node is written ascending the tree, until the root inode is updated. Writing the root last maintains atomicity of the update. A flash-memory block is the unit for erasures and is usually larger than the file-system block. LogFS handles this disparity by packing multiple file-system blocks into a single flash-memory block. A "sum" entry at the end of the flash-memory block records what data is stored in it. When the flash-memory block has all its file-system blocks moved or deleted, it can be erased and used for new data. For peak usage of the flash-memory drive, it is necessary to compact data so that flash-memory blocks are full of useful data. This is accomplished by garbage collection. LogFS's garbage collection strategy relies on file data being placed in a certain way into flash-memory blocks: a flash-memory block will hold only file data from the same level in the inode tree. LogFS can garbage collect the top level of the trees using just 1 empty flash-memory block. It can garbage collect the top 2 levels of the trees using 2 empty flash-memory blocks. And can garbage collect all N levels of the trees using N empty flash memory blocks. The algorithm is exponential time in the worst case, but the worst case is rare and the algorithm requires reserving only a handful of flash-memory blocks.

Worked examples

Example 1 — a first encounter with LogFS

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

In research: LogFS appears in 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 LogFS 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: LogFS is common in secondary-school and first-year university syllabi. It links to neighbouring topics Disk file systems, Embedded Linux, Flash file systems, so understanding it makes those chapters shorter.
In everyday life: Look for LogFS 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 LogFS in 20 minutes

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

Frequently asked questions

What is LogFS in simple terms?

LogFS is a Linux log-structured and scalable flash file system, intended for use on large devices of flash memory. It is written by Jörn Engel and in part sponsored by the CE Linux Forum.

Why does LogFS matter?

Because it connects several 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 LogFS?

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 LogFS.